Stress corrosion and hydrogen embrittlement of two precipitation hardening stainless steels

Abstract

Straining of smooth tensile specimens of two precipitation hardening stainless steels over a wide range of controlled potentials in natural sea water has revealed regimes of low ductility above about — 200 mV(SCE) and below — 600 mV(SCE). The apparent loss inductility in the anodic region was attributable to dissolution and was found to be associated with pitting and crevice corrosion effects that were exacerbated when straining was conducted in acidified sea water. The embrittlement observed at more cathodic potentials, which gave rise to transgranular or intergranular failure depending upon the material and its heat treatment, is believed to be due to hydrogen that is evolved at such potentials. The occurrence of embrittlement by hydrogen in these materials, to produce similar modes of failure, was confirmed by testing smooth specimens ajtercathodic charging and both smooth and precracked specimens in gaseous hydrogen. Cracking of precracked specimens in gaseous hydrogen is complicated by the presence of δ ferrite stringers in the microstructure, which can give rise to delamination perpendicular to the principal plane of fracture if the stresses ahead of the crack are not reduced significantly by hydrogen embrittlement.     

Effect of applied potential on hydrogen embrittlement of weld simulated HSLA–80 steel in sea water

Abstract

Hydrogen embrittlement behaviour of an HSLA–80 steel in the weld simulated, grain coarsened heat affected zone condition, in synthetic sea water under cathodic charging in the applied potential range of -600 to -1400 mV(SCE) has been studied using a slow strain rate technique. Loss of ductility, as reflected in the percentage elongation and percentage reduction in area values, was substantial at and beyond -800 mV(SCE). The material in the weld simulated condition showed more susceptibility than the as received material, which is considered to be a result of increased strength and a bainitic–martensitic microstructure. Fracture showed both microvoid coalescence and quasi-cleavage features and was indicative of hydrogen induced void nucleation. Both hardening and softening effects on hydrogen charging were experienced.     

Hydrogen embrittlement of cathodically protected high strength steel in sea water and seabed sediment

Abstract

Hydrogen embrittlement tests were carried out using double cantilever beam and slow strain rate tensile specimens to measure the susceptibility of a 900 MPa yield strength steel in different marine environments, ranging from sterile sea water and filtered natural sea water to open sea water and seabed sediment. The cathodic protection potential and the presence of biologically generated sulphides in seabed sediment were shown to be important factors that controlled the extent of hydrogen uptake and embrittlement. Threshold stress intensities K_th were measured for each condition and the optimum cathodic protection potential to control corrosion with the minimum risk of embrittlement was established.     

En24钢在海水中的应力腐蚀

用恒位移弯曲试验,慢应变速率试验、动电位扫描和浸蚀试验研究了En24钢在海水中的应力腐蚀特性。结果表明,En24钢的环境敏感断裂倾向随强度降低而降低;强度低于临界值,则不出现应力腐蚀裂缝。外加电位对En24钢的环境敏感特性有重要影响,裂缝扩展平台速度于-900mV处出现极小值。依据裂缝扩展动力学、断口形貌、物理冶金参数和电化学等方面的判据,认为En24钢在海水中的应力腐蚀与电位密切相关。低于-900mV,属氢致开裂;高于-900mV,则由阳极溶解过程所制约;在一定电位范围内,不排除两种机理共同作用的可能性。     

Bimetallic Corrosion Effects on Mild Steel in Natural Environments

Abstract

Corrosion rates of mild steel when coupled to each of 31 other metals and alloys have been determined in 5 natural environments using disc-type specimens. These environments were industrial, urban/rural and marine atmospheres, natural water and sea water. The relative order of effect on the corrosion rate of mild steel in each environment has been tabulated. The results show that the performance of mild steel in a given couple can differ appreciably in different environments.     

Characterisation of calcareous deposits on freely corroding low carbon steel in artificial sea water

Abstract

Calcareous deposits were formed on steel under cathodic protection conditions in artificial sea water at various potentials from ?0·900 to ?1·400 V(SCE). The deposition calcareous layers were characterised by electrochemical impedance spectroscopies, scanning electron microscopy observations and X-ray diffraction analyses. At 20°C, the deposits were composed of calcite CaCO₃ when formed at various potentials in solution 1, of brucite Mg(OH)₂ and aragonite CaCO₃ when formed at potentials from ?0·900 to ?1·200 V(SCE) and only of brucite when formed at potentials E??1·300 V(SCE) in solution 2. Magnesium seems to influence the corrosion behaviour of freely corroding steel by causing calcium carbonate to precipitate as aragonite. Aragonite is more effective in covering the surface than calcite and is therefore more functional in preventing oxygen from reaching the steel surface, thereby lowering the corrosion rate.     

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 investigation of 460 low alloy steels exposed in the natural deep-sea environment

ABSTRACT

The corrosion behaviours of 460 low alloy steels were investigated by the field exposure corrosion experiments with various long periods and 1200–3000?m depths in the natural deep-sea environment. XRD analysis results indicate that corrosion products of β-FeOOH, γ-FeOOH and Fe₃O₄ increase and the corrosion product of α-FeOOH decreases with the exposure sea depth extending. Electrochemical experiment results reveal the corrosion status is aggravated with the exposure sea depth extending: corrosion potentials shift negatively to a minimum self-corrosion potential of ?0.590?V (vs. SCE) and charge transfer resistances decrease to a minimum of 278?Ω?cm². Corrosion behaviours analysis shows that the average corrosion rate decreases with the time increasing and reaches a maximum value of 101?μm?a^?1. The maximum pitting depth varies between 63.0?μm and 138?μm. Gray relational analysis shows that the corrosion rate of 460 steel is most influenced by the pressure and dissolved oxygen concentration of deep sea, exhibiting a positive correlation relationship.     

Development of high strength low alloy steels for marine applications: Part 2: Simulation of marine exposure of steel by means of laboratory tests in flowing sea water

Abstract

Sixteen experimental high strength low alloy steels were exposed to natural flowing sea water in the laboratory and in the harbour of den Helder at a depth of 8 m for a period of 2 years. Differences in the corrosion rates of the various steels were less pronounced in the laboratory tests than in the harbour. The effects of the alloying elements aluminium, chromium: and molybdenum on general corrosion and pitting were found to be in general agreement with those obtained from earlier exposure tests at depths of 45 and 90 m in the North Sea. A fairly good correlation was found between the general corrosion rates measured from weight losses and the results obtained from electrochemical techniques based on determination of polarisation resistance.     

Influence of strain Bacillus cereus bacterium on corrosion behaviour of carbon steel in natural sea water

This paper examines the corrosion behaviours of carbon steel immersed in sterile natural sea water with and without strain Bacillus cereus. Electrochemical studies, including Tafel plots and electrochemical impedance spectroscopy (EIS) were performed to evaluate the variation of the corrosion behaviour of carbon steel in medium containing B. cereus as compared to the sterile control samples. The results of Tafel plot measurements showed significant reduction in the corrosion rate in the presence of bacterial biofilm produced by strain B. cereus. The EIS data showed that the charge transfer resistance is greater in a medium containing B. cereus and increases with immersion time.     

Corrosion of CuNi30Fe in artificial solutions and natural sea water: influence of biofouling

Abstract

The electrochemical behaviour of CuNi30Fe in natural sea water, artificial sea water, NaCl+Na₂SO₄, and NaCl solutions has been compared. Potentiodynamic polarisation curves, open circuit potential versus time plots, and current transients were recorded. Significant differences between the electrochemical responses of the alloy in the electrolytes used were detected. Corrosion products after various exposure times in-natural sea water and in artificial solutions were analysed using SEM and EDAX. Attachment of micro-organisms after exposure in natural sea water was examined by SEM. The effect of biofilms at early stages of formation on the corrosion process was also studied. The protective characteristics of the complex layers formed on the CuNi30Fe surface were found to be altered by microbial adhesion to the metal surface.     

锰铜合金腐蚀裂缝在3.5%NaCl中的电化学行为

<正> 锰铜系高阻尼合金在铸态下具有优良的机械性能与阻尼性能,是一种新的舰艇螺旋浆材料。该合金的主要成份是:35～36%Cu,4～4.5%Al,2.5～3.5%Fe,1～2%Ni,余量 Mn,实艇试验表明,采用这种合金对降低螺旋浆的振动和噪声有明显的效果。但是,锰铜合金在海水介质中有 SCC 敏感性。实验室试验得出,该合金在3.5%NaCl 中的 K_(?)cc 约等于26kgf/mm~3 /2。现场试验也发现,经焊补的锰铜合金螺旋浆很快在焊缝周围出现开裂。本工作沿用前人的方法,采用闭塞电池模拟腐蚀裂缝,初步探讨了锰铜合金在海水介质中发生 SCC 的原因及可能防止的方法。     

Behaviour of Couples of Aluminium and Plastics Reinforced with Carbon Fibre in Aqueous Salt Solutions

Abstract

Reaction potential measurements have been made on a range of carbon fibre reinforced plastic (CFRP) materials in aqueous chloride solutions. In 5% aqueous NaCl the CFRP slowly reached an equilibrium potential, normally about 300 mV positive to SCE. No correlation of potential with the type of carbon fibre or the resin matrix of the CFRP was found. The potential of aluminium and aluminium alloys is about 600–700 mV negative to SCE.

Polarisation studies showed that for CFRP – aluminium alloy couples the polarisation occurs almost exclusively at the CFRP. Very high corrosion currents are passed in aqueous chloride solutions. The effects of chloride ion concentration, pH, temperature, oxygen concentration, and nature of cations and anions in solution have been briefly studied.     

Electrochemical corrosion characteristics of Ni-Resist type 2 in sea water containing ferrous sulphate

Abstract

The corrosion characteristics of Ni-Resist type 2 alloy in sea water have been determined in relation to dissolved oxygen (DO) content, pH, and concentration offerrous sulphate (injected to inhibit corrosion of copper based alloy condenser tubes). The alloy underwent uniform cathodically controlled corrosion so that relatively high pH and low DO provide an acceptable environment. Compared with similar pH conditions in the absence of added iron, Fe²⁺ dosing of aerated sea water to ≤100 ppm at pH<5 increased the corrosion rate, whereas dosing at 1000-2400 ppm (pH<5) decreased it. Addition of ferrous sulphate to deaerated sea water further suppressed the already low corrosion rates in this medium. Even under aggressive conditions of anodic polarisation, the alloy showed near uniform attack of the major constituent metals, leaving, however, a graphitised surface. The possible stagnation of a concentrated ferrous sulphate solution injected into the condenser circulating cooling water system has been considered. It is also concluded that ferrous sulphate dosing does not alter the stress corrosion cracking susceptibility of the alloy.     

New strain of anaerobic bacteria and its association with corrosion pitting of X52 pipeline steel

Abstract

Studies of microbiologically induced corrosion have been carried out in connection with sea water pipelines used in the oil fields of the Southern Region of Mexico. The bacteria used in the studies were obtained from a sea water pipeline belonging to a secondary oil recovery system and were grown in an API RP 38 liquid medium. The morphological and chemical characteristics of the bacteria isolated from the growth medium were investigated and phylogenetic analysis showed that the most closely related bacterial strain is clostridium sphenoides. However, the existence of significant differences in the phylogenetic codes indicates that the bacteria from the sea water pipeline belongs to a new species of sulphate reducing bacteria. Coupons of API 5L X52 pipeline steel exposed to these bacteria developed corrosion pits which did not form during the exposure of similar coupons to the sterile growth medium. What is more, bacteria from the biofilm formed on the pitted coupons contained iron, which was absent from bacteria cultured in growth medium in which there were no steel coupons. These results demonstrate the existence of an association between the new species of sulphate reducing bacteria and the development of pitting corrosion in X52 pipeline steel.     

Enhanced corrosion resistance of titanium foil from nickel, nickel-molybdenum and palladium surface alloying by high intensity pulsed plasmas

The corrosion resistance of titanium, surface-treated by high intensity, nitrogen-plasma pulses to alloy the surface with nickel, nickel-molybdenum or palladium is determined in 0.1 M H₂SO₄ at 80 °C for immersion times up to 100 h. The purpose of the study is to identify surface treatments that have the potential to extend the life of the titanium foil window used in the clean-up of flue gases, including removal of SO₂, NO_x and volatile organic compounds, employing electron beam technology. The alloy layers, less than 1 μm thick, were produced using either deposition by pulsed erosion or pulsed implantation doping (PID) modes, with various conditions examined for each mode. The results revealed major improvements in the corrosion resistance of titanium, typically by about two orders of magnitude, following the various treatments, which are explained by the promotion of passivity of the titanium associated with a shift in the open-circuit potential. Thus, the open circuit potential of titanium was about −780 mV (SCE), compared with −300 to −250 mV (SCE) for nickel alloying, −130 to 50 mV (SCE) for nickel-molybdenum alloying and 400-470 mV (SCE) for palladium alloying. The PID process tended to result in slightly more positive potentials.     

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.     

Influence of Surface Condition on the Corrosion of Aluminium Brass

Abstract

The corrosion behaviour of aluminium brass, with four different surface treatments (highly and lightly oxidised, pickled and sandblasted), was studied in natural sea water, both stagnant (aerated) and flowing.

Electrochemical measurements and weight loss determinations showed that the oxide film produced during industrial annealing had little effect on the corrosion rate as compared with a pickled surface, in both stagnant and flowing conditions. Sandblasted surfaces were inferior.

Measurements of currents flowing in galvanic couples formed between oxidised and stripped metal were small and decreased with time, but the oxidised surfaces appeared to be slightly more vulnerable than the pickled ones to localised corrosion. Some further work is needed here.     

Electrochemical aspects of Ti–Ta alloys in HBSS

Corrosion behaviour of the studied Ti–Ta alloys with Ta contents of 30, 40 and 50 wt% together with the currently used metallic biomaterials commercial pure titanium (Cp–Ti) was investigated for biomedical applications. All the samples were tested by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in Hank's balanced salt solution (HBSS) with and without albumin protein at 25 °C. Very low corrosion current densities and passive current densities (in the order of 10^?6 A/cm²) were obtained from the polarization curves, indicating a typical passive behaviour for all the samples in HBSS with and without albumin. The EIS technique was applied to study the nature of the passive film formed on all the samples at various imposed potentials ?500 mV (SCE), 0 mV (SCE), 500 mV (SCE) and 1000 mV (SCE). The equivalent circuit (EC) used successfully to describe the behaviour of the samples suggests a single passive film present on the metals' surface in HBSS with and without albumin. The results showed that the presence of albumin in HBSS had an influence on the zero current potential (ZCP), polarization resistance (R_p) and capacitance (C). The presence of albumin protein in HBSS improved slightly the corrosion resistance of the entire sample. The experimental results confirm that the electrochemical behaviour of the studied Ti–Ta alloys is better to that of Cp–Ti, suggesting their promising potential for biomedical applications.     

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.