Corrosion of ferrous alloys in deep sea environments

Abstract

The deep sea corrosion of ferrous alloys that undergo uniform corrosion without forming a passive layer is related to the amount of dissolved ox ygen available in the sea water. The performance of some ferrous alloys at depths of 500, 1200, 3500, and 5100 m in the Indian Ocean has been studied. The results of atomic absorption spectroscopy have revealed that the corrosion product present on mild steel coupons is FeOOH. The results of experiments in shallow water have shown that micro- and macrobiological growths play a significant role in the corrosion of ferrous materials. However, in deeper waters, the absence of macrofouling was evident, and corrosion was not related to any biological product but mainly to the electrochemical reaction of these alloys with sea water. The results of morphological studies by SEM on the surfaces of ex posed specimens are also presented.     

Cathodic protection requirements for high strength steel in sea water assessed by potentiostatic weight loss measurements

Abstract

A potentiostatic weight loss technique has been used to compare the corrosion rates of Weldox 700 high strength low alloy steel exposed to both natural and sterile sea water at potentials ranging from the free corrosion condition to - 1000 mV (SCE). Anodic polarisation curves with Tafel constants of 54 and 64 mV per decade were fitted to the data and the minimum levels of protection required to limit the corrosion rate to 0·001 mm per year were found to be - 770 and - 790 mV (SCE) in natural and sterile sea water, respectively. Lower corrosion rates occurred in filtered natural sea water than in sterile sea water but the highest rates were recorded on specimens ex posed to a seabed sediment. The reasons for this behaviour are discussed in terms of the biofilms and the numbers of sulphate reducing bacteria present on the surfaces.     

Dibutyl thiourea as corrosion inhibitor for acid washing of multistage flash distillation plant

Abstract

The effect of the additive dibutyl thiourea (DBTU) on the corrosion of 70Cu–30Ni alloy and of carbon steels in HCl solutions in distilled water and in sea water at pH 1·8–2·0 was observed using the weight loss technique at temperatures between 19 and 60°C. In the absence of DBTU the corrosion of the Cu–Ni alloy is characterised by an induction period which is longer in the pure acid than in acidified sea water. Following the induction periods, the loss in weight varies linearly with time and is temperature dependent in the pure acid, but not in sea water. On the other hand, the weight loss of carbon steel in both media varies linearly with time directly from the moment of immersion. The rate of dissolution in pure HCl is greater than in acidified sea water and both are temperature dependent. The effect of concentration of DBTU on the dissolution of the two alloys in both media was tested at 60°C. In all cases inhibition of corrosion increased with additive content. The inhibition efficiencies in presence of 300 ppm DBTU amounted to 78 and 30% for Cu–Ni and 87 and 75% for carbon steel in pure HCl and acidified sea water, respectively. Weight loss experiments carried out under modified conditions suggested decomposition of DBTU when in contact with the metal surface. The results of the present investigation reveal that DBTU is not a particularly effective inhibitor for the acid washing of multistage flash distillation units.     

Effect of heat treatments on Cu-Ni 70/30 alloy pitting susceptibility in sea water at different temperatures

A series of experiments was carried out on Cu-Ni 70/30 commercial alloy specimens presenting different microstructures. The microstructure of homogeneized specimens was modified with annealing treatments by which grain size significantly increased and intermetallic compounds precipitated, thus creating some dishomogeneity in the alloy. Free corrosion and electrochemical tests were carried out at 20, 40, 60, 80°C in quiescent sea water at pH 8.2 with dissolved oxygen (D.O) content ranging from 6.5 ppm (at 20°C) to 3.0 ppm (at 80°C). The corrosion products adherent to the metallic surface were analyzed with chemical and XPS methods. The following could be observed:

• with increasing temperature, a decrease in corrosion rate and selective copper dissolution was observed in homogeneized and annealed specimens
• the annealed specimens have the highest corrosion rate in the whole temperature range and undergo pitting corrosion at low temperature.

     

Steel pipe corrosion under flow conditions-III. Effect of sulphate ion

Data are presented on the corrosion of commercial iron piping in flowing aerated water containing sodium sulphate (26–470 ppm SO₄^2?) at 125°F as weight loss-time curves for flows, corresponding to Reynolds numbers in the range 21,000–63,000. It was observed that sulphated water produced uniform attack with no pitting corrosion. Increased corrosion always resulted with increased sulphate ion concentration over the flow range considered. Although the sulphate ion was not detectable in the corrosion product layer, its presence in solution significantly altered the physical form and nature, such as roughness, porosity and adhesion, of the corrosion product layer. The double resistance model recently proposed provided a useful basis for assessing the relative resistances afforded by the corrosion product layer and the liquid film or damped turbulence layer. It was found that the mass transfer coefficient in the damped turbulence layer controls the overall mass transfer process for the exposure time, flow rates, and sulphate ion concentration ranges examined.     

Mathematical modelling of the electrochemistry in corrosion fatigue cracks in steel corroding in marine environments

A mathematical model has been developed to describe the mass transport and electrochemical conditions in a corrosion fatigue crack in steel in 3.5% NaCl and in sea water for both freely corroding and anodic polarization conditions. Mass transport by advection (fluid flow induced by the movement of the crack walls), diffusion and ion migration was considered. Anodic and cathodic processes, hydrolysis reactions (including hydrolysis of alloying elements) and buffering reactions were included in the model. The pH value developed within the crack at a temperature of 5°C was between 7.0 and 8.5 for a wide range of conditions, with the maximum value controlled by the buffering associated with deposition of ferrous hydroxide. The lower pH values corresponded to relatively high ferrous ion concentration and were obtained for combinations of high R values (minimum/maximum load) and low frequencies for which convective mixing with the bulk solution was minimized. The presence of chromium in the steel at the 1 wt% level had only a small effect on the crack tip pH value in deep cracks but could lower the pH considerably (to about 4.0) in very shallow cracks (2.5 × 10⁻² cm) if the potential was about −600 mV(SCE). The potential drop in the crack was relatively small (<30 mV) for a wide range of conditions at the free corrosion potential for structural steel in sea water ( −690 mV(SCE)) but increased markedly with anodic polarization with the effect most pronounced for deep cracks. Comparison of the model predictions with experimental measurements showed very good agreement with respect to crack tip pH and potential.     

Corrosion behaviour of pure Mg,AS31 and AZ91 in buffered and unbuffered sulphate and chloride solutions

Abstract

The low corrosion resistance of magnesium limited its application in industrial affairs. The main corrosive anions for magnesium are sulphate and chloride. This paper deals with their effect at low concentrations on the corrosion behaviour of Mg and Mg based alloy (AS31 and AZ91) in presence/absence of buffer solutions (pH 8). The electrochemical measurements used are open circuit potential and potentiodynamic polarisation. The results exposed that in the absence of a protective layer, general corrosion is observed to occur at greater rate than when sulphate is present; however, the general corrosion is transformed to localised corrosion faster in presence of chloride. In the presence of a protective layer, the localised corrosion is perceived in the case of chloride more than sulphate. The presence of aluminium in the alloys has two contradictory actions. One increases the passivity of the formed layer and the other increases the localised corrosion. The percentage of aluminium in the alloy controls these actions.     

The hot corrosion of Co-25Cr-10Ni-5Ta-3Al-0.5Y alloy (S-57)

A cobalt-base alloy, Co-25 Cr-10 Ni-5 Ta-3 Al-0.5 Y (S-57), was subjected to hot corrosion in Mach 0.3 burner rig combustion gases at maximum alloy temperatures of 900 and 1000°C. Various salt concentrations were injected into the burner; 0.5, 2, 5 and 10 parts per million synthetic sea salt and 4 parts per million sodium sulphate (Na₂SO₄). The extent of corrosion was determined by measuring the maximum depth of corrosion in the alloy and the corrosion process was studied by metallography, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis. While S-57 was found to possess only moderate oxidation resistance at these temperatures, this alloy resisted significant hot corrosion attack under all but the most severe test conditions. The process of the hot corrosion attack under the most severe conditions of this study was primarily sulphidation.     

Effects of pH and chloride concentration on pitting corrosion of AA6061 aluminum alloy

Effects of pH solution and chloride (Cl⁻) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.003 wt% and 5.5 wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting E_pit and corrosion E_cor potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH = 2) and alkaline chloride solutions (pH = 12) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH = 6).     

Corrosion of stainless steels in marine conditions containing sulphate reducing bacteria

Abstract

The corrosion behaviour of a range of austenitic and duplex stainless steels has been studied in sea water containing sulphate reducing bacteria under conditions where the influence of an external aerated steel surface is absent. By means of a combination of anodic polarisation, cathodic polarisation, galvanic coupling experiments, microscopy, and microanalysis, the complex corrosion behaviour has been rationalised. It has been demonstrated that severe corrosion, involving a number of localised and general features, can occur in the low oxygen conditions, and good correlation has been obtained between accelerated electrochemical tests and long term corrosion occurring without the intervention of electrochemical monitoring.     

Effect of heat treatments on 8090 AlLi alloy pitting susceptibility in sea water

The pitting susceptibility of 8090 Al-Li alloy in sea water, after different heat treatments, was investigated. Free corrosion and electrochemical tests were carried out at 25°C, in quiescent sea water at pH = 8.2 and dissolved oxygen = 6.5 ppm. The microstructure was examined by metallographic microscopy and by X-Ray microdiffractometry, while the corrosion layer was characterized by chemical methods and by Infrared and X-Ray photoelectron spectroscopies. The following was observed:

• aging treatments lead to a non homogeneous microstructure which increases the average corrosion rate as well as pitting susceptibility;
• heat treatments do not affect the average passive film composition;
• in all examined states, Al-Li alloy 8090 is subject to localized corrosion which takes place preferentially at the grain boundaries.

     

Exploring corrosion protection of La-Fe-Si magnetocaloric alloys by passivation

Magnetocaloric La(Fe,Si)₁₃-based alloys are promising materials for magnetic cooling systems but their limited corrosion resistance in water-based heat transfer fluids is critical. The corrosion behavior of as-cast and annealed La-Fe-Si alloy samples was analyzed in comparison to that of La and Fe for evaluation of the impact of alloy chemistry and microstructure. Electrochemical studies were conducted in defined electrolytes starting with aerated distilled water (pH = 6) for assessing the influence of pH value changes and anion contaminations. Specifically, forced flow electrolyte conditions were applied which are closer to operation conditions of real magnetocaloric regenerator beds than stagnant ones. Corroded sample surfaces were analyzed with SEM to assess damage mechanisms. The reactive nature of the alloy constituents determines the high corrosion activity and limited passivation ability of La-Fe-Si alloys. Their exposure to distilled water is particularly detrimental under stagnant conditions as local fluid acidification enhances corrosion processes. These are based on galvanic coupling between the phases with different corrosion activities: La-rich phases > La(Fe,Si)₁₃-based matrix > alpha-Fe(Si). Laminar fluid flow is beneficial for alloy surface passivation. But anion contaminants like sulfate or hydrogen phosphate ions counteract the weak passivity in flowing distilled water. While acidic conditions lead to instable corrosive states, a pH value control of the heat transfer fluid at alkaline conditions is effective for stable passivity of the alloy surface. Also, the applicability of a phosphate conversion coating treatment in 0.15 M NaH₂PO₄ (pH = 4) was evaluated and prospects of this approach are discussed.     

ESCA investigation of iron-rich protective films on aluminium brass condenser tubes

The object of this investigation was to examine the nature of the protective oxide layers on an aluminium brass condenser tube which had been in service at Fawley under an operating regime in which the cooling water was regularly dosed with ferrous sulphate for protection against pitting corrosion. The protective films have been analysed by means of XPS (ESCA) and found to contain mainly Cu⁺ and Fe³⁺ ions. Of more interest were extensive areas in which the iron deposits are non-adherent and non-protective. XPS analyses show that these deposits are separated from the underlying brass by a layer rich in zinc and magnesium but containing no iron. Some indications are given of how this intervening layer could have formed.     

Effect of sulphides on corrosion of Cu–Ni–Fe–Mn alloy in sea water

Abstract

The corrosion behaviour of Cu–30Ni–2Fe–2Mn commercial alloy (similar to C71640) in quiescent, hermetically closed sulphide polluted and unpolluted natural sea water at 25°C was investigated. The corrosion resistance was examined using free corrosion and electrochemical tests and the surface film was characterised by chemical analysis and X-ray photoelectron spectroscopy. The different susceptibilities to corrosion were closely linked to the initial sulphide concentration; the accelerated attack occurring with an initial sulphide concentration of 4 ppm correlated with the amount of dissolved oxygen in solution and with the chemical composition of the corrosion layer.     

Influence of hydrostatic pressure on pitting of aluminium in sea water

Abstract

The effect of hydrostatic pressure on the corrosion of aluminium in sea water oj pH 8·2 containing 7ppm dissolved oxygen has been investigated at 20°C. On increasing the pressure from 1 to 300 atm, the corrosion rate increased, an effect attributable to an increase in the rate of the anodic reaction while the cathodic rate remained unchanged. The observed increased susceptibility to pitting corrosion with increasing pressure was found to correlate with the presence of increasing quantities of SO^2?₄ and Cl^? ions in the oxidation layer. The introduction of electron acceptors such as SO^2?₄ and Cl^? into n type compounds such as aluminium oxides increases the density of lattice dejects and promotes breakdown oj the barrier provided by the oxidation layer.     

Behaviour of Zinc in Sea Water in the Presence of Sulphides

Abstract

The effect of sulphides on the corrosion of zinc in artificial sea water has been studied over the pH range 6·5 to 8·6. A comparison of the weight loss and potential values of specimens in aerated and deaerated sea water, with or without the addition of sulphide, has shown that at pH values > 7·2 the presence of sulphides increases the corrosion, while at lower pH values it is partially inhibited. This is related to changes in the concentration of S^2- ions in the solution. The corrosion products formed in the more corrosive solutions were shown to consist mainly of ZnS, which is believed to cause loss of adhesion of the corrosion products on the zinc surface, and thus to accelerate the corrosion.     

Cu-Ti合金在模拟S2-污染海水中的腐蚀行为研究

通过浸泡实验与电化学测试,研究了固溶态Cu-4wt.%Ti合金在模拟污染海水(含S2-的3.5wt.% NaCl溶液)的腐蚀行为。借助扫描电子显微镜(SEM)、X射线衍射(XRD)以及X射线光电子能谱仪(XPS)对铜钛合金表面腐蚀产物进行测试分析。结果表明：含Cl-溶液中铜钛合金的腐蚀形式为点蚀,点蚀坑尺寸较小,分布均匀。添加S2-后点蚀更容易被诱发,点蚀坑尺寸较大。当NaCl溶液中的S2-浓度达到60ppm时,点蚀坑在合金表面相互连接,呈现出均匀腐蚀的形态;S2-<和Cl-对铜钛合金的腐蚀存在竞争吸附,S2-吸附性强对铜钛合金的腐蚀剧烈;在含S2-的NaCl溶液中腐蚀产物主要为CuS、Cu2S、Cu2O以及Cu2(OH)3Cl。S2-浓度较大时会导致溶液中OH-浓度增加,使腐蚀产物膜层厚度与致密度增加,因此当S2-浓度达到100ppm时铜钛合金能够发生钝化从而减缓腐蚀。     

Sea water corrosion behaviour of T2 and 12832 copper alloy materials in different sea areas

ABSTRACT

Field exposure test was carried out to investigate the sea water corrosion behaviour and rules of T2 and 12832 copper alloy materials in different sea areas. The corrosion morphology characteristics and corrosion products properties were analysed using different techniques, and the main environmental factors influencing their corrosion rates were discussed by applying grey relational analysis. Reaults showed that with the decrease of the latitude, the corrosion rate of T2 increased, and grey relational analysis indicated that temperature had a significant influence on it. For 12832 copper alloy, the corrosion rate was lower than that of T2 on the whole. Different degree of biofouling occurred on 12832 copper alloy, and the surface fouling conditions (including sediment deposition) had a crucial impact on its corrosion behaviour. In summary, 12832 copper alloy had a better corrosion resistance, but accompanied by de-Mn and de-Al corrosion.     

Effects of Hydrostatic Pressure on the Corrosion of Copper in Sea Water

Abstract

A study has been made of the effect of hydrostatic pressure, ranging from 1 to 300 atm. (0·102 to 30·6 N.mm^?2) on the corrosion of copper in sea water at pH 7·8 and at a temperature of 10°c. The experimental technique ensured that the content of dissolved oxygen in the sea water was virtually constant over the pressure range, at a value of 7·0 ppm. The same test series was also carried out in a 3·5% NaCl solution.

The weight loss of the copper was found to increase with increase in pressure, both in sea water and in NaCl, and reached a maximum at a pressure of 150 atm. (～15 N.mm^?2) in both solutions. The increased pressure has no influence on the anodic dissolution process for copper, but accelerates the cathodic process. Protective films adhering to the corroded surfaces are identical for the two solutions at ambient pressure. However, at higher pressures adherent corrosion products form only in NaCl solutions. These products were found to be the same as those formed at ambient pressure.     

Pitting Corrosion in Copper Tubes – Cause of Corrosion and Counter-Measures

Abstract

An investigation of failures of hard-drawn copper water pipes (phosphorus-deoxidised copper) in service due to pitting corrosion was conducted from November, 1962 to February, 1965. Fifteen cases were reported. All those about which information could be obtained came from hot water installations and occurred in water with a low pH (≤7) and a HCO_3- content of, at the most, 100 mg/l but generally below 50 mg/1. Failures not due to pitting corrosion (i.e. caused by erosion and corrosion or corrosion fatigue) occurred in waters with a higher pH and higher HCO₃- content.

A laboratory investigation into the ability of the corrosion products to counteract further corrosion in different types of water was also carried out, using an electrolytic cell which, in principle, was a model of an active pit in a copper tube. This led to the following conclusions, which are in good agreement with the results obtained from the examination of service failures:

If the pH value of the water is high enough, the copper dissolved by the corrosion can be precipitated as basic copper salt. At low pH values such precipitation does not take place.

If the [HCO₃^?]/[SO₄^2?] ratio in the water is high, dissolved copper can be precipitated as basic copper carbonate in the neighbourhood of the corrosion site and counteract further corrosion.

At a low [HCO₃^?]/[SO₄^2?] ratio, crusts of basic copper sulphate will be precipitated at some distance from the corrosion site and may lead to a high corrosion rate.

Pitting is not likely to occur in hot water tubes of hard copper if the pH is ≥ 7·4 and the [HCO₃^?]/[SO₄^2?] ratio ≥1 (the concentrations given in mg/1). The critical values mentioned are approximate and may be adjusted in the light of future experience.