Service life of corrosion pitted pipes subject to fatigue loading and hydrogen embrittlement

The structural service life of brittle material pipes with exterior corrosion pits is likely to depend on crack initiation and crack development and this may be influenced by pressure loading fluctuations and the possibility of material hydrogen embrittlement. Recently developed methods are used to estimate the cracking pattern, the failure state of the crack development from external pits and the rate of Hydrogen-Assisted Cracking under fluctuating loadings. The effect of hydrogen from the surrounding environment on the cracking rate is formulated using a generalized form of Paris' law. The depth of cracks initiated from surface pits is estimated as a function of pipe age. A realistic example is presented and the results discussed.     

基于氧化特性分析的IF钢连退麻点缺陷产生机理

首钢现行工艺下IF钢热卷铁皮厚度达到10 μm以上,边部铁皮略薄,中部铁皮较厚,随着卷取温度的升高,铁皮厚度增加且对应酸洗时间延长;不同卷取温度下的热卷酸洗后均发现小麻坑缺陷。本文利用差热分析手段研究了IF钢的高温氧化机理,发现IF钢抗氧化性低,随温度升高铁皮增厚明显,精轧区间以FeO铁皮结构为主,在1 150 ℃左右发生明显的内氧化,界面形成大颗粒氧化质点。综合分析得出：IF钢带钢连退后麻点缺陷产生的主要机理为热卷的铁皮较厚,热轧过程压入钢板表面所致。为此,提出了控制措施,即降低热轧过程温度,改变层冷模式,加大精轧用水量,提高精轧轧制速度,降低冷轧酸洗速度等,有效减少了麻点缺陷的发生概率。     

Corrosion Behavior of 5A05 Aluminum Alloy in NaCl Solution

The corrosion behavior of 5A05 aluminum alloy in 3.5 wt.%NaCl solution was investigated by scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS),Fourier transform-infrared(FT-IR)spectroscopy and electrochemical impedance spectroscopy(EIS)test,and the corrosion mechanism was also discussed.The results showed that the corrosion rates of the 5A05 alloy were low and decreased with the increase in immersion time.Under the conditions of exposure studied,this alloy sufered from pitting corrosion that took place from or around the intermetallic particles existing in the alloy.The number and size of the hemispherical corrosion pits on the sample surfaces increased with the increase of the test time.The dark-grey layer of corrosion products formed on 5A05 aluminum alloy in 3.5%NaCl solution contained many microcracks.Furthermore,XPS and FT-IR analysis proved that the corrosion products were mainly composed of Al2O3,Al(OH)3 and AlCl3.     

Ni-Cr-Mo-Cu合金耐晶间腐蚀及点蚀性能研究

在Cr、Mo成分一定的情况下,通过改变铜的质量分数(1％～5％),制备新型Ni-Cr-Mo-Cu耐蚀合金.利用化学浸泡法、电化学法(极化曲线法、循环伏安法)对其耐晶间腐蚀以及耐点蚀能力进行分析研究.实验结果表明,w(cu)=1％～5％的Ni-Cr-Mo-Cu合金具有优良的耐晶间腐蚀以及点蚀性能,相对而言w(cu)=2％的镍基合金耐晶间腐蚀以及点蚀性能较差,因此说明合适的铜含量可以提高镍基合金的耐晶间腐蚀和点蚀性能;并且试验合金晶间腐蚀与点蚀的电化学行为和特征与其浸泡腐蚀的结果是吻合的.     

Corrosion behaviour of 304LN activated tungsten inert gas and flux-cored arc weld metals

Activated tungsten inert gas (A-TIG) and flux-cored arc (FCA) weld metals were prepared using 304LN stainless steel plate. The weld metals were thermally aged at 923, 973 and 1023?K for 100?h to study the decomposition of initial δ-ferrite in A-TIG (～10 ferrite number (FN)) and FCA (～5 FN) weld metals into secondary phases like M₂₃C₆ carbides, χ and σ. Ferrite number is the measurement of δ-ferrite based on the principle of magnetic property using ferritescope. Preliminary microstructural studies revealed the formation of carbides in FCA weld metals aged at 923?K for 100?h, which was correlated with higher carbon content (0.04?wt-%), and also ageing at higher temperature transformed δ-ferrite into χ/σ phases. However, A-TIG weld metals showed the transformation of δ-ferrite mainly into χ/σ phases. The δ-ferrite transformation kinetics was found to be sluggish in A-TIG weld metals compared to FCA weld metals. This difference was attributed to the difference in the carbon contents of A-TIG and FCA welds. Activated tungsten inert gas weld metals showed better uniform and pitting corrosion resistance compared to FCA weld metals in as-deposited and thermally aged conditions. Presence of higher amount of initial δ-ferrite content in A-TIG weld metal helped diffusion of minor alloying elements like sulphur and phosphorous into it, thereby reducing their microsegregation at the δ/γ interface boundaries and subsequent pitting corrosion attack. Thus, A-TIG welding process was found to be superior compared to FCA welding process.     

Influence of tempering treatment on microstructure and pitting corrosion resistance of a new super ferritic–martensitic–austenitic stainless steels with 17%Cr

Supermartensitic stainless steels (SMSSs) allow high mechanical strength with better corrosion resistance and toughness than conventional martensitic stainless steels. The SMSS steels with 12–13%Cr have been studied and applied in the oil and gas offshore production. The increase of Cr content, and the addition of Mo and W is now being investigated to increase mechanical and pitting corrosion resistance. In this work, a new 17%Cr steel, with Mo and W additions was studied. Depending on the final tempering treatment, the steel has a complex microstructure of austenite, ferrite, martensite and precipitates. The pitting corrosion resistance also depends on the microstructure produced by tempering. It was found that the pitting potential slightly decreases with the increase of tempering temperature and is further decreased by the double-tempering treatment. The pits initiate and grow preferentially in the martensite or tempered martensite islands, due to the lower Cr, Mo and W contents of these areas.     

Corrosion Behavior of Extruded near Eutectic Al—Si—Mg and 6063 Alloys

In this work,a comparison study on corrosion behavior of extruded near eutectic Al—12.3%Si—0.26%Mg and 6063 alloys has been carried out by mass loss test in 4%H₂SO₄ aqueous solution in the open air and potentiodynamic polarization test in 3.5 wt.%NaCI aqueous solution.Results indicate that the corrosion resistance of the near eutectic Al—Si—Mg alloy is less than that of 6063 alloy.Macro/microscopy and scanning electron microscopy results clearly show the difference of the corrosion progress of these two alloys in 4%H₂SO₄ aqueous solution.The corrosion type of 6063 alloy is pitting corrosion.The Mg₂Si and AlFeSi particles and surface defects act as nucleation sites for pitting,and the amount and distribution of them have a significant effect on the pitting behavior.For the near eutectic alloy,there are two types of corrosion cells. One is between the extruded primaryα-AI and the eutectic,the other is between the eutectic Al and eutectic Si particles.Combination of these two types of corrosion cells leads to a lower corrosion resistance,a higher mass loss of the near eutectic alloy compared with 6063 alloy,and the formation of the paralleling corroded grooves.     

Role of alloyed silicon and some inorganic inhibitors in the inhibition of meta-stable and stable pitting of Al in perchlorate solutions

The study of both meta-stable and stable pitting events on the surface of pure Al and three Al–Si alloys, namely (Al + 6%Si), (Al + 12%Si) and (Al + 18%Si) alloys, was carried out in deaerated neutral NaClO₄ solutions of various concentrations (10⁻⁴–10⁻² M). Measurements were carried out under the effect of various experimental conditions using potentiodynamic anodic polarization and potentiostatic techniques. The results presented below showed that meta-stable pits (appeared as oscillations in current) form at potentials close to the pitting potential (E _pit) and during the induction time for stable pit formation. Various factors affecting the rate of meta-stable and stable pits were studied. The presence of Si as an alloying element in Al reduces the rate of formation of meta-stable pits, corresponding to a reduction in the probability of developing stable pits, and an increase in the pitting potential results. The inhibitive effects of chromate, silicate, molybdate and tungstate on pitting corrosion in Al were also studied. Results obtained showed that these known inhibitors retard both meta-stable and stable pitting events. This makes attainment of stable pit growth more difficult in presence of these inhibitors.

钴元素对SUS304不锈钢耐点腐蚀性能的影响

开发红土镍矿是解决镍资源危机的重点,由于红土镍矿中钴和镍伴生,导致不锈钢中钴元素的存在。本文以SUS304不锈钢钢板和电解钴为原料,采用真空感应冶炼炉和三高石墨铸模浇铸不同钴含量试样,利用线切割机加工腐蚀试样,进行三氯化铁点腐蚀试验。试验结果表明：钴元素的加入增大了铸态SUS304不锈钢点腐蚀敏感性,降低了铸态SUS304不锈钢耐点腐蚀性能,当钴含量小于0．3％时,点腐蚀速率没有增加,当钴含量为0．4％和0．5％时,点腐蚀速率明显增大。     

Effects of selected water chemistry variables on copper pitting propagation in potable water

The pit propagation behavior of copper (UNS C11000) was investigated from an electrochemical perspective using the artificial pit method. Pit growth was studied systematically in a range of HCO₃⁻, SO₄²⁻ and Cl⁻ containing-waters at various concentrations. Pit propagation was mediated by the nature of the corrosion products formed both inside and over the pit mouth (i.e., cap). Certain water chemistry concentrations such as those high in sulfate were found to promote fast pitting that could be sustained over long times at a fixed applied potential but gradually stifled in all but the lowest concentration solutions. In contrast, Cl⁻ containing waters without sulfate ions resulted in slower pit growth and eventual repassivation. These observations were interpreted through understanding of the identity, amount and porosity of corrosion products formed inside and over pits. These factors controlled their resistive nature as characterized using electrochemical impedance spectroscopy. A finite element model (FEM) was developed which included copper oxidation kinetics, transport by migration and diffusion, Cu(I) and Cu(II) solid corrosion product formation and porosity governed by equilibrium thermodynamics and a saturation index, as well as pit current and depth of penetration. The findings of the modeling were in good agreement with artificial pit experiments. Malachite, bronchantite, cuprite, nantokite and atacamite corrosion products were both observed in experiment and predicted by the model. Stifling and/or repassivation occurred when the resistance of the corrosion product layer became high enough to lower the pit bottom potential and pit current density such as 10^-5 A/cm² could be attained with thick and dense layer. The ramifications of these findings towards pit propagation characteristics in potable waters will be discussed with improved insight into the roles of Cl⁻ and SO₄²⁻ ions.