Die Bedeutung des Oxalsäure-Tests für die Prüfung der Korrosionsbeständigkeit nichtrostender Stähle

The meaning of the oxalic acid etch test for testing the corrosion resistance of stainless steels In the oxalic acid etch test according to ASTM A 262 practice A, precipitations of phases rich in chromium and molybdenum which can occur in stainless steels, are preferentially dissoved. The behaviour of such phases in the oxalic acid etch test was investigated taking precipitations of carbide M₂₃C₆, s?-phase, χ-phase and Laves-phase in stainless steels AISI 304 L and 316 L as examples. The chemical composition of these was evaluated with a scanning transmission electron microscope (STEM) by EDS. With coarser precipitations, it was possible to support this analytical method by EDS of metallographic cross sections in a scanning electron microscope (SEM). In oxalic acid, critical threshold potentials exist above which the above mentioned phases are preferably attacked, furthermore critical pH values, below which no selective attack of the precipitated carbides and intermetallic phases occurs. The numerical values of the threshold potentials as well as the critical pH values were evaluated. When testing stainless steels in the oxalic acid etch test, the steel specimens are polarized to a highly positive potential in the very trans passive range. In this potential range the corrosion rate of stainless steels increases with increasing chromium content, while in the active and passive range the corrosion rate decreases with increasing chromium content. Other than the nitric-hydrofluoric acid test, the copper-copper sulfate-sulfuric acid test, and the ferric sulfate-sulfuric acid test, the oxalic acid etch test does therefore not indicate any chromium depletion. Hence, an intergranular attack also occurs when precipitations of carbides rich in chromium are present at the grain boundaries of austenitic stainless steels with the carbides being precipitated without any chromium depletion of the areas adjacent to the grain boundaries. Sensitized austenitic stainless steels which are susceptible to intergranular corrosion due to the precipitation of chromium rich carbides and chromium depletion of the areas adjacent to the grain boundaries, can suffer intergranular SCC in high temperature aqueous environments when additionally critical conditions with respect to the mechanical stress level and the oxygen concentration in the environment are given. For the detection of sensitized microstructures, the oxalic acid etch test must be valued critically due to the dependence of the corrosion rate on the chromium content mentioned above, and is obviously by far less suited than the conventional tests for establishing resistance to intergranular corrosion in sulfuric acid-copper sulfate solutions with additions of metallic copper (Strauß test, severe Strauß test).     

Elektrochemische Untersuchungen zur Hochtemperaturkorrosion von Chrom-Nickel-Stählen in Alkalisulfatschmelzen bei 700°C

Electrochemical investigations into the high temperature corrosion of chromium nickel steels in alkali sulfate melts at 700°C Specimens from austenitic stainless steels, with and without susceptibility against intergranular corrosion attack, decarburized and carburized, and stabilized with Ti or Nb, have been corroded in eutectic mixture of alkali sulphates at 700°C at constant potentials. Furthermore additions of 10% NaCl and 1% SO₃ have been investigated. Any selective corrosion of both carbides and Cr depleted zones did not take place. With decreasing potentials there was a preferred attack at grain boundaries – especially in the case of NaCl additions. At very negative potentials an “active” corrosion state exists without formation of protective oxide layers with the consequence of fissured surfaces and internal oxidation. The effect is supported by NaCl addition and by Ni as alloying element.     

Effects of cooling rate on the precipitation behavior of grain boundary carbide and corrosion resistance of 5Cr15MoV stainless steel

High-carbon and high-chromium alloy steels are prone to pitting and intergranular corrosion, which reduces corrosion resistance. The precipitation behavior of the carbides of high-carbon and high-chromium alloy steels is one of the main factors affecting pitting and intergranular corrosion of stainless steel. In this study, 5Cr15MoV stainless steel was heated to 1,200℃ and then cooled by cooling rates varying from 25 to 150°C/min. The precipitation behavior of grain boundary carbides of 5Cr15MoV steel at different cooling rates, and its effect on the corrosion resistance of materials was studied. The results show that the carbides of 5Cr15MoV steel mainly precipitate along the grain boundaries, which leads to the formation of chromium-depleted zones near the grain boundaries and reduces resistance to intergranular corrosion. It has been found that a higher cooling rate shortens the width of the Cr-depleted zone near the boundary from 0.871 to 0.569 μm, reduces the Cr-concentration gradient near the grain boundary from 36.422% to 12.667%, and suppresses the nucleus growth rate of grain boundary carbides. As the cooling rate increases, the corrosion current density decreases from 13.29 to 2.42 μA/cm². The corrosion rate is the lowest, while the cooling rate is 150°C/min. The corrosion rate decreases from 218.339 to 158.488 mm/a. The phenomenon of intergranular corrosion and pitting corrosion was found to be weakened; and thereby, it is shown that an intensive cooling rate can improve the corrosion resistance of 5Cr15MoV steel.     

氮对高纯奥氏体不锈钢耐晶间腐蚀性能的影响

采用电化学、化学浸渍、俄歇电子能谱分析、物理化学相分析等方法研究了氮对高纯奥氏体不锈钢000Cr19Ni14耐敏化态晶间腐蚀和非敏化态晶间腐蚀性能的影响,并探讨了其作用机理.结果显示,高纯奥氏体不锈钢加氮合金化(≤0.20%)基本上消除敏化态晶间腐蚀,敏化处理不会引起晶界贫铬;当氮含量不超过0.087%时,对非敏化态晶间腐蚀影响很小,而超过0.087%时,由于在晶界氮元素的偏聚以及氮化铬的析出加速非敏化态晶间腐蚀.     

Interkristalline Korrosion ferritischer Chromstähle mit rd. 17 Gew.-% Chrom nach Glühen im Temperaturbereich um 500 °C

Intergranular corrosion of ferritic 17% chromium stainless steels after heat-treatment in the 500 °C temperature range After stabilizing heat-treatment at 750°C, the non-stabilized, ferritic 17% chromium stainless steel Mat.-Nr. 1.4016 (X8Cr17) still contains a sufficient high concentration of carbon dissolved in solid solution, that after heat-treatment in the 500 °C temperature range carbides rich in chromium of the M₂₃C₆-type are precipitated, causing a relatively weak pronounced susceptibility of the steel to intergranular attack. The susceptibility to this type of attack can be detected by testing specimens in the sulfuric acid-copper sulfate-test with increased concentration of sulfuric acid as compared with the DIN standard 50914, followed by metallografic examination of the specimens. The susceptibility to intergranular corrosion of the material investigated occurring after heat-treatment in the low temperature range, which until now is unknown, is described in terms of a Rollason-diagram as it is commonly used for austenitic chromium-nickel stainless steels. As it is to be expected, the stabilized 17% chromium steels Mat. No. 1.4510 (X8CrTi17), 1.4511 (X8CrNb17) and 1.4523 (X8CrMoTi17) are resistant to intergranular corrosion after heat-treatment at low temperatures.     

Sensitization Behavior of Type 409 Ferritic Stainless Steel: Confronting DL-EPR Test and Practice W of ASTM A763

Stainless steels employed for manufacturing automotive exhaust systems must withstand severe thermal cycles, corrosive environment due to urea decomposition, and welding operations. AISI 409 ferritic stainless steel can be considered a low-cost alternative for this application. However, depending on the manufacturing conditions during welding cycles, this material can be sensitized due to the precipitation of chromium carbides at grain boundaries. In this work, the intergranular corrosion resistances of the AISI 409 ferritic stainless steel were evaluated after annealing at 300, 500, and 700 °C for 2, 4, and 6 h. Solution-annealed samples were also tested for comparison purposes. Two methodologies were used to assess the sensitization behavior of the 409 stainless steel samples: the first one was based on the ASTM A763 (practice W), while the second one was based on the double-loop electrochemical potentiodynamic reactivation test. It was possible to identify that the annealing treatment performed at 500 °C was more critical to the occurrence of intergranular corrosion.     

Der Einfluß des Prüpotentials und der Wärmebehandlung auf das Korrosionsverhalten niob- und titanstabilisierter ferritischer Chromstähle mit rund 17% Chrom

The influence of test potential and heat treatment on the corrosion behaviour of ferritic chromium steels stabilized with niobium or titanium By means of corrosion-chemical, electrochemical, metallographical and electronmicroscopical investigations, the influence of the electrode potential and the heat treatment on the corrosion behaviour of steels of the X 8 CrTi 17 and X 8 CrNb 17 has been determined. With ferritic 17 pC chromium steels, a distinction must be made between two different types of grain boundary corrosion:

• (1) Grain boundary corrosion on steels which were quenched at high temperatures and therefore sensitized, unstabilized or understabilized, due to the segregation of chrome-rich carbides at the grain boundaries, causing a chromium reduction in their vicinity (typical inter-crystalline corrosion), and
• (2) Grain boundary corrosion on steels quenched at high temperatures but fully stabilized, due to the chemical dissolution of the type MX carbonitrides segregated at the grain boundaries during quenching (carbide corrosion).

The heat treatment conditions conducive to carbide corrosion were determined, and the correlation of this type of corrosion with the potential was ascertained by potentio-static tests in sulphuric acid and compared with the behaviour of synthetic carbides. The different corrosion behaviour of the ferritic chromium steels quenched at high temperature and stabilized with niobium and titanium, respectively, is attributed to the different chemical dissolution rates of the carbo-nitrides segregated. The findings also provide an explanation of the corrosion behaviour of sensitization-annealed, stabilized austenitic chrome-nickel steels in acid solution.     

Influence of Ti,C and N concentration on the intergranular corrosion behaviour of AISI 316Ti and 321 stainless steels

Intergranular corrosion behaviour of 316Ti and 321 austenitic stainless steels has been evaluated in relation to the influence exerted by modification of Ti, C and N concentrations. For this evaluation, electrochemical measurements – double loop electrochemical potentiokinetic reactivation (DL-EPR) – were performed to produce time–temperature–sensitization (TTS) diagrams for tested materials. Transmission (TEM) and scanning electron microscopy (SEM) were used to determine the composition and nature of precipitates. The addition of Ti promotes better intergranular corrosion resistance in stainless steels. The precipitation of titanium carbides reduces the formation of chromium-rich carbides, which occurs at lower concentrations. Also, the reduction of carbon content to below 0.03 wt.% improves sensitization resistance more than does Ti content. The presence of Mo in AISI 316Ti stainless steel reduces chromium-rich carbide precipitation; the reason is that Mo increases the stability of titanium carbides and tends to replace chromium in the formation of carbides and intermetallic compounds, thus reducing the risks of chromium-depletion.     

Relationship Between Microstructure and Corrosion Behavior of Cr12Ni3Co12Mo4W Ultra-High-Strength Martensitic Stainless Steel

The effect of tempering temperature on the microstructure and corrosion behavior of Cr12Ni3Co12Mo4W ultra-high-strength martensitic stainless steel was investigated using transmission electron microscopy, atomic force microscopy, X-ray diffraction, and electrochemical tests. The microstructures of the ultra-high-strength martensitic stainless steel consisted of some retained austenite and lath/plant martensite with the carbides distributed within the matrix and at the grain boundaries. Tempering of the steel for 4 h at various temperatures resulted in various carbide grain sizes and different amounts of the retained austenite. The results showed that larger carbide grains led to diminished corrosion resistance, whereas larger amounts of the retained austenite resulted in improved corrosion resistance. The steels exhibited good corrosion resistance in 0.017 mol/L NaCl solution and exhibited pitting corrosion in 0.17 mol/L Na Cl solution. The martensite and prior austenite crystal boundaries dissolved in solution with pH 1.     

Elektronenmikroskopische Untersuchung der Carbidausscheidungen in korrosionsbeständigen Stählen

Electronmicroscope study of carbide precipitation in stainless steels Extraktion replicas and thin metal foils can be used for the preparation of specimes for electron microscope investigation. Extraktion replicas enable the size of precipitated carbide particles and their distribution in the metal structure to be established. With the thin metal foils the precipitation mechanism and orientation relations between precipitate and base material can be studied. The experimental techniques are explained for two applications. In this connection the relation has been confirmed between chromium carbide precipitation at grain boundary and intercristalline corrosion of austenitic CrNi steel. It has been found that it is not carbide morphology but the continuity of the carbide network which plays the predominate role. In addition, direct evidence has been obtained of the fact that the knife-line corrosion found in the vicinity of weldseams is mainly due to Ti carbides occuring in Ti-stabilised steels.     

Zur Prüfung von Schweißplattierungen mit austenitischen Bandelektroden der Art X2 CrNiNb 2413 auf Beständigkeit gegen interkristalline Korrosion

Testing of weld-claddings with austenitic band electrodes of the type X2CrNiNb24 13 for resistance against intercrystalline corrosion Two-phase, austenite- and ferrite-containing weld claddings of the 24 Cr-13 Ni-type may become susceptible to intercrystalline corrosion after heat-treatment in the 550°C temperature range due to a sensitization of the austenite. Thus, both austenite/austenite- and austenite/ferrite grain boundaries are attacked with the attack starting from the austenite phase in the latter case. Testing the resistance to intercrystalline corrosion in a sulphuric acid/copper sulphate media in the presence of copper turnings according to DIN 50914, the attack of grain boundaries ferrite/austenite becomes evident only if the specimens are bent after testing. Non-bent specimens do not show any indication of grain boundary attack by light-optical or even electron-microscopical metallographic examination. At non-bent specimens, the attack may become recognizable with considerably prolonged (several hundred hours) testing time. Referring to the investigations performed, the mechanism of intercrystalline corrosion of two-phase, ferritic-austenitic stainless steels and the influence of heat-treatment on susceptibility to intergranular attack are discussed.     

Investigation of intergranular corrosion of 316L stainless steel diffusion bonded joint by electrochemical potentiokinetic reactivation

Electrochemical potentiokinetic reactivation technique (EPR) was employed to assess degree of sensitization in 316L stainless steel diffusion bonded joint (DBJ). The result showed the degree of sensitization of DBJ was much smaller than that of base material (BM). No chromium carbides precipitated at grain boundaries in DBJ after 100 h treatment at 650 °C, while chromium carbides could be seen clearly in the BM after 8 h treatment, indicating that DBJ has better intergranular corrosion resistance than BM. Diffusion bonding technique will not increase intergranular corrosion susceptibility of 316L DBJ. Reactivation potential has the biggest effect on sensitization.     

Evaluation of sensitization in stainless steel 304 and 304L using nonlinear Rayleigh waves

Austenitic stainless steels have a wide range of applications in the energy industry, but the corrosion resistance of these stainless steels can be reduced by sensitization, particularly in the heat affected zones in welds. Sensitization is the formation of chromium carbide precipitates along the grain boundaries, causing the formation of a zone of chromium depletion around the grain boundary. Since chromium is the primary alloying element that makes stainless steel corrosion resistant, this chromium depleted zone is susceptible to intergranular stress corrosion cracking (IGSCC). Sensitization occurs when a stainless steel is exposed to a high temperature for an extended time period, such as during welding. The objective of this research is to determine the sensitivity of nonlinear ultrasound to the presence of sensitization by using nonlinear Rayleigh waves to quantitatively track the sensitization of 304 and 304L stainless steels as a function of holding time at 675 °C. The effect of the carbon content of the alloys (304 versus 304L) to the sensitization process and the measured nonlinearity parameter, β are investigated. Annealing of these specimens isolates the effect of just sensitization, removing the presence of cold work which can also affect the material nonlinearity. Complementary electrochemical potentiodynamic reactivation (EPR) measurements and microscopy are used to confirm the absence or presence of sensitization. The results show that the acoustic nonlinearity parameter is sensitive to the presence of chromium carbide precipitates in sensitized austenitic stainless steels.     

Controlling grain boundary energy to make austenitic stainless steels resistant to intergranular stress corrosion cracking

Intergranular corrosion and intergranular stress corrosion cracking are the two localized corrosion mechanisms that are of concern to the typical applications of austenitic stainless steels in industries. Until recently, the common understanding was that a higher frequency of random boundaries increases the susceptibility, caused by a sensitization heat treatment or by operating temperatures, of austenitic stainless steels to both intergranular corrosion and intergranular stress corrosion cracking. A recent study demonstrated that extreme randomization of grain boundaries leads to a considerable improvement of resistance to both sensitization and intergranular corrosion. This work is a continuation of Ref. 1 and relates the effects of grain boundary randomization to intergranular stress corrosion cracking: the results show a trend consistent with earlier observations on intergranular corrosion. It is shown that there is improvement in resistance to intergranular stress corrosion cracking with extreme randomization of grain boundaries.     

Korrosion in der Wärmeeinflußzone geschweißter chemisch beständiger Stähle und Legierungen und ihre Verhütung

Corrosion in the heat-affected zone of welds in chemically resistant steels and alloys, and respective preventive measures As to weldability, chemically resistant steels and alloys can be classified as follows: (1) weldable without any restriction; (2) weldable only with reduced wall thickness; (3) weldable only with ultimate thermal treatment. These restrictions are due to the precipitation of chromium carbide and intermetallic phases at the grain boundaries; this effect gives rise to a Cr and Mo depletion and, finally, to intercrystalline corrosion susceptibility. In view of the fact that weldability requires the delay of the precipitation of chromium carbides at 650°C for at least 1 hour, and of intermetallic phases at 900°C for at least 10 min it is possible to reduce the carbon content and/or to add stabilizing elements (Ti, Nb). It should be taken into account, however, that the precipitation behaviour is not a function of carbon concentration, but rather of carbon activity which, again, depends from the overall composition of the respective alloy. This activity is increased by Ni and Si, while Mn and N function as decelerators and, consequently, contribute to weldability. In the case of the steel X 3 CrNiMoN 17 13 5 the addition of N inhibits not only the carbide precipitation but also the precipitation of the Chi-phase (at 950°C). In the case of the alloy NiMo 16 Cr the gradual reduction of the contents of secondary constituents has resulted in a practically pure ternary system characterized by high precipitation resistance: carbides are precipitated only after 5 hours at 800°C.     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

In situ Beobachtung lokaler Korrosionserscheinungen mit Hilfe von elektrochemischen AFM‐Untersuchungen

In situ observation of local corrosion events with electrochemical AFM investigations The combination of scanning force microscopic (AFM) und electrochemical investigations allowed the in situ observation of local corrosion events. An example for such events is the intergranular corrosion (IK) of austenitic stainless steels. There is a clear relationship between the precipitation and intergranular corrosion behaviour of austenitic steels. The results of the EPR‐Test verify this relationship. An improvement of the detection of increasing intergranular corrosion susceptibility could be obtained by simple modifications of the EPR test parameters. The test conditions for the electrochemical in situ AFM investigations were found in pretests following the principle of the EPR test. In situ AFM images of an intergranular attack were made by use of a special electrochemical cell. On this way local changes on the surface of the specimen can be observed in situ, that means during the polarization.     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.     

Mechanism of Intergranular Corrosion of Austenitic Stainless Steels—Literature Review

Abstract

The literature on intergranular corrosion of austenitic stainless steels is reviewed, with particular reference to the theories of intergranular corrosion and the connexion between carbide morphology and the corrosion susceptibility. In the Strauss solution, intergranular corrosion is due to the formation of a chromium-depleted region adjacent to a continuous grain boundary carbide. In a strongly oxidising atmosphere, such as nitric acid contaminated with Cr⁶⁺ ions, the chromium-depleted layer appears to have little effect on the corrosion sensitivity and corrosion proceeds by transpassivity. The healing effect is associated with the formation of discontinuous carbides, a finding which is compatible with all the theories of intergranular corrosion. However, an investigation is to be carried out to examine more closely the relationship between the carbide morphology and intergranular corrosion susceptibility.     

Advanced Mg‐Mn‐Ca sacrificial anode materials for cathodic protection

Addition of calcium to the Mg‐Mn sacrificial anodes enhanced the anode efficiency by promoting the uniformly distributed corrosion along the grain boundaries and increased the driving potential by intrinsic electronegative potential of α‐Mg. The reason for the uniform intergranular corrosion is that Mg₂Ca precipitate (cathode) at grain boundaries is galvanically coupled to α‐Mg matrix (anode). Larger anodic area limits the localized nature of the intergranular attack.