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Saturday, 28 December 2013


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Friday, 27 December 2013




Over several decades, many thousands of tons of the copper-nickel alloy UNS C70600 have been used as a seawater piping material in marine engineering. In order to provide reliable service performance of the material, this paper discusses its metallurgical properties, the relevant mechanisms of general, localized and erosion corrosion, the unique befouling resistance and the performance in polluted water. The guidelines for the production of UNS C70600 products, the design of pipe work, the limits of the alloy and useful service recommendations are outlined.

Keywords: UNS C70600, corrosion resistant alloys, seawater, piping systems, service recommendations


The choice of an appropriate material for seawater service is a difficult decision that has to be made by a designer prior to specification of the system. Since a broad range of conditions will usually be imposed on the piping material, the impact of seawater on material performance is determined by numerous variables such as condition of the material, system design, fabrication procedure, various seawater temperatures and flow regimes, biological activity, and presence of oxidizing compounds. Further factors that are relevant in choosing a material for a seawater piping system are: physical and mechanical properties, availability, material costs, ease of fabrication and maintenance, anticipated design-life and previous design experience.

Over several decades, many thousands of tons of the copper-nickel alloys UNS C71500 and UNS C70600 have been installed in different marine engineering structures for the shipbuilding, offshore, power and desalination industries. These alloys, which have been applied for seawater piping and heat exchangers, are adopted by various standards. UNS C71500 is predominantly used for military submarine service due to its higher strength and maximum allowable flow rate, as well as low magnetic permeability. However, the wider commercial application of this alloy is limited to a certain extent because of its higher material cost. The work-horse, therefore, is the UNS C70600 (CuNi 90/10, cupronickel). This alloy reveals a well-balanced combination of characteristics allowing its widespread and economical use. To ensure the further reliable application of the material, there is a need for a detailed discussion on its properties. In particular, attention should be drawn to the quality of CuNi 90/10 products, the performance in waters containing hydrogen sulfide and the prevention of erosion as well as galvanic corrosion.

This paper is based on a literature review and experience of KME as a manufacturer of copper-nickel piping systems. It describes the relevant corrosion mechanisms and provides useful service recommendations particularly for UNS C70600.


Metallurgical considerations

Copper and nickel have similar atomic radii and lattice parameters and so the phase diagram is relatively simple. At all temperatures, Cu-Ni alloys are represented by a single phase face centered cubic structure. The absence of phase transformation during thermal cycles reduces the effect of welding on mechanical characteristics and the corrosion resistance of the material. This crystallographic structure reveals very good ductility and impact strength even at temperatures well below freezing point. The slow diffusion rate for nickel in copper leads to concentration gradients in the melt and consequently increases a segregation tendency in the cast structure at normal cooling rates (Figure 2). Thus, to provide a uniform establishment of protective oxide layers, the homogenization of the segregated structure is required by hot forging or cold working with a subsequent recrystallisation anneal.

Table 1 compares the common international compositional standards for cupronickel. Some specifications monitor more strictly the iron content which is an essential alloying element and responsible for the improvement in corrosion resistance of the alloy. The alloying range between 1.5 and 2.0 wt.% Fe provides optimum resistance in flowing seawater (Figure 3)3. To provide the appropriate quality of cupro-nickel products, the cooling rate from the solution annealing temperature must keep the precipitation of iron containing particles to a minimum. The quality of the piping can be easily accessed by measurement of the relative magnetic permeability, which should be lower than 1.5.4 As for all metallic materials, if the CuNi 90/10 has to be welded, the maximum limits for some impurities such as lead, sulfur, carbon, and phosphorus should be carefully controlled.

As a confirmation of above considerations, Figure 4 illustrates one of many weld necks installed on an offshore unit operating off the coast of South America. A great number of weld necks downstream valves and pumps suffered corrosion attack. The system was operated at ambient temperature and at a flow rate below 3,5 m/s. The cross-section of the weld neck shows a cast structure. The chemical analysis of the weld neck revealed the presence of 0.952 % Fe, 0.012 % Mn, and 0.087 % S.

General Corrosion

The corrosion behavior of copper-nickel depends on the presence of oxygen and other oxidizers because it is cathodic to the hydrogen electrode. During the primary corrosion reaction, a cuprous oxide film is produced that is predominately responsible for the corrosion protection. The products of corrosion reactions can react with compounds in seawater e.g. to CuCl23Cu.(OH)2 or Cu2(OH)3Cl and in so doing build a multi-layered oxide structure. The corrosion rate quickly decreases significantly over a few days, with one study indicating that the associated discharge of copper ions was reduced tenfold during 10 min and 100-fold in the first hour. The long-term general corrosion rates of UNS C70600 have been found to continuously decrease with time of exposure to below 2.5 µm/yr.6 However, establishment of a mature film takes from 27 to 35 months at temperatures of 15-17°C. At 27°C, common inlet temperature for the Middle East, the establishment of the protective film within a few hours was reported.8 In spite of a wide utilization of cupronickel, the positive effect of the seawater temperature9 is still not well understood and more research is required on this area.

It is important to ensure appropriate formation of the protecting film and, thus, to avoid premature failures. The following recommendations work well during hydrotesting and commissioning:

·         The system should be cleaned from dirt, lubricants and debris. Introduction of solid matter should be avoided by installation of strainers or screens.

·         The usage of clean seawater or fresh water for hydrostatic testing is advisable. If polluted water has been used, it should be disposed and the pipe work should be rinsed properly with clean seawater or fresh water. If subsequent stagnant conditions are expected, blow-drying of the system is advisable.

·         During commissioning of a new or retubed system, continuous exposure to clean seawater for up to 3 months, depending on water temperature, is needed to establish a mature protective film.

·         A system with continuous pumping activity, such as a cooling system, can be commissioned under normal operating conditions. In seawater and water containing high levels of suspended matter, the minimum flow rate should not be below 0.5-1 m/s in order to prevent the formation of deposits.

·         The commissioning of a system with intermittent flow, such as a fire fighting system, should be conducted in seawater free from suspended matter, e.g. in water from the open sea. However, the sea-water has to be replaced by oxygenated seawater within 4-5 days to avoid putrefaction.


The bacteriostatic properties and resistance of cupronickel to macrofouling are well recognized and widely used. There are two different aspects to which these characteristics can be attributed:

On the one hand, there is a gradual release of cuprous ions that is not tolerated by many marine organisms. It has been shown, that the establishment of a bacterial layer on the surface of cupronickel is significantly retarded. The natural corrosion rate of cupronickel is sufficient to decelerate the initial development of the biofouling layer.

On the other hand, the cupronickel generates a passive film, which consists of several different layers. A thin cuprous oxide layer, which is formed on the bulk metal, provides sufficient corrosion resistance. The top layers, which are formed during secondary corrosion reactions, have a weakly adherent porous structure which reduces the release rate of cuprous ions. This may allow some organism settlement to take place on the surface in quieter conditions although, due to weak the adherence of the top layers, biofouling sloughs off periodically revealing the protective inner cuprous oxide layer again.

Figure 5 compares the appearance of cupronickel and steel plates exposed under similar conditions. The significant difference in the accumulation of biological mass on steel samples is evident. Costly cleaning procedures and chlorine treatment of seawater become unnecessary. However, it has been widely recognized that, if cathodically protected, the biofouling resistance of cupronickel is decreased.

Localised Corrosion

In clean natural seawater or in seawater chlorinated to levels sufficient to control the biological metabolism, UNS C70600 is resistant to localized attack. Due to its biofouling resistance, the number of potential sites for pitting attack is limited, even in slowly moving seawater. However, in polluted water containing hydrogen sulfide, pitting usually takes place in the form of wide and shallow pits. The undercut type of pitting attack normally associated with stainless steel is not common for CuNi 90/10.

Only a limited amount of information is available on failures of copper-nickel due to crevice corrosion. Theoretically, the crevice corrosion behavior of the alloy is generally controlled by an ion concentration cell mechanism where the accumulation of copper ions in the crevice leads to ennoblement. Thus, if encountered, the corrosion attack would tend to take place in the region adjacent to the crevice, which is exposed to the bulk water.

Under-deposit corrosion is really crevice corrosion beneath deposits. There is a tendency for deposition of suspended matter at flow rates below 0.5-1 m/s. Cupronickel performs well as a material for firefighting systems with predominantly quiet conditions. However, if the region under deposits become anaerobic, contributing to the establishment of sulfate reducing bacteria, the situation can become critical.

Since, stagnating conditions should be avoided in seawater containing high levels of suspended particles or biological matter; it is advisable to use the water from the open sea in stand-by systems. In addition, the frequent refreshment of seawater is preferable at least once every 4-5 days. The chlorination of seawater could improve the water quality by control of microbiological activity. During prolonged shut-down periods of the plant, it is recommended to rinse the system with clean seawater and keep it dry.

Effect of Polluted Water

Most accelerated corrosion problems and premature failures of UNS C70600 have been related to the activity of sulfate reducing bacteria (SRB) and presence of hydrogen sulfide originating from biochemical reactions. It was demonstrated14 that in seawater containing no sulfide, the free corrosion potential of cupronickel lies on the noble side of hydrogen evolution. In the presence of sulfide, however, the corrosion potentials are shifted to negative values. Therefore, hydrogen evolution becomes possible as a part of the cathodic reaction. Sulfur can be reduced to sulfide at the cathodic site. The elemental or colloidal sulfur contained in seawater may be reduced to sulfide. HS- reacts with Cu+ and produces a non-protective black cuprous sulfide which is poorly adherent and results in enhanced probability of erosion attack.

Eiselstein et al demonstrated that samples pre-exposed to aerated water corroded much slower even than fresh material in aerated polluted water. This indicated that corrosion films formed in sulfide-free environments offered some protection against accelerated attack although not longer than for three days. Finally, samples pre-exposed to aerated polluted and then exposed to aerated unpolluted water were re-passivated in less than five days. In contrast, Kirk and co-worker reported that protective films formed on cupronickel during four months exposure to clean seawater provided nearly total corrosion immunity in seawater containing up to 0.5 ppm H2S.

From general experience, UNS C70600 installed on offshore systems or seagoing vessels provides sufficient resistance to sulfide induced corrosion. However, it is desirable to replace the water in systems with stagnating conditions every 4-5 days with oxygenated clean seawater. For ships, it is recommended to fill the system with clean seawater prior to entering, while within and when leaving the harbor. Care should be taken if there is a risk of prolonged exposure to sulfide polluted water as is commonly the case in harbors and brackish water.

It has been recognized that the presence of Fe2+ in seawater, which originates from additions of ferrous sulfate (FeSO4) or installation of stimulated iron anodes, can reduce the extent of sulfide induced corrosion.

It was indicated that protection was due to establishment of a protective film consisting primarily of lepidocrocite (g-FeO.OH). Two possibilities for film formation were suggested: lepidocrocite was electrophoretically deposited on the surface from g-FeO.OH colloid formed in the solution, or, through an intermediate step, ferrous ions are transported to cathodic sites and ultimately oxidized to g-FeO.OH. The substance acted as a cathodic inhibitor increasing the cathodic polarization. In another study18, considering the zeta potentials and colloidal chemistry of FeSO4, it was postulated that the film formation takes place due to direct attraction of lepidocrocite from the colloid by Cu2O.

However, it has been recognized that extensive application of ferrous ions can result in formation of a bulky scale on the tube surface leading to deterioration of heat exchanger performance. Therefore, the control of treatment efficiency and periodical mechanical cleaning of the heat exchanger might be required. Sato19 recommended a gradual decrease in dosing levels after the initial film formation. The concentration and type of dosage capable of preventing sulfide induced corrosion are given in Table 2. It is advisable to stop the treatment with ferrous ions one hour before chlorination since the simultaneous treatments were reported to be ineffective.

It should be emphasized that during normal service on offshore units or seagoing vessels, additional ferrous sulfate dosing is seldom required. However, if exposure to polluted water is going to occur (e.g. when entering port), a reasonable additional precaution might be to apply dosing prior to entering, while in and after leaving port.20 In addition it has been reported that application of ferrous sulfate treatment is beneficial in combating the erosion of condenser tubing.


Erosion-corrosion is a combined process, which is partly the mechanical impact of a moving medium over a metal surface, and partly electrochemical processes. In the case of copper alloys, it has been generally recognised that increasing flow velocities have no significant effect on the corrosion rate until a critical velocity – the so called breakaway velocity – is reached. Depending of the inner pipe diameter, the maximum flow rates in CuNi 90/10 systems should to be conservatively limited to 3.5 m/s. It has to be emphasized, that no erosion failures are known in clean seawater within the above velocity range. Moreover, the reported erosion failures are usually associated with the basic design mistakes of the piping system.

The mechanism of erosion-corrosion is affected by hydrodynamic characteristics of the flow depending on the thickness of both the velocity boundary layer and the diffusion boundary layer at similar average flow velocities. As a result, the increasing boundary layer causes a decreased concentration gradient and thus reduces the mass transport. Consequently, if the corrosion process is determined by the mass transport from or to the surface, it can be expected that an increasing pipe diameter results in lowering erosion-corrosion rates and an increased breakdown velocity. Efird estimated that the critical velocity was 4.4 m/s for a tube 0.03 m in diameter and 6 m/s for a tube 3 m in diameter at 27°C. The calculated critical shear stress was 43.1 N/m² for UNS C70600 in seawater indicating the conditions, under which the passive film is formed, must be considered.

Severe conditions might be expected if the pipe opening has a foreign body lodged in it causing throttling of the flow and leading to an abnormal increase in local flow velocity. Therefore, entrance of debris has to be prevented by installation of strainers. Also growth of macrofouling accumulations must be prevented by appropriate biocide treatment.

Campbell evaluated the effect of controlled bubble size (1.0 and 2.3 mm) on erosion-corrosion resistance and compared these results with bubble free water using a jet impingement apparatus. The deleterious effect decreases with smaller bubble size. To minimize this effect of gas bubbles, consideration should be given to the position of inlet boxes and the construction of venting systems.

It appears reasonable to suppose that an occasional reduction in velocity may contribute to repassivation and thickening of the surface film. Knutsson presented results of an examination of flow regimes over 12 months using a copper alloy. From 1 to 25 % flow duration, no attack was observed at 11.9 m/s. Continuous flow produced results of <15 µm/yr. at 6.1 m/s and 76 µm/yr. at 11.9 m/s. Therefore, for emergency situations, as in fire fighting systems which do not experience frequent pumping activity, the flow velocities up to 10-15 m/s are reasonable for UNS C70600. During two years of testing with sand loaded natural seawater by means of a once-through loop including pipes from 4” to 7” operated intermittently, no appreciable corrosion attack was detected at velocities up to 7 m/s.

Negative effects of erosion are preventable by good design which is also necessary to improve the efficiency of the piping system. The general guidelines for reduction of friction loss depend on the flow velocity, pressure drop in the system due to the geometry of bends and valves, the required pumping power, and the probability of erosion-corrosion:

·         The layout of the system should be as direct as possible.

·         Control the flow with the least number of valves.

·         Ask the valve manufacturer for data related to the effect of valve geometry on the pressure drop in the system. In most instances, there are considerable variations for nominally similar valves.

·         Consider the effect of r/d-ratio of elbows and the effect of sudden enlargement and contractions on the pressure drop.

·         When fittings are used, specify long radius and full-form types.

·         Cut the gasket flush with the inner surface of the pipe.

·         Provide a minimum distance of 5 x I.D. between a pump or a valve and a bend.

The exact adjustment of pipe ends and outlet is important to avoid protrusions that can lead to erosion damage in service. In all cases, 100 % weld penetration without excessive penetration of the root into the tube cavity is required to avoid causing any turbulence of the fluid in service. Mismatching of pipe-ends should not exceed half of the wall thickness; however, it should be less than 2 mm.

Effect of Chlorination

Chlorination of seawater is the most common method to control biofouling and it has been reported that continuous chlorination to a residual free chlorine level of 0.25 ppm can be 100 % effective against fouling.

It has also been reported that, in the presence of 0.25 ppm free chlorine, the corrosion of CuNi 90/10 increased during 30 days of exposure but the effect of the chlorine weakened subsequently. Kirk and co-worker stated that according to general experience no negative effect of chlorine concentrations 0.2-0.5 ppm was indicated on the corrosion behavior of copper-nickel alloy during many years in coastal power and process industries. In spite of the biofouling resistance of copper-nickel, the chlorine treatment extended the intervals between mechanical cleanings to restore the performance of heat exchangers from 2 months without chlorination to up to 1 year under chlorinated conditions in coastal plants.

Francis published the results of tests related to the effect of chlorine additions in the range between 0.3 and 4.0 ppm on corrosion and jet impingement tests (jet velocity 9 m/s for 2 months) of UNS C70600 exposed to natural seawater. The products formed on cupronickel during chlorination led to appreciable anodic and cathodic polarization, and, thus, somehow to an improvement in corrosion resistance. Nevertheless, these products impaired the mechanical resistance of the copper-nickel surface leading to an increase in impingement susceptibility. For continuous and intermittent chlorine additions, the concentrations of 0.3 and 0.5 ppm respectively were recommended. Finally, the author pointed out that his results require more additional research.

Another study proposed a possible mechanism for the effect of free chlorine on corrosion performance of CuNi 90/10 in highly polluted brackish water containing appreciable amounts of planktonic and sessile sulfate reducing bacteria. It was assumed that the corrosion process in the presence of chlorine was controlled by transformation of Cu2O layer into secondary compounds such Cu2(OH)3Cl due to the high oxidizing power of the medium. These secondary products are not well adherent to the surface and easily removed allowing further Cu2O formation. Unfortunately, no mass loss or depths of corrosion attack were presented in this paper.

Obviously, more research is needed on the effect of chlorination on CuNi90/10. However, it can be concluded that an over-chlorination of seawater should be avoided.

Galvanic Corrosion

The avoidance of galvanic corrosion is a principal design consideration. Stainless steels undergo significant potential variations depending of chlorine and oxygen content, temperature, as well as presence of a biofilm, e.g. up to +800 mV SCE in seawater containing 0.5-1.0 ppm free chlorine to less than –400 mV SCE in deaerated seawater. In contrast, UNS C70600 reveals only small changes (Figure8). It has been reported that the corrosion potential of this alloy remains in the range between 0 and –300 mV SCE in natural aerated seawater at 10 and 40°C, in seawater flowing at 3 to 15 m/s at same  temperatures, and in seawater containing 0.5 ppm free chlorine at 15 °C. Figure 9 demonstrates the galvanic series of different commercial alloys in flowing seawater. UNS C70600 has a central position in the series. It is nobler than aluminum alloys, carbon and galvanized steel and less noble than stainless steels and titanium alloys. It can be coupled to tin and aluminum bronze.

Bardal et al studied coupling of CuNi 90/10 with high-alloy stainless steel in natural and chlorinated seawater. During connection of the metals in chlorinated water, no significant effect of galvanic corrosion was found. The difference was attributed to the establishment of biofilm on high-alloy stainless steel in natural seawater providing a much higher cathodic efficiency. However, precautionary measures should be taken if the chlorination might be turned off. For prevention of galvanic corrosion, it is recommended to use compatible materials wherever possible. However, in multi-material systems, the combination of different metals is often unavoidable. Thus the electrical contact of UNS C70600 with aluminum, nickel and titanium alloys, carbon and stainless steels should be avoided by application of commonly applied protection measures.

Stress-Corrosion Cracking

It has been reported36 that cupronickel failures due to stress-corrosion cracking are unknown in seawater polluted with ammonia. The alloys are also immune to chloride and sulfide stress corrosion cracking. Thus, no stress relief treatment is required for cold worked or welded material.


The merits of the UNS C70600 as an appropriate alloy for seawater pipework can be attributed to various aspects. First of all, it is a simple alloying system with a single phase face centered cubic structure providing excellent hot and cold workability. The absence of phase transformations during welding contributes to its easy weldability with no need for extensive post-weld treatments. However, the chemical composition and the manufacturing of cupronickel products must comply with international standards.

The low uniform corrosion rates of the alloy allow the specification of thinner walled piping and, therefore, provide weight saving. Cathodic protection is not usual for UNS C70600 piping. The alloy is resistant to biofouling and does not reveal sensitive corrosion potential variations under different seawater conditions. Such a combination of features leads to the improved resistance to localized corrosion and the elimination of extensive monitoring procedures associated with chlorination and higher seawater temperatures which other alloy systems might require. The alloy has high resistance to crevice corrosion and is resistant to stress corrosion cracking under marine conditions.

For lower seawater temperatures, protective surface films may take up to 3 months to fully mature. For this reason, hydrotesting and commissioning recommendations have been given. In addition, to avoid premature failures in the presence of hydrogen sulfide, precautionary measures are needed. The given practical recommendations should be followed to avoid corrosion problems. The susceptibility to erosion- corrosion and galvanic corrosion can be eliminated by recommended design considerations.

Corrosion of Monel-400 in Aerated Stagnant Arabian Gulf Seawater after Different Exposure Intervals

Corrosion of Monel-400 in Aerated Stagnant Arabian Gulf Seawater after Different Exposure Intervals

The corrosion of Monel-400 after varied exposure periods in naturally aerated Arabian Gulf seawater (AGS) has been carried out using gravimetric, cyclic potentiodynamic polarization, chronoamperometry, open-circuit potential, and impedance spectroscopy measurements along with SEM/EDX investigations. Gravimetric data within 160 days showed that the weight loss increased, while the corrosion rate decreased with time. SEM/EDX investigations after 160 days immersion indicated that dissolution of Monel takes place due to the selective dissolution of Ni. The electrochemical measurements confirmed the gravimetric data and proved that severity of uniform corrosion of Monel decreases, while pitting one increases with increasing the exposure period.

Keywords: corrosion; Monel-400 alloy; EIS; polarization; SEM; weight loss


Monel-400 is one of the most important nickel based alloys that contains about 60-70 percent nickel, 20-29 percent copper and small amounts of iron, manganese, silicon and carbon. It is a solid solution alloy that can only be hardened by cold working. This alloy was discovered due to the efforts of Robert Crooks Stanley, who worked for the International Nickel Company (INCO) in 1901. It was installed as a sheet roofing membrane in 1908. In the late 1920s, Monel-400 was begun to be used for grocery coolers, countertops, sinks, laundry and food preparation appliances, roofing and flashing.

Monel-400 is characterized by its good corrosion resistance, good weldability and high strength. Therefore, it has been used extensively in many applications such as chemical processing equipment, gasoline and fresh water tanks, crude petroleum stills, valves and pumps, propeller shafts, marine fixtures and fasteners, electrical and electronic components, de-aerating heaters, process vessels and piping, boiler feed water heaters and other heat exchangers, and etc [1-4]. For that when a piece of equipment needs to stand up to interior or exterior corrosive, Monel-400 is the fail-safe solution. It also has higher maximum working temperatures than nickel (up to 540 °C, and its melting point is 1300‒1350 °C), which makes it the preferred metal for boiler feed water heaters and other heat exchangers.

Although Monel-400 is known for its ability to stand up to tough corrosive elements, pitting corrosion of Monel-400 occurs when it is exposed to stagnant salt water such as seawater [5]. The corrosion rate of this alloy decreases sharply with increasing nickel content in the alloy. A series of Cu-Ni alloys have been studied in natural sea water and in chloride solutions under different conditions [6‒12]. Some of these studies have [7] reported that selective electrodissolution of nickel is predominant; while others [11] have found that the dissolution of copper depends on the composition of the alloy under investigation.

The objective of the present work was to study the anodic dissolution of Monel-400 in the aerated stagnant solutions of Arabian Gulf seawater after varied exposure periods. The experimental work has been carried out using weight-loss measurements after varied immersion periods of 5-160 days. The study was also complemented by a variety of electrochemical techniques along with surface morphology and elemental analysis investigations. Since, pitting corrosion is one of the most destructive forms of localized corrosion and the ability of corrosive ions that present in sea water on the breakdown of a passive film might form on the surface of Monel-400, a particular attention was paid to the effect of stagnant AGS solutions on the pitting corrosion of the alloy.


The natural sea water (AGS) was obtained directly from the Arabian Gulf at the eastern region (Jubail, Dammam, Saudi Arabia), and was used as received. An electrochemical cell with a three-electrode configuration was used for electrochemical measurements. Monel-400 rod and sheet (were purchased from Magellan Metals, USA, with the following chemical composition, Ni–63.0% min, Cu– “28-34%” max, Fe–2.5% max, Mn–2.0%, Si–0.5% max, C–0.3% max, and S–0.024%) were used in this study. The Monel rod was used as a working electrode. A platinum foil and a Metrohm Ag/AgCl electrode (in 3 M KCl) were used as counter and reference electrodes, respectively. The weight loss experiments were carried out using rectangular Monel-400 coupons cut from the Monel sheet. The coupons had dimensions of 4.0 cm length, 2.0 cm width, and 0.4 cm thickness and the exposed total area of 54.02 cm2. were grinded successively with metallographic emery paper of increasing fineness of up to 800 grits, and then polished with 1, 0.5 and 0.3µm alumina slurries (Buehler). The electrodes were then washed with doubly distilled water, degreased with acetone, washed using doubly distilled water again and finally dried with pure air. The coupons were weighed and then suspended in 300 cm3 solutions of Arabian Gulf seawater for different exposure periods between 5 and 160 days. The losses in weight per area (ΔW, and the corrosion rates (KCorr, millimeters/year (mmpy)) over the exposure time were calculated as has been reported before [13, 14]. The SEM investigation and EDX analysis were obtained on the surface of a Monel-400 coupon after its immersion in AGS solution for 160 days. The SEM images were obtained by using a JEOL model JSM-6610LV (Japanese made) scanning electron microscope with an energy dispersive X-ray analyzer attached.

The Monel-400 rods for electrochemical measurements were grinded and polished as for the Monel coupons. The diameter of the working electrode was 1.2 cm with a total exposed area of 1.13 cm2. Electrochemical experiments were performed by using an Autolab potentiostat (PGSTAT20 computer controlled) operated by the general purpose electrochemical software (GPES) version 4.9. The cyclic potentiodynamic polarization (CPP) curves were recorded by scanning the potential in the forward direction from -800 to +800 mV then backward from +800 to -800 mV against Ag/AgCl again at the same scan rate, 3.0 mV/s. Chronoamperometric (CA) experiments were carried out by stepping the potential of the Monel-400 electrode at +100 mV versus Ag/AgCl for 120 min. For the PPC and CA experiments, the curves were recorded after the electrode immersion in AGS for 0, 24, and 72 h before measurements. Electrochemical impedance spectroscopy (EIS) tests were performed after 1, 24, and 72 h of the electrode immersion at corrosion potentials (ECorr) over a frequency range of 100 kHz –10 mHz, with an ac wave of  5 mV peak-to-peak overlaid on a dc bias potential, and the impedance data were collected using Powersine software at a rate of 10 points per decade change in frequency.


3.1. Weight-loss data and SEM / EDX investigations

The variations of (a) the weight loss (ΔW) and (b) the corrosion rate (KCorr) vs. time for Monel-400 coupons in 300 cm3 of AGS are shown in Fig. 1. The values of ΔW and KCorr were calculated as reported in the previous work [13, 14]. It is clearly recognized that the values of ΔW increased from 510‒5 g/cm2 after 5 days to 5510‒5 g/cm2 after 160 days immersion in the AGS solution at the same condition. This is due to the contentious dissolution of Monel-400 surface under the influence of the high salinity of the Arabian Gulf (47000 ppm). On the other side, the values of KCorr decreased with increasing time, which indicates the development then the accumulation of corrosion products and/or oxides on the Monel surface. These formed components partially protect the surface by reducing its dissolution and so decreasing aggressiveness of AGS by limiting the contact of the active Monel surface to AGS test solution. This agrees with the previous studies [15, 16] and proves the poor performance of Monel-400 in freely aerated stagnant AGS.

Figure 2 shows SEM/EDX investigations for Monel-400 surface after its immersion in AGS solutions for 160 days where, (a) the SEM micrograph for a large area of the surface and (b) the corresponding EDX profile analysis for the selected area on the SEM image, respectively. It is obvious from Fig. 2a that the surface has a flat area covered with corrosion products in addition to numerous pits, which have almost similar round shapes and different diameters.

Figure 1. Variations of the weight loss (a) and corrosion rate (b) versus time for Monel-400 coupons in open to air stagnant Arabian Gulf seawater.

The atomic percentage of the elements found in the selected area of image (a) by the EDX profile, were 41.80% C, 35.28% O, 12.75% Ni, 6.33% Cu, 2.86% Cl, 0.49% Fe, 0.29% S, and 0.19% Mn. The low contents of Ni and Cu and the high percentages of C and O suggest that the alloy surface is covered with corrosion products that have different compounds, complexes and oxides. The presence of chloride, sulphur and iron besides carbon also suggest that the surface is having scales deposited from the seawater.

In order to understand the mechanism of pitting corrosion of Monel-400 in the AGS at this condition, the SEM/EDX investigations were obtained for the corrosion products around the pits as well as inside the pits. Fig. 3 depicts, (a) SEM micrograph represents a pit on the Monel-400 surface after its immersion in freely aerated stagnant AGS solution for 160 days and (b) the corresponding EDX profile analysis taken for the corrosion products around the pit as selected on the SEM image, respectively. The atomic percentage of the elements found around the pit shown in the SEM image (a), were 46.31% O, 20.76% C, 5.01% Ni, 19.12% Cu, 8.26% Cl, and 0.55% S with no Fe and Mn. The high level of O and C provide that the surface of Monel-400 around the pit is covered with a thick oxide layer with other corrosion products. The very low content of Ni compared to Cu is due to the selective dissolution of Ni, while Cu tends to form oxide and chloride. The presence of C and S indicate that the area around pits has scale and corrosion products resulted from the components of the

Figure 2. (a) SEM micrograph for a large area of Monel-400 surface after its immersion in freely
aerated stagnant AGS solution for 160 days and (b) the corresponding EDX profile analysis
taken in the selected area of the SEM image, respectively.

Figure 4 shows (a) SEM micrograph represents an extended area inside a pit was formed on the surface of Monel-400 that has been immersed in freely aerated stagnant AGS solution for 160 days and (b) the corresponding EDX profile analysis taken inside the pit as selected on the SEM image, respectively. The SEM image shows that the formed pit is deep and wide with corrosion products deposited in some areas inside it. The atomic percentages of the components found inside the pit were found to be 58.92% Cu, 17.01% Ni, 15.73% O, 6.96% Cl, 0.95% Fe, and 0.43% S. The very high content of copper (almost double of its natural presence in the alloy) as well as the very low Ni content inside the pit confirms the selective dissolution of Ni with copper enrichment. The poor presence of oxygen inside the pit compared to its percentages on the surface (35.28%) and around the pit (46.31%) also specifies that the aggressive ions such as Cl─ displace the oxygen at its weakest bond with metal on the alloy surface and initiate pitting corrosion.

Figure 3. (a) SEM micrograph represents a pit on the Monel-400 surface after its immersion in freely aerated stagnant AGS solution for 160 days and (b) the corresponding EDX profile analysis taken for the corrosion products around the pit as selected on the SEM image, respectively.

The presence of Cl─ increases the potential difference across the passive film, thereby enhancing the rate of nickel ions diffusion from the nickel-film interface to the film-solution interface, forming cation vacancies at the Monel-film interface [5]. When the concentration of Cl─ is high enough, voids develop at the nickel-film interface. Continued growth of such voids results in the localized collapse of the passive film, which will dissolve faster than other regions of the passive film, leading to pit growth and ultimately substrate alloy dissolution [17]. It is worth to mention that the maximum pit depth measured in stagnant natural seawater for Monel-400 was 1.067 mm deep as reported in a three-year study at the Inco Test Facility [15]. The attacked regions were copper-rich while the regions around the active sites had higher Ni concentrations. This agrees with our work and also the work reported by Gouda et al. [18] and Little et al. [19] that the corrosion of Monel-400 undergoes through the selective leaching of nickel from the alloy leaving a spongy copper –rich material in the base of the pit.

Figure 4. (a) SEM micrograph represents an extended area for a pit on the Monel-400 surface after its immersion in freely aerated stagnant AGS solution for 160 days and (b) the corresponding EDX profile analysis taken inside the pit as selected on the SEM image, respectively.

According to Little et al. [19] chlorine and sulphur from seawater accumulate within the pit and react with the iron and nickel in the alloy, which is why the percentage of Fe and Ni are lower inside the pit compared to their natural presence in the alloy.

3.2. Cyclic potentiodynamic polarization (CPP) measurements

CPP experiments were carried out after Monel immersion in AGS solutions for 0 h, 24 h, and 72 h in order to understand the mechanism of Monel dissolution after varied exposure periods. This technique was also used to report the effect of time on the change of corrosion potential (ECorr), Corrosion current (jCorr), pitting potential (EPit), pitting current (jPit), polarization resistance (RP), and corrosion rate (KCorr) of Monel-400 in the test medium.

Figure 5. Cyclic potentiodynamic polarization curves for Monel-400 after 0 h (a), 24 h (b), and 72 h (c) immersion in Arabian Gulf seawater, respectively.

The CPP curves for the Monel electrode after (a) 0 h, (b) 24 h, and (c) 72 h immersion in AGS solutions, respectively are shown in Fig. 5. Blundy and Pryor [9] have reported that the anodic reaction of Monel-400 is the selective dissolution of nickel, particularly at high potential values. Gouda et al. [18] with Arabian seawater and Little et al. [19] with Gulf of Mexico water have also demonstrated that the anodic dissolution of Monel-400 occurs due to the intergranular corrosion and selective dealloying of iron and nickel, especially in the presence of sulfate-reducing bacteria. It is clearly seen from Fig. 5a that an active dissolution of the alloy occurred with increasing potential in the anodic side. It is also seen that there is a peak on the anodic branch at which the current decreased with increasing the applied potential. This peak was appeared due to either the formation of a passive oxide film [20, 21] or the accumulation of corrosion products on the electrode surface. The sudden increase of the current after the formation of the peak is due to the breakdown of the passive film formed on the Monel-400 surface by the attack of aggressive ions presented in the seawater such as chlorides and lead to the occurrence of pitting corrosion [22]. The further increase of the current with potential is caused by the agglomeration of chloride ions inside the pits leading to pit growth and ultimately substrate alloy dissolution [5].

Table 1. Corrosion parameters obtained from CPP curves shown in Fig. 6 for the Monel-400 in aerated stagnant Arabian Gulf seawater after different exposure intervals.

Increasing the immersion time of the electrode in the AGS solution to 24 h before measurements (Fig. 5b) decreased both the anodic and peak currents and even eliminated the peak when the immersion time was increased to 72h as shown in Fig. 5b and 5c, respectively. The corrosion parameters obtained from Fig. 5 are shown in Table 1. It is seen from Table 1 that the values of jCorr, jProt and jPit decreased with increasing the immersion time. Also, the values of ECorr, EProt, and Epit increased to the more negative values. This effect also increased the values of polarization resistance (RP) and decreased the values of corrosion rate (KCorr), which were calculated as previously reported [13, 23-25].

3.3. Chronoamperometric measurements

The variation currents versus time for Monel-400 electrode that has been immersed in the AGS solutions for (1) 0 h, (2) 24 h, and (3) 72 h, respectively before stepping the potential to 100 mV for 120 min are shown in Fig. 6. The highest current values for Monel-400 in AGS solutions were recorded when the measurements were carried out after the first moment of the electrode immersion, curve 1. It is clearly observed that the current increased with time till the end of the run. Increasing the immersion time to 24 h, (curve 2) led to decreasing the absolute current and further current decreases were recorded when the time was increased to 72 h (curve 3). This decrease in the absolute current with increasing immersion time of Monel before applying the constant potential can be explained by the formation of a passive oxide film and/or corrosion products; this film gets even thicker as the time increases. The formation of such species on the surface decreases the uniform attack of the Monel-400 and so decreases the absolute current under the applied potential.

Figure 6. Chronoamperometric curves for the Monel-400 electrode that has been immersed in Arabian Gulf seawater for 0 h (1), 24 h (2) and 72 h (c) before stepping the potential to 100 mV vs. Ag/AgCl before measurements.

On the other hand, the increase of current values with time when the potential was stepped to 100 mV is due to the dissolution of the film formed on the Monel-400 surface, due to the preimmersion of the electrode in the solution, leading to the occurrence of the pitting corrosion. The higher the absolute currents the higher the number of small and narrow pits. This means that increasing the exposure time before measurement leads to decrease the number of pits at the same time it increases the width and depth of the pits formed. Pits develop [5, 26] at sites where oxygen adsorbed on the alloy surface is displaced by an aggressive species such as Cl─ ions that are presented in the AGS solution. This is because Cl─ ions have small diameters allows it to penetrate through the protective oxide film and displace oxygen at the sites where metal-oxygen bond is the weakest [5, 17].

3.4. Open-circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements

The variation of the OCP versus time (72 h) for Monel-400 electrode in AGS is shown in Fig. 7. It is clearly seen that the potential values slightly increased toward the more negative direction in the first 10 h due to the dissolution of Monel-400 by the aggressive ions attack that present in the AGS on the electrode surface. This slight negative potential shift decreased with increasing the immersion time up to the first 45 h, which might be due to the formation of corrosion products including oxides on the surface.

Figure 7. The change of the open-circuit potential versus time for Monel-400 in Arabian Gulf seawater.

The formation of such corrosion products partially protected the alloy surface, which is why the potential decreased in the positive direction again and till the end of the test. This very slight positive shift in the OCP values decreased the corrosion rate by decreasing the uniform attack of the alloy with time. This decrease in corrosion rate might result not only from the formation of corrosion products but also because of the ability of AGS solution to form scales on the Monel-400 surface. The EIS measurements were carried out to determine kinetic parameters for electron transfer reactions at the Monel-400/electrolyte interface and to confirm the data obtained by CPP and CA measurements. Typical Nyquist plots (a), Bode (b), and phase angle (c) for Monel-400 after its immersion for (1) 1 h; (2) 24 h; and (3) 72 h, respectively in AGS are shown in Fig 8. It is clear from Fig. 8a that only single semicircles are observed for the Monel electrode in AGS for the different exposure intervals. The chord length pertaining to the high frequency (HF) loop observed in Nyquist diagram increased as the immersion time before measurements increased. This increase of the HF chord is due to the decrease in the electrochemical active areas by the accumulation of corrosion products and oxides at the Monel surface. The diameter of the semicircle is significantly increased with increasing the time of exposure to 24 h and further to 72 h. It has been reported that the semicircles at high frequencies are generally associated with the relaxation of the capacitors of electrical double layers with their diameters representing the charge transfer resistances [24, 27]. The Nyquist spectra shown in Fig. 8a were analysed by fitting to the equivalent circuit model shown in Fig. 9 and was also used previously to fit the impedance data obtained for Monel-400 in simulated seawater [28]. The parameters obtained by fitting the equivalent circuit shown in Fig. 9 are listed in Table 2.

Table 2. EIS parameters obtained by fitting the Nyquist plots shown in Fig. 8a with the equivalent circuit shown in Fig. 9 for the Monel-400 in aerated stagnant Arabian Gulf seawater after different exposure intervals.

Figure 8. Nyquist (a), Bode (b) and phase angle (c) plots for Monel-400 at OCP (ECorr ± 5 mV) after its immersion in Arabian Gulf seawater for 1 h (1), 24 h (2), and 72 h (3).

Figure 9. The equivalent circuit used to fit the experimental data presented in Fig. 8a.

According to usual convention, RS represents the solution resistance between the Monel surface and the platinum counter electrode, Q the constant phase elements (CPE) and contain two parameters; a pseudo capacitance and an exponent (an exponent close to 0.5), the RP1 accounts for the resistance of a film layer formed on the Monel-400 surface, Cdl is the double layer capacitance, and RP2 accounts for the charge transfer resistance at the alloy surface, i.e. the polarization resistance. It is seen from Fig. 8 and Table 2 that the values of RS, RP1 and RP2 increased as the immersion time of Monel in AGS before measurements was increased. The polarization resistance measured by EIS in this case is a measure of the uniform corrosion rate as opposed to tendency towards localized corrosion. The increase of the resistances (RP = RP1 + RP2) in this case is attributed to the formation of a passive film and/or corrosion products, which gets thicker with time and could lead to the decrease in jCorr and KCorr and also the increase in RP values we have seen in CPP experiments, Fig. 5 and Table 1, under the same conditions. The CPE, Q, is almost like Warburg impedance with its n-values smaller than 0.50, suggesting that the formed corrosion products and oxide layer on the Monel-400 surface block the mass transport acting like a resistor. The values of YQ decreased with time indicating that the dissolution of Monel is also limited by mass transport. That was again confirmed by the decrease of the double layer capacitance, Cdl, values with increasing the immersion time. The results presented by Nyquist plots and Table 2 were also supported by the increase in the impedance of the interface with increasing the exposure interval of Monel before measurement as shown in Fig. 8b. It has been reported [29‒37] that the increase of the impedance and low frequency values means the increase of the passivation of the surface. Further confirmation is also provided by the increase of the maximum phase angle (Fig. 8c) with time at the same conditions. In general, the EIS data is in good agreement with the electrochemical (CPP and CA) experiments and weight loss measurements.


The corrosion of Monel-400 alloy in stagnant Arabian Gulf seawater (AGS) has been studied by using gravimetric and electrochemical measurements in addition to SEM/EDX investigations. The loss in weight data indicated that Monel-400 suffers both general and localized corrosion. Pitting corrosion occurred for Monel due to the attack of corrosive species such as Cl─ to the weakest oxygen-Alloy bond and that a selective dissolution of Ni leads to the propagation of the formed pits as shown by SEM/EDX investigations. Cyclic polarization, OCP and EIS measurements revealed that the increase of exposure time decreases the corrosion current and increases the polarization resistance as well as shits pitting and protection potentials to the more negative direction. Chronoamperometric curves proved that the severity of pitting corrosion increases and the uniform attack decreases for Monel with increasing immersion time. In general, the electrochemical measurements confirmed the data obtained by gravimetric ones that the uniform attack decreases, while pitting corrosion increases with increasing the exposure time of Monel in the AGS solution.

Wednesday, 25 December 2013

EEMUA 144 90/10 Copper Nickel C70600 Alloy Piping for Offshore Applications - Specification: Tubes Seamless and Welded


Seamless Pipes
Seamless pipes are in accordance with EEMUA–144. They are manufactured from hot extruded shells followed by cold work and annealing.

Welded Pipes
Longitudinally welded pipes are in accordance with EEMUA–144. They are manufactured from hot rolled or cold rolled and annealed sheet or plates in accordance with BS 2870, BS 2875, ASTM B171 or ASTM B402. Mechanical testing is carried out in accordance with the standards above. The pipes are supplied in “as welded” condition.

Dimensions are based on EEMUA –144. However, the pipe diameters range from ½ in./16 mm to 36 in./914 mm. Although the pipe dimensions of 38 in./965 mm and 40 in./1016 mm are not included in the EEMUA 144 – 1987 they are available on request as they are commonly specified. The corresponding wall thicknesses of the pipes comply with the pressure containment requirements of ASME B31.3 as well as the requirements of the International Association of Classification Societies. Pipes with other wall thicknesses are available on request.

See notes 1-4 for seamless and notes 2-4 for welded pipes.

Weld Preparation
For wall thickness less than 3 mm, the pipes are supplied with plain weld ends. Larger thicknesses are supplied with the weld bevel of 37 ½°±2 ½°.

Nominal (in)
Specified (mm)
Specified Wall Thickness (mm)
Theoretical Weight/Meter (kg)
16 bar
20 bar
16 bar
20 bar

Nominal (in)
Specified (mm)
Specified Wall Thickness (mm)
Theoretical Weight/Meter (kg)
16 bar
20 bar
16 bar
20 bar

Note 1
The pipe sizes up to including 4 in./108 mm are based on BS 2871: Part 2: Table 3 for outside diameters and their tolerances to allow for the use of capillary and compression fittings and brazed (and welded) slip-on flanges. The wall thickness of the 16 bar range have been increased to match the 20 bar range for mechanical strength.

Note 2
The pipe size 6 in./159 mm up to 16 in./419 mm are also based on BS 2871: Part 2: Table 3 for specified diameters but the tolerance have been applied to the inside diameters for facilitate alignment of matching weld preparations.

Note 3
The ovality of the finished pipe doesn't exceed 2% of the difference of the maximum and minimum diameter measured on the same cross section.

Note 4
Up to including 4 in./108 mm, the wall thickness doesn't vary by more than 10 % specified therein. For diameters from 6in./159 mm and larger, the wall thickness is not less than 12.5 % of the specified value.

The pipes with other dimensions than mentioned herein are available on request. Please contact us for more information.