Corrosion characteristics of the wrought Ni‐Cr‐Mo alloys

This paper concerns the wrought, nickel‐chromium‐molybdenum (Ni‐Cr‐Mo) alloys, a family of materials with a long history of use in the chemical process industries. Their attributes include resistance to the halogen acids and resistance to pitting, crevice attack, and stress corrosion cracking in hot, halide salt solutions. The purpose of this paper is to characterize the performance of the Ni‐Cr‐Mo alloys in several key chemicals, using iso‐corrosion diagrams. These indicate the expected corrosion rates over wide ranges of concentration and temperature. Furthermore, the differences between individual Ni‐Cr‐Mo alloys, and their behavior relative to the stainless steels are defined. The data indicate benefits of both a high chromium content and a copper addition, as used in Hastelloy® C‐2000® alloy.     

Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

The effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine-containing atmosphere using air mixed with the salt-vapor mixture of NaCl–KCl–CaCl₂ at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo-free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr₂O₃ scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr₂O₃ scale, resulting in internal chromium chloride precipitates and a Cr-depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high-temperature chlorination of Ni–Cr alloys in salt-vapor-containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.     

合金元素对铸造Fe-Cr铁素体不锈钢耐浓硫酸腐蚀性能的影响

采用电化学技术、浸泡腐蚀及能谱分析等研究了合金元素对铸造Fe-Cr铁素体不锈钢耐浓硫酸腐蚀行为的影响。结果表明:随铬含量的增加,Fe-Cr合金的耐浓硫酸腐蚀性能增强,单一的铬合金化不能使Fe-Cr合金在60℃,98%H2SO4中自钝化;钼能促进Fe-Cr25-Mo合金的钝化和自钝化,随钼含量的增加,Fe-Cr25-Mo合金的耐浓硫酸腐蚀性能增强;辅助合金元素镍、铜可促进Fe-Cr25Mo2合金的钝化和自钝化,而钛、铌的影响不大。     

The structure and properties of electroless Ni–Mo–Cr–P coatings on copper alloy

In the present study, the quaternary Ni–Mo–Cr–P alloy coatings were deposited on copper alloy by an electroless deposition process. Crystallization behavior and the effect of heat‐treatment on hardness and corrosion resistance of Ni–Mo–Cr–P deposits were detailedly investigated. X‐ray diffraction (XRD) analysis shows that as‐deposited Ni–Mo–Cr–P coatings are Ni–Mo–Cr–P solid solution and mixed crystal structure; the trend of microcrystallinity increases with the introduction of additional types of metal element; Ni–Mo–Cr–P alloy coatings start to occur in the crystallization with the heat‐treatment temperature increasing. With an increase in the annealing temperature, the hardness improves and reaches the maximum value at 500 °C. Further, it is found that Ni–Mo–Cr–P coatings have superior corrosion resistance than Ni–P and Ni–Mo–P deposits after the analysis of electrochemical measurements. Moreover, corrosion resistance increases before annealed at 400 °C, but heat‐treatment at higher temperatures has a negative effect on the corrosion resistance of Ni–Mo–Cr–P alloy coatings.     

Corrosion behaviour of alloy 31 – UNS N08031 – under conditions of oil & gas production

The corrosion behaviour of alloy 31 (UNS N08031‐31Ni – 27Cr – 6.5Mo – 1.2Cu – 0.2N – bal. Fe) was tested in laboratory and field tests in seawater with and without additions of CO₂ and/or H₂S in slow strain rate tests, and in SSC (Sulphide Stress Corrosion) tests according to NACE MR0175. The results demonstrate a high resistance of alloy 31 to localised corrosion. Due to the high chromium and molybdenum concentration, its resistance to pitting and crevice corrosion in chloride‐contaminated seawater is significantly higher than that of alloy 28 and alloy 825 and it equals that of typical nickel base alloys like alloy 625. Alloy 31 is not sensitive to chloride‐induced stress corrosion cracking, either with or without H₂S, or sulphide stress cracking. Alloy 31 is approved for sour gas applications up to LEVEL VI in NACE MR 0175. The combination of properties makes alloy 31 an attractive choice for components in oil and gas production including wirelines, umbilicals, tubing, piping and topside application.     

Thermodynamic characteristics and numerical modeling of internal nitridation of nickel base alloys

Kinetics and thermodynamics of the process of internal nitridation of various nickel‐base alloys have been investigated in oxygen‐free nitrogen atmospheres. Furthermore, the influence of formation and spalling of a protective oxide scale on the internal nitridation behavior of the alloys was studied by isothermal and cyclic oxidation tests in air. In general, nitridation kinetics of model nickel‐base alloys of the system Ni‐Cr‐Ti was found to obey a parabolic rate law indicating that the nitridation process is diffusion‐controlled. The temperature dependence of the nitridation rate constants is well described by an equation of the Arrhenius type. A thermodynamic calculation of the Ni‐Cr‐Ti‐Al‐N system was used to determine the nitrogen solubility in respective alloys as a function of temperature and alloy composition. The results show that a higher chromium content gives rise to an increase in the nitrogen solubility of Ni‐Cr‐Ti alloys leading to an increased nitridation rate in accordance with the experimental observations. From the calculated values for the nitrogen solubility, the diffusion coefficients of nitrogen were assessed using Wagner's classical theory of internal oxidation. A computer model of internal nitridation was developed that combines a commercial thermodynamic software (ChemApp) with a finite‐difference diffusion calculation. It was found that this model describes the internal nitridation process in reasonable agreement with the experimental results and allows to treat the case of simultaneous formation of different nitrides. The dependence of internal nitridation behavior on spalling and cracking of the oxide was incorporated into the simulation on the basis of simple assumptions showing that this calculation method successfully applies also to complex internal corrosion processes.     

铸铁在海水中的腐蚀行为

报告了18种铸铁在天然海水和流动海水中的腐蚀试验结果,总结了它们在海水中的腐蚀行为,普通铸铁在天然海水及流动海水中的腐蚀速度与碳钢接近,低合金铸铁在海水中的腐蚀行为与普通铸铁相似。CrSbCu铸铁在海水中的腐蚀比普通铸铁轻,添加Ni,Ni-Cr,Ni-Cr-Mo,Ni－Cr-Cu,Ni-Cr-Re,Cu-Sn-Re,Cu-Cr,Cu-Al等的低合金铸铁在海水中的腐蚀速度与普通铸铁无明显差别,加入少量Ni,Cr,Mo,Cu,Sn,Sb,Re等合金元素可减小铸铁在海洋大气区的腐蚀速度,高镍铸铁在天然海水及流动海水中的腐蚀均较轻。     

显微组织对Cu—Cr—Ni合金高温氧化行为的影响

研究了两种单／双相Cu-Cr-Ni合金的高温氧化行为。结果表明,合金氧化动力学偏离抛物线规律,其瞬时抛物线速率常数随时间延长而降低。两种合金表面氧化膜的结构差别较大,单相合金表面形成－连续的Cr2O3层,双相合金表面氧化膜外层是一边疆的CuO层,Ni和Cr的氧化发生在合金内部,这种合金与氧化物共存的混合内氧化与经典的内氧化明显不同,氧化层最里面形成了一连续的CrO3膜,抑制了合金的进一步氧化。     

The use of potentiostatic techniques in the development of improved stainless steels for chemical plant

It is possible by potentiostatic methods to exactly forecast the behaviour of unusual material/corrodent systems; such methods may, consequently, be used for the development of corrosion resistant alloys. A new alloy on iron base (with, %, 25 Cr, 5.5 Ni, 3.0 Cu, 2.5 Mo) combines very good mechanical properties with resistance to general corrosion, pitting and crevice corrosion. Industrial applications include impellers for pumps operating in acid slurries (30% P₂O₅, 30% CaSO₄ as solids, 2% H₂SO₄, HF, H₂SiF₆; 60–80°C, three years without corrosion); pump impeller for sulfuric acid and other corrosive products shipped in tank ships; impeller for a sea water pump (no corrosion after 4 years' operation).     

Influence of Cr Content on the Corrosion of Fe–Cr Alloys: The Synergistic Effect of NaCl and Water Vapor

The corrosion behavior of pure Fe and Fe–Cr alloys with different Cr content in the presence of a solid NaCl deposit and water vapor at 600°C was studied. Results indicated that the corrosion of pure Fe was severe even in air at 600°C and the scale formed on the surface was compact and uniform. However, with a solid NaCl deposit on its surface, the corrosion of pure iron in air was suppressed to some extent, but the presence of water vapor in the atmosphere causes accelerated corrosion. Under the synergistic effect of NaCl and water vapor, the corrosion of pure iron is accelerated more significantly. In contrast with the known effect of Cr content on the oxidation of Fe–Cr alloys, an increasing Cr content in Fe–Cr alloys increases the corrosion rate of the alloys under the synergistic effect of solid NaCl and water vapor. A mechanism to explain the effect of water vapor and NaCl on the corrosion of pure iron and Fe–Cr alloy is proposed.     

Materials performance in diethylene glycol dibenzoate environments containing sulphur compounds

The effect of para-toluene sulphonic acid (PTSA) on the corrosion of steels and nickel-based alloys in diethylene glycol dibenzoate (DEGDB) was investigated. Chromium is the major alloying element that reduces the corrosion rate. With increasing Ni content in metals, the rate increases. Mo alloying element alone cannot provide sufficient protectiveness from corrosion but Mo can act with Cr to protect metals effectively from dissolution. It is recommended that high Ni content metal without alloying with Mo and Cr is inadequate to be a construction material for sulphur-compound environments.     

New Ni-Cr-Mo alloy demonstrates high-temperature structural stability with resultant increases in corrosion-resistance and mechanical properties

Equipment made of NiCrMo alloys of the Ni 16 Cr 16 Mo W and Ni 22 Cr 16 Mo types is used extensively in the chemical industry, in particular because the corrosion resistance in the as-welded condition could be improved in recent years by controlling C and Si contents to very low levels. A new type of alloy — Hastelloy Developmental Alloy C-455 — has high resistance to carbide and intermetallic phase precipitation at high temperatures. By reducing the amount of these phases it has been possible to considerably improve the high temperature properties, the corrosion resistance and the mechanical properties of the alloy. The new material may be exposed to sensitizing range temperatures (550–1090 °C) for a long time without becoming susceptible to corrosion: it is, therefore, readily weldable. Contrary to NiCrMo alloys known so far there are almost no problems when the material is exposed to hydrochloric and sulfuric acids.     

Nitridation in NH3‐H2O‐mixtures

Exposures were conducted of iron, nickel, ferritic 1‐18%Cr steels, austenitic 18%Cr‐9%Ni‐ and 20%Cr‐31%Ni‐steels and a 16%Cr‐Ni‐base alloy at 500°C in He‐30%H₂O and 70%H₂O‐30%NH₃, to compare the corrosion behaviour of these materials in water vapor as in conventional power plants with their behaviour in a NH₃‐H₂O mixture, i.e. under conditions of the “Kalina‐cycle”. After 50 h in He‐H₂O generally a dense oxide scale had grown on iron and on the steels, whereas the scale grown in NH₃‐H₂O was porous, due to initial formation of the γ′‐ and ε‐nitrides, which are converted to Fe₃O₄ later. The porous scale allows internal nitridation of the Cr‐steels, nitrogen is transferred into the metal phase and reacts to finely dispersed CrN‐precipitates. This process causes stresses in the material and formation of cracks. The higher the Cr‐content of the material, the worse is the damage of the materials surface. Least corrosion damage occurs for iron and the 1%CrMo‐steel, however, the inward penetration of nitridation is greatest, and after 5 years on the low Cr‐steel a layer of about 15 mm would be embrittled by internal nitridation, formation of γ′ and ε‐nitride layers and external oxidation. Nickel is strongly damaged by intermediate formation of instable Ni₃N, which causes internal stresses and cracking, but also pore formation by its decomposition. The surface region of the 15%Cr‐Ni‐base alloy is also destroyed by internal nitridation and extrusion of Ni‐particles, while for this material the inward penetration of nitridation is relatively slow due to the low solubility and diffusivity of N in Ni and Ni‐alloys.     

氮及热处理制度对028合金微观组织的影响

借助Thermo-Calc热力学计算软件,分析了028合金主要元素对氮饱和溶解度和平衡相析出温度的影响,设计了不同氮含量的试验合金。采用光学显微镜、扫描电镜等方法研究了不同固溶温度下试验合金的微观组织。结果表明,Cr、Mo和Mn提高氮的饱和溶解度,Ni、Cu降低氮的饱和溶解度;Ni显著降低σ相析出温度,Cr、Mo显著提高σ相析出温度。随着固溶温度的升高,晶粒的等轴性提高,在1080 ℃固溶时,试验合金充分固溶获得再结晶完全的等轴晶。当固溶温度超过1110 ℃时,试验合金回复再结晶完全,晶粒迅速长大;固溶温度相同条件下,氮含量越高,试验合金的平均晶粒尺寸越小,说明氮可以起到细化晶粒的作用。     

Long‐term exposure of austenitic steels and nickel‐based alloys in lignite‐biomass cofiring

The aim of this study was to assess the long‐term impact that the addition of biomass provokes on superheater materials exposed to fireside corrosion environments. Alloys covering a broad range of commercially available materials were investigated. Their corrosion kinetics under different corrosive deposits and atmospheres was evaluated, and their corrosion products analyzed to deepen understanding of the underlying corrosion mechanisms. Therefore, three nickel‐based alloys and three austenitic steels containing 20–24 wt.% Cr were tested at 650°C for 7,000 hr. The long‐term exposure shows new mechanistic aspects of Type II hot corrosion that were revealed by accelerated material depletion. The formation of Ni–NiS eutectic and the formation of a Cr depleted zone close to the substrate corrosion product interface are indicative of the breakaway occurrence. Differences in the corrosion behavior are related to the balance of Ni, Mo, Co, and Cr and can serve as the material selection argument. The evaluation concluded with the finding that alloys presenting Mo and Ni might be preferentially used in fireside corrosion in the presence of biomass, whereas the use of austenitic steels suffer less corrosion if no biomass is present in the corrosive atmosphere.     

Gaseous corrosion of alloys and novel coatings in simulated environments for coal,waste and biomass boilers

The reduction of emissions from power generation plants is a key part of the Kyoto Protocol. Reduced emissions per unit of power produced can be achieved via increased thermal efficiency and this can be achieved by increasing steam parameters (i.e. temperature and pressure). Increased steam parameters in turn leads to accelerated corrosion of boiler components. Biomass and solid waste fuels introduce a number of aggressive species into process environments that result in enhanced rates of boiler degradation. This paper reports on studies, both theoretical and experimental, of the corrosion behaviour of high‐alloy steels and Ni‐base alloys as well as coatings for use in high efficiency coal and/or biomass‐ and waste‐fired power plants. Coatings produced within the SUNASPO project have been laboratory tested in gaseous atmospheres representative of coal combustion, biomass combustion and waste incineration. Laboratory tests were carried out mainly in the temperature range 500 °C to 800 °C. Initial results showed the poor performance of traditional uncoated low‐alloy boiler steels P91 (9% Cr) and HCM12A (12% Cr), as well as the higher alloy steel, 17Cr/13Ni. Results show the beneficial effects of coatings containing Al, Si, Al + Si, Al + Ti and Al + B in reducing the rate of corrosive attack. In a combustion product gas containing 100 ppm HCl and 1000 ppm SO₂, aluminizing affords corrosion resistance of low‐alloy steels such as HCM12A and P91 similar to that of Alloy 800 over 1000 h of test. The presence of Al inhibits internal, sometimes localized corrosion by promoting the formation of a protective surface oxide layer even at relatively low temperatures. The results of experiments in simulated coal; biomass and waste atmospheres are presented and discussed in terms of both corrosion kinetics and mechanisms of degradation.     

Corrosion of alloys and their diffusion aluminide coatings by KCl:K2SO4 deposits at 650 °C in air

P91 ferritic‐martensitic steel, 17Cr–13Ni and alloy 800 austenitic stainless steels and Inconel 617 alloy have been aluminised to form Fe₂Al₅, (Fe,Ni)Al and Ni₂Al₃ aluminide coatings. These alloys and their corresponding coatings were subjected to corrosion in air by 50:50 mol/mol K₂SO₄/KCl deposits at 650 °C for 300 h. With the exception of the Inconel 617 alloy, significant metal losses (>180 µm) were recorded. These losses were planar for P91 alloy but involved internal corrosion for the two austenitic steels. The (Fe,Ni)Al and NiAl coatings on the austenitic steels and the Inconel 617 alloy were significantly corroded via intergranular and internal chloridation–sulphidation–oxidation. In contrast, the Fe₂Al₅ coating on the P91 alloy coating was virtually unattacked. For the alloys, the relative extents of corrosion damage can be explained in terms of the stability and volatility of metal chlorides formed. For the coatings, STEM/EDS analyses enable clear linkages to be made between the presence and number of Cr‐rich particles on coating grain boundaries and the corrosion damage observed for the coatings.     

An Evaluation of Nickel-rich Alloys In Wet Process Phosphoric Acid

The corrosion behavior of materials in the production of wet process phosphoric acid is a function of acid concentration, temperature and purity. The effects vary depending on the type of process and raw materials used to manufacture phosphoric acid. Laboratory corrosion studies are presented for several nickel-rich alloys in pure C. P. grade and wet process phosphoric acid manufactured by the sulfuric acid digestion of phosphate rock. The low concentration (30 010 P20,) acids contain contaminants which make the acid more corrosive than the 56O10 P, O, phosphoric acid. At moderate sorption temperatures (< 140 °C) several nickel-rich alloys demonstrated excellent corrosion resistance. Hot wall corrosion tests were employed to simulate heat transfer conditions in superphosphoric acid processes. The hot wall tests demonstrated excellent correlation between the metal skin temperature and the resulting corrosion rate of the specimen. Excessive corrosion rates were experienced with the nickel-rich alloys at metal skin temperatures >145°C with the crude phosphoric acids. Electrochemical studies were used to demonstrate that the crude acid can become unstable at elevated temperatures and result in excessively high cor-rosion attack of the nickel-rich alloys. Hastelloy alloy G and Haynes alloy No. 625 demonstrated best overall corrosion resistance in the wet process acids, especially at higher temperatures.     

不锈钢,铜和铝合金酸性土壤腐蚀行为研究

研究了1Cr13,1Cr18Ni9Ti,H62黄铜和Ly11铝合金在酸性土壤中的腐蚀行为,结果表明,Ly11铝合金的腐蚀率比不锈钢的腐蚀率约大50倍,发现Ly11铝合金在不同种的酸性土壤中的腐蚀率与其腐蚀产物的相结构有一定的对应关系     

Kinetics of phase separation in iron-based ternary alloys. II. Numerical simulation of phase separation in Fe–Cr–X(X=Mo, Cu) ternary alloys

Numerical simulations were performed using a model based on the Cahn–Hilliard equation in order to investigate asymptotic behavior of a minor element associated with phase decomposition of the major element in Fe–Cr–Mo and Fe–Cr–Cu ternary alloys. Bifurcation of peaks of Mo along peak tops of Cr concentration occurs in an Fe-40at.%Cr–5at.%Mo alloy at temperatures between 750 and 850 K. Bifurcation of peaks of Cr is also shown in an Fe–40at.%Mo–5at.%Cr alloy. The asymptotic behavior of Mo or Cr along a trajectory of a peak top of Cr or Mo concentration depends on the sign of the second derivative of the chemical free energy with respect to the concentrations of Cr and Mo. In an Fe–40at.%Cr–5at.%Cu alloy,the amplitude of Cu along peak tops of Cr concentration decreases with time, Simulated asymptotic behavior of Mo and Cr in Fe–Cr–Mo ternary alloys and Cu in an Fe–Cr–Cu alloy is in good agreement with that predicted by the theory proposed by the present authors.