Oxalic Acid Etch Test Research Articles

Intergranular Corrosion in a Martensitic Stainless Steel Detected by Electrochemical Tests

Abstract Quenched and tempered martensitic stainless steel UNS S41000 was tested electrochemically for susceptibility to sensitization in specimens quenched from 975°C (1,248 K) and tempered for 2 h at different temperatures between 300°C (573 K) and 700°C (973 K). Besides an oxalic acid etch test, the following tests were performed using a potentiostat and 1 N sulfuric acid (H2SO4) solution: potentiokinetic polarization, potentiostatic etch, and electrochemical potentiokinetic reactivation in the double-loop version (DL-EPR). Tested surfaces were observed metallographically. The maximum susceptibility to intergranular corrosion was observed in the condition tempered at 550°C (823 K), and a sensitized structure was detected. For lower tempering temperatures, steel was less sensitized, or not at all, and for higher tempering temperatures, steel was less sensitized. All tests except for the oxalic acid etch were able to evaluate quantitatively different degrees of sensitization as a function of tempering te...

Technical noteEffect of ceramic heat insulating paste on degree of sensitisation of AISI type 304 stainless steel

Abstract Double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests and oxalic acid etch tests were conducted to evaluate the effect of a ceramic heat insulating paste on the degree of sensitisation of AISI type 304 stainless steel. Steel strips in the as received condition, partially and totally protected with the ceramic heat insulating paste, were heat treated in a furnace at 700°C for intervals of 15, 30, and 60 min. Measurements of the ratios of the reactivation current to the anodic current indicated that the ceramic heat insulating paste effectively prevented sensitisation of the steel for the first 15 min at 700°C and reduced the degree of sensitisation for specimens heat treated for 30 and 60 min. Microstructural examination of specimens after DL-EPR tests and oxalic acid etch tests provided additional evidence for the effectiveness of the ceramic paste. The decreased degree of protection at 30 and 60 min was due to dehydration and subsequent cracking of the protective paste layer on...

Technical noteComparative study of intergranular corrosion behaviour of three high manganese austenitic stainless steels

The intergranular corrosion behaviour of three commercially available austenitic stainless steels of high Mn content have been investigated using the oxalic acid etch test and the double loop electrochemical potentiokinetic reactivation technique. Time-temperaturesensitisation diagrams indicate that replacement of Ni by Mn promotes a significant resistance to intergranular corrosion along with the presence of nitrogen in the alloy.

Some aspects of electron beam welded and laser welded ferritic Fe-22Cr-3Mo-5Ni-0.3Ru alloy

Ferritic Fe-22Cr-3Mo-5Ni-0.3Ru alloy was welded by electron beam (EB) and laser techniques. Tensile tests, impact tests, oxalic acid etch, electrochemical potentiokinetic reactivation (EPR) and potential-time measurements were employed to investigate the mechanical properties, microstructural features and corrosion behaviour of the EB and laser welded metals. The oxalic acid etch tests revealed that the microstructure of the EB and laser welded metals consisted of columnar grains and cells, respectively. These structures were formed directly from the substrate by epitaxial solidification. Furthermore, the oxalic acid tests also showed that the EB and laser welded metals were immune to intergranular corrosion. Similar results were obtained by double loop EPR tests.

Intergranular corrosion resistance of microstructure- modified type 316 austenitic stainless steel

Cold working and a double aging treatment was used to produce a microstructure with fine nuclei of carbides distributed throughout the grains to improve the intergranular corrosion (IGC) resistance of austenitic stainless steel. The treatment was carried out on type 316 stainless steel as follows: cold working (20,30, and 40% reductions in thickness), sensitization (923 KJ5 h), and aging each for 1173,1223,1273, and 1323 K/l h, respectively. Specimens in the solution annealed condition (0% cold work) were also given the above treatment. All of the specimens were resensitized at 923 KJ5 h and tested for IGC resistance as per ASTM A262, Practice A(oxalic acid etch test) and Practice E (24 h immersion in boiling Cu-CuSO.4-H2SO4 and the U- bend test). Microhardness measurements were also carried out on all specimens. The results indicated that at an optimum treatment (30% cold work + sensitization + aging) all the specimens showed improved IGC resistance. The 0 and 20% cold worked specimens showed improvement at higher aging temperatures only. Specimens undergoing 40% cold work exhibited a decrease in IGC resistance. Compared to as-cold-worked specimens, an improvement in IGC resistance was obtained with 30% cold working.

塑性加工を受けたオーステナイト系ステンレス鋼溶接継手部の低温鋭敏度の予測

A nondestructive method has been investigated to predict the time when the degree of sensitization of a welded austenitic stainless steel joint having a plastic strain exceeds a critical degree of sensitization at a low temperature.The possibility of determining the plastic strain ratio of the welded joint from its hardness, the relationship between the time and the temperature of the accelerated tests needed for the degree of sensitization of the welded joints to exeed the critical degree of sensitization, and the sensitizations of the welded joints and the laboratory test specimens of the same intergranuler etch ratio and hardness as those of the welded joint were studied.The material used in this experimant was type 304 stainless steel plates having a 10 or 30% plastic strain ratio. To measure the degree of sensitization, the EPR test was used. The 10% oxalic acid etch test was used to determine the intergranuler etch ratio.From these experimental tests, the following conclusions were obtained. It is possible to determine the plastic strain ratio from the hardness. The relationship between the time and the temperature of the accelerated test needed for the degree of sensitization of the welded joint having a plastic strain to exceed a critical value is linear on the Arenius plots. It is possible to predict the time needed for the degree of sensitization of the welded joint to exceed a critical value by using the laboratory test specimen.

塑性加工を受けていないオーステナイト系ステンレス鋼の溶接部の低温鋭敏化度の予測方法

The nondestructive method to predict the time when the austenitic stainless steel welded joint exceeds a critical degree of sensitization at a low temperature is investigated by the mathematical model and experimental tests.The material used in this experiment is type 304 stainless steel without cold working. To measure the degree of sensitization, the electrochemical potentiokinetic reactivation (EPR) test was used. To determine the intergranular etch ratio, the 10% oxalic acid etch test was employed.From the mathematical analysis, the following results were obtained. The time when welded joint exceeds a critical degree of sensitization is predicted from the results of the accelerated test with the test pieces having the intergranular etch ratio similar to that of the welded joint at a temperature below 873 K. In the case where the intergranular etch ratio of welded joint is less than 0.7, this method is useful.From these experimental tests, we were able to predict the time using the method based on the mathematical analysis.

Effect of nitrogen contamination on intergranular corrosion of stabilized ferritic stainless steels

AbstractThree different alloys of AISI Type 444 (18Cr–2Mo) ferritic stainless steel, stabilized to different levels with titanium and/or niobium, were melted as welds or weld simulations with deliberate additions of nitrogen to the argon shielding gas in order to simulate the effects of accidental contamination by nitrogen during production welding. The oxalic acid etch test (ASTM A262–A) was used to assess the susceptibility of the melts prepared in this way to intergranular attack, and the extent of attack was quantified. It is shown that a clear correlation exists between the extent of intergranular corrosion and the amount of nitrogen taken up by the steel on melting. The implications of sensitization by nitrogen contamination both for steel design and integrity of production welds are discussed.MST/415

Four Nondestructive Electrochemical Tests for Detecting Sensitization in Type 304 and 304L Stainless Steels

Three different electrochemical reactivation tests are compared with etch structures produced in the electrolytic oxalic acid etch test. These nondestructive tests are needed to evaluate welded sta...

Potentiodynamic Reactivation Method for Detecting Sensitization in AISI 304 and 304L Stainless Steels

Abstract Electrochemical methods have been proposed as a nondestructive, quantitative means for determining the degree of sensitization associated with the chromium-depleted zones surrounding precipitates of chromium carbides in stainless steels. The use of one such method, the electrochemical potentiokinetic reactivation (EPR) test, has been investigated on AISI 304 and 304L stainless steels. Its reproducibility, the effects of scan rate, solution temperature, surface finish, and nonmetallic inclusions were examined. Measurements made with the EPR method were compared with those obtained with the oxalic acid etch test, the ferric sulfate - 50% sulfuric acid test, and the copper sulfate -16% sulfuric acid test of ASTM A 262. Both the EPR and the oxalic acid etch test are nondestructive and can therefore be applied to equipment in the field. It was found that the reproducibility of the EPR test is good and that comparable results can be obtained with a variety of instruments. Results obtained from a round robin ASTM test program are included. The EPR test provides a sensitive, nondestructive, quantitative measure of relatively small degrees of sensitization to intergranular attack. However, when at least some of the grains are completely surrounded by carbide precipitates, the EPR test is less sensitive (it “saturates”) in differentiating between a range of degrees of sensitization than are the ferric sulfate or copper sulfate tests. Even though it is not a quantitative test, the oxalic acid etch test can also be used as a nondestructive method for identifying material which is free of carbide precipitates, i.e., completely free of sensitization.

Etching Method for Determining Susceptibility to Intergranular Attack in Ferritic Stainless Steels

Abstract An etching test has been developed for determining the susceptibility of stainless steels to intergranular corrosion after either heat treatment or welding. For Types 430, 430 Ti, 430 Nb, 434, 444, and 446 ferritic stainless steels, the test consists of etching a polished sample for five minutes in a boiling 60% (by volume) sulfuric acid plus 120 g/liter copper sulfate solution. The same test method can also be applied to Types 304 and 316 austenitic stainless steels. However, for Type 316 stainless steel, a ten minute etch in a boiling 50% (by volume) sulfuric acid plus 120 g/liter copper sulfate solution instead of the above test condition should be utilized. This test is to be used in the same manner as the oxalic acid etch test (ASTM A262-A) for stainless steels, but gives superior results for ferritic alloys and requires less equipment.

Sensitization of Austenitic Stainless Steels, I. Controlled Purity Alloys

Abstract This paper presents a study of the effects of carbon, nitrogen, molybdenum, and manganese on the sensitization of high-purity austenitic stainless steels of composition similar to Type 304. The modified Strauss test (ASTM-A262-E), the oxalic acid etch test (ASTM-A262A) and analytical electron microscopy were used to determine the degree and nature of sensitization in the steels. The alloy compositions are considered in terms of effective chromium content, and from plots of this parameter versus sensitization time a strong effect of carbon content is seen. Additions of nitrogen, molybdenum, and manganese are found to delay sensitization at any given carbon concentration. In the case of nitrogen, the amount of improvement depends on both the carbon content and the sensitization temperature. Strong evidence is presented that nitrogen acts to retard the nucleation and/or growth of carbides at grain boundaries and hence increase the time necessary for sensitization. Molybdenum appears to increase the ease with which the steel passivates; thus more chromium depletion is required before sensitization will be detected. The combination of molybdenum plus nitrogen is found to be particularly effective in retarding nucleation and/or growth of carbides; molybdenum alone does not have this effect. Only limited experimental evidence is presented concerning the role of manganese. Beneficial effects of this element are primarily seen at low sensitizing temperatures, where manganese assists nitrogen in slowing carbide formation.

Development of an Etching Technique for Determining the Intergranular Corrosion Resistance of Type 430 Stainless Steel

Abstract An etching technique has been developed for determining susceptibility of Type 430 stainless steels to intergranular corrosion after either heat treatment or welding. The test consists of a five minute chemical etching in boiling 60% (by volume) sulfuric acid plus 120 g/liter of copper sulfate (CuSO4·5H2O). Unlike austenitic stainless steels, the microstructures of most nonsensitized ferritic stainless steels contain chromium carbide precipitates. Hence, the use of the oxalic acid etch test (the ASTM Standard A262-A) for the intergranular corrosion of ferritic stainless steels is questionable, since the chromium carbide itself in nonsensitized samples may be attacked by the oxalic acid. The test described in this paper can be used as a substitute for the oxalic acid etch test for Type 430 stainless steel.

Effect of Heat Treatment, Composition and Microstructure on Corrosion of 18Cr-8Ni-Ti Stainless Steels in Acids

Abstract Titanium additions to 18Cr-8Ni steels can effectively eliminate susceptibility to intergranular attack associated with the precipitation of chromium carbides. However, these additions also drastically reduce the grain size of these steels and promote the formation of sigma phase. Both of these factors modify the behavior of titanium-stabilized steels in corrosive environments. A detailed investigation of the influence of heat treatment and microstructure on the corrosion resistance of four titanium-stabilized, AISI 321, heats is described. Incomplete stabilization of carbon by titanium leads to precipitation of chromium carbides on heating in the range of 800 F to 1600 F. This type of susceptibility is readily detected in the oxalic acid etch test, the nitric-hydrofluoric acid test, the copper sulfate-sulfuric acid test, the ferric sulfate-sulfuric acid test, and in the nitric acid test. A second type of susceptibility to intergranular attack is associated with the formation of sigma phase, which may precipitate in a sub-microscopic form during heating in the range of 1150 to 1550 F. It is detected only in the nitric acid test and, to a lesser extent, in the ferric sulfate-sulfuric acid test. The action of various acid solutions on titanium-stabilized stainless steels are compared by means of metallographic studies of microstructures and of progressive corrosion, and by measurements of weight loss and change in electrical resistance of corroding specimens. The data obtained are used to explain the action of acids on these steels and to recommend procedures for evaluation testing.