Pitting,Crevice and stress corrosion resistance of high chromium und molybdenum alloy stainless steels

This paper presents new data on the resistance of recently developed high-alloy stainless steels to localised corrosion in chloride solutions. Pitting potential was determined in artificial sea water, and critical pitting temperature CPT in very aggressive FeCl₃ solution. Critical crevice corrosion temperature CCT was tested in the same FeCl₃ solution. Stress corrosion measurements, made in a more familiar NaCl solution by the drop evaporation method, demonstrate that alloy stainless steels with high chromium and molybdenum have very long failure times, comparable with those of nickel alloys found to be SCC-resistant under practical conditions. Stainless steels of 20 Cr 25 Ni 6 Mo type showed the best resistance to localised corrosion.     

Kühlwasserseitige Korrosionsbeständigkeit von metallischen Werkstoffen zur Handhabung von Schwefelsäure

Cooling water side corrosion resistance of high alloyed materials for handling of process side sulfuric acid The approved materials for use in sulfuric acid alloy 825 (German material No. 2.4858) and alloy 20 (German mater. No. 2.4660) have only a low resistance against localized corrosion in chloride containing water and are unsuitable for handling of sulfuric acid. The newly developed austenitic Cr-base alloy, alloy 33, (X1CrNiMoCuN 33-32-1, German mater. No. 1.4591) with 33 % Cr, 31 % Ni, 0,6 % Mo and 0.4 % N should have an excellent resistance against pitting and crevice corrosion additional to its high sulfuric acid resistance, too, because its Pitting Resistance Equivalent No. calculated according to PREN = %Cr + 3,3 · %Mo + 30%N runs to 50. Pitting and crevice corrosion properties of the alloy 33 are tested in comparison to those of reference materials in high chloride containing solutions (1M NaCl, artificial and modified sea water, 10% FeCl₃ · 6H₂O; 500 g/l CaCl₂ ). Pitting potentials and potentials of repassivation of pitting, critical temperatures of localized corrosion (FeCl₃-test, CaCl₂-test, artificial sea water), potentials of repassivation of crevice corrosion as well as depassivation pH values of crevice corrosion following Crolet have been determined. The results confirm that the localized corrosion behaviour of the alloy 33 corresponds to its PREN. With regard to pitting corrosion alloy 33 is comparable with the special stainless steel alloy 31 (mater. No. 1.4562), with regard to crevice corrosion it is comparable with alloy 926 (German mater. No. 1.4529).     

Pitting and crevice corrosion behaviour of high alloy stainless steels in chloride‐fluoride solutions

The localised corrosion resistance (pitting and crevice corrosion) of two high alloy stainless steels, namely superduplex (SD) and superaustenitic (SA), has been studied in chloride‐fluoride solutions at pH values ranging from 2 to 6.5. The pitting potential (E_pit) and crevice potential (E_cre) have been calculated for these test media using electrochemical techniques (continuous current). The Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) are in both materials lower then the room temperature. In spite of this fact and due to the high repassivation rate, the resistance of these materials to localised corrosion is high in the tested media. At the highest tested concentration of aggressive anions and pH 6.5 both materials undergo a generalised attack.     

Einfluß von Chrom,Molybdän und Stickstoff auf die Korrosionsbeständigkeit des Ni-freien,austenitischen Edelstahles Macrofer 2515MoN (W.-Nr.: 1.4653)

Influence of chromium, molybdenum and nitrogen on the corrosion resistance of the Ni-free, austenitic stainless steel Macrofer 2515MoN (German Alloy No. 1.4653) Nitrogen alloyed, Ni-free, austenitic stainless steels comprising of more than 1 wt.-% nitrogen are a new group of alloys with promising properties. They show a very interesting combination of high strength and toughness with a high corrosion resistance. This combination of properties make the alloys not only suitable for fasteners but also for parts for medical and dental applications. This work shows the influence of chromium, molybdenum and nitrogen on the corrosion resistance of Fe25Mn-alloys in media typical for the above mentioned applications. According to these results Fe25Mn-alloys with appr. 20 wt.-% chromium, about 3 wt.-% molybdenum and appr. 1,3 wt.-% nitrogen have an excellent corrosion resistance in Ringer solution, artificial saliva and artificial sweat. The critical pitting temperature (CPT) as well as the critical crevice temperature (CCT) with 61°C respectively 37°C tested according ASTM G 48A provided significantly higher temperatures when compared to the commercially well established Ni-austenite X6CrNiMoTi17-12-2 (German Alloy No. 1.4571).     

Elektrochemische Methode zur Prüfung des Korrosionsverhaltens austenitischer CrNiMo-Stähle im Schweißnahtbereich

Electrochemical method for testing of the corrosion behaviour of austenitic CrNiMo-stainless steel weldments Austenitic CrNiMo stainless steel welds can be more susceptible to pitting and crevice corrosion than the base material owing to segregation and precipitation in the heat affected zone, in the high temperature zone and in the welded zone. Suitable test methods are needed to optimize welding technology. Comparison of potential curves (DIN 50919), comparison of critical pitting corrosion temperatures in FeCl₃- or other model solutions, visual estimation after longterm corrosion tests with or without electrochemical load are discussed. A small measurement cell heated from the rear with circulating and temperature controlled electrolyte is shown. With its help separate areas of a weld can be electrochemically investigated. Welds of the materials X 2 CrNiMoN 17 12 2 (1.4406), X 2 CrNiMoN 17 13 5 (1.4439) and X 1 NiCrMoCu 31 27 4 (1.4563) are tested in acidic NaCl solutions in the range of 25 to 75°C. Critical pitting corrosion potentials are obtained for base materials, heat affected zone and weld material. An influence of the welding energy is probable, but cannot be proved in this case without statistical certainty.     

Influence of the surface state on the Initiation of Crevice Corrosion on stainless steels

Results from sea-water and laboratory exposure tests with multiple crevice assemblies are presented. The results from tests on austenitic and ferritic-austenitic stainless steel samples subjected to various surface treatments demonstrate that an acid treatment greatly improves the crevice corrosion resistance of previously ground surfaces. Pickling in dilute sulphuric acid and passivation in nitric acid has thereby virtually the same beneficial effect as the common nitric + hydrofluoric pickling acid. The beneficial effect of pickling and passivation is related to the removal from the surface of sulphide inclusions, being potential nucleation sites for the initiation of discrete pitting attacks preceding the onset of crevice corrosion. Potentiodynamically determined pitting potentials and results from testing in 10% FeCl₃ · 6 H₂O according to ASTM G 48–76 show poor correlation with results from sea-water exposures.     

Neue Erkenntnisse auf dem Gebiet der ferritischen rostfreien Stähle

Recent developments in ferritic stainless steels The pitting resistance of ferritic stainless steels in HCl is visibly improved by Mo, in particular in the case of vacuum-melted material. In this context the ratio Cr:Mo = 25:2 is superior ta Cr:Mo = 17:3; addition of Mo prevents, beyond that, crevice corrosion. Ti increases resistance in the Strauß test but not in the Huey test, while Nb turns out to have a positive effect in either test. Steels containing Cr: Mo = 17:l are certainly still susceptible to pitting, but no longer to stress corrosion cracking in boiling MgCl₂, solution; stress corrosion cracking is not observed in 55% boiling Ca(NO₃)₂, and 25% boiling NaOH, but after annealing at 980 °C intercrystalline corrosion takes place. The test duration required for establishing cracking susceptibility is considerably shorter with ferritic than with austenitic steels (100 and 1000 to 2000 hours respectively).     

Das Ausscheidungsverhalten von hochlegierten austenitischen Stählen mit 6% Molybdän und sein Einfluß auf die Korrosionsbeständigkeit

Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance The high-alloy austenitic steels with 6 to 7% Mo, 20 to 21% Cr and 18 to 25% Ni are increasingly used in seawater and chemical applications. This is due to the excellent resistance to pitting and crevice corrosion in chloride-containing neutral and acidic environments. It is the high chromium and molybdenum content which provides the excellent corrosion behaviour but, at the same time favors the tendency to precipitation of intermetallic phases. Therefore, time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N. The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl₃ solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 1000°C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 950°C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. The precipitates are interpreted to be chi-phase. After very long annealing times additionally small amounts of Laves phase appear. Neither carbides nor nitrides were observed. In spite of the rapid precipitation, sensitization in terms of the 50 m?m grain boundary penetration criterion is observed not before 0.7 h at 850 °C and not before about 2 h at 800°C in case of the 25% Ni/0.19% N steel. After about the same times of annealing also the critical pitting temperature as observed in the FeCl₃-test is dropping below 50°C. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. If high heat inputs will occur during manufacturing because of hot forming operations or welding of heavy sections, or if more severe test conditions are a requirement, a steel with 25% Ni and about 0.2% N (UNS N 08925, Cronifer hMo) is recommended due to its retarded precipitation and sensitization behaviour when compared to steels with only 18% Ni (UNS S 31 254). Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material.     

Pitting and crevice corrosion of superaustenitic stainless steels

Superaustenites are mainly used in offshore applications, oil production and chemical industry. Most important types of localised corrosion of these steels are pitting and crevice corrosion. Investigated materials were N08028, S31254 and three modified alloys. Chromium content of investigated alloys varied between 20 and 27%, molybdenum between 3.2 and 6.0%, nitrogen between 0.1 and 0.36% and copper between 0 and 1.1%. For means of comparison stainless steel AISI 316L has been included in the study. Pitting and crevice corrosion of these highly corrosion resistant steels has been investigated by use of standardized tests. Critical pitting temperature and critical crevice temperatures were determined according to ASTM G 48, Methods C and D, respectively. Electrochemical measurements for determination of pitting potentials were done according to ASTM G 61 as well as for determination of critical pitting temperatures according to ASTM G 150. Results are presented as function of MARC (Measure of alloying for resistance to corrosion) defined by Speidel since linear correlation coefficients were higher when compared to conventional PREN. Results obtained by different testing methods must not be compared directly. Every test however is sensitive to microstructural defects like precipitations and segregations that decrease corrosion resistance. The higher alloyed a material is, the higher is its tendency to form microstructural defects, and the more difficult is it to reach its theoretical corrosion resistance at given chemical composition.     

Influence of cooling rate on microstructure evolution and pitting corrosion resistance in the simulated heat-affected zone of 2304 duplex stainless steels

Pitting corrosion resistance of 2304 duplex stainless steel heat-affected zone with different cooling rates has been studied by potentiostatic critical pitting temperature (CPT) in 1.0 M NaCl. The results showed that, as cooling rate decreased from 100 to 10 °C/s in the temperature range of 1350–800 °C, the austenite fraction increased from 27.8% to 35.7%, and the CPT value increased from 14 to 19 °C. The morphologies after the CPT tests showed pitting occurred preferentially in the ferrite phase for all specimens. Moreover, relationship between pitting corrosion resistance and microstructure evolution was further discussed.     

Zur Korrosionsprüfung von Schweißverbindungen hochlegierter CrNiMo-Stähle und NiCrMo-Legierungen

On the corrosion testing of weldments of high alloyed CrNiMo-stainless steels and NiCrMo-alloys Weluments of high-alloyed CrNiMo stainless steels and Nicro alloys can he more susceptible to localized corrosion than the solution annealed basic material owing to segregations and precipitations in the heat affected zone, the high temperature zone and/or in the weld. To investigate these differences the FeCl₃-test (10% FeCl₃ · 6aq), the test “green death” (11.5% H₂SO₄, 1.2% HCl, 1% CuCl₂, 1% FeCl₃) as well as chronopotentiostatic tests in artificial sea water or in 3% NaCl-solution are used. In particular for testing the highest alloyed materials a CaCl₂ test was developed (4.5M CaCl₂, chronopotentiostatic test in duration of 8 to 10 hours at + 200 mV (SCE)), which can be carried out to a temperature of 115°C at atmospheric pressure. The aggressivity increases in the range FeCl₃-test, “green death”-test, CaCl₂-test. Matching and graduated over-alloyed weldments (TIG, heat input of 7 and 15.5 kJ/cm) of materials 1.4529, 1.4562, 2.4856, 2.4819 (german materials No.) are comparingly examined in various tests, of materials 1.4406, 1.4539, 1.4439 and 1.4563 (german materials No.) only matching weldments in the FeCl₃-test. In strongly oxidizing media only a highly over-alloyed performed weldment (filler material 2.4607, german material No.) produces the best corrosion behaviour, measured as the critical temperatures of localized corrosion. Measurements of critical current densities of passivation can be used for investigations of corrosion behaviour of weldments, too. Critical current densities of passivation are showing a tendency to inverse proportion to the critical temperatures of localized corrosion. Suitable electrolytes are among others 0.2M H₂SO₄ + 1M NaCl + 10^?3% KSCN, N₂-bubbled, 25 to 60°C and xM H₂SO₄ + 4M NaCl + 10^?3% KSCN (x = 0.05 to 1), 25°C, in contact with air. An influence of heat input at the welding is indicated in the test of localized corrosion, but it is only small. It is sometimes more clearly shown at measurements of passivation.     

Loch- und Spaltkorrosion von Chrom- und Chromnickelstählen in chloridhaltigen Lösungen

Pitting and crevice corrosion of stainless steels in chloride solutions In practice stainless steels in chloride containing waters are found to be susceptible to crevice corrosion and pitting. Corrosion tests were carried out on AISI 304 L stainless using a simulated crevice and the compositions of the electrolyte in the crevice determined. Long term potentiostatic tests were used to determine the critical potentials for crevice corrosion (U_S), for various steels in sodium chloride solutions at different concentrations and temperatures. The steels studied were 22 CrMo V 121, X 22 CrNi 17 and AISI 304 L. Like the critical pitting potential (U_L), U_S was found to have a strong dependence on the chloride content of the external solution. At higher concentrations the two potentials were similar. At lower concentrations the U_S was lower than U_L. The knowledge of these critical potentials together with well known rest potentials for a steel in an electrolyte of known concentration, allows conclusions to be drawn about its susceptibility to pitting and crevice corrosion. The method is suitable also for other passive metals.     

Corrosion resistance and mechanical properties of nickel-bearing ferritic stainless steels

Additions of nickel to ferritic steels containing 25–28% Cr and 2–4% Mo increased the impact toguhness especially when more than 2% Ni was present. The effect of nickel content increased up to 4% Ni, the largest addition studied. Steels stabilized with niobium had lower transition temperatures then did corresponding steels stablizied with titanium. Steels containing 4% Ni required annealing at 1050 C to avoid intermetalic compounds. It was also noted that nickel reduced the upper shelf energy in the Charpy impact test and eliminated a sharp transition from ductile to brittle behaviour. No definite effect of nickel on pitting potential was pound but steels in the series 25Cr-3.5 Mo-Ni-Ti consistenly had more noble pitting potentials and greater resistance to crevice corrosion than the 28 Cr-2Mo-Ni-Ti steels. Nickel contents of 1 or 2% tended to improve crevice corrosion resistance while larger nickel contents were somewhat ditrimental. Nickel strongly reduced critical current densities for passivity both in l N H₂SO₄ and in l N HCL and yielded corresponding increases in resistance to corrosion by these acids. Although 1% Ni or more caused the annealed steels to be susceptible to stress corrosion cracking in MgCl₂ boiling at 140 C, while the as-Welded steels containing 4% Ni did not crack in boiling 25% Nacl at pH 1.     

氮元素对25-6Mo3型双相不锈钢耐孔蚀及缝隙腐蚀性能影响的研究

从孔蚀与缝隙腐蚀两方面考察氮对25-6Mo3型双相不锈钢耐蚀性能的影响。结果表明:随含氮量增加,材料的孔蚀阻力增大,缝隙腐蚀速率下降,且存在关系式:CCT(℃)=17.3+78.4N％(wt)。含氮量越高,氮在钝化膜中的富集程度越大,对于含0.28％N的钢,其最高富集系数可达14;随含氮量增高,孔蚀位置从Y相转移到α相,含0.096％N是孔蚀位置发生转移的分界点。模拟闭塞区试验表明,含氮量越高,闭塞溶液的最终pH值越大,用Nessler特性试剂检验,发现0.28％N钢的闭塞液中存在着NH,从而证实了双相钢中同样存在着氮元素的缓蚀效应。可见氮元素的作用是通过膜内富集,改善γ相耐蚀性能以及与H~+结合形成NH_4~+从而抑制闭塞液酸度变化达到缓蚀效应而得以实现的。     

Pitting and Intergranular Corrosion Resistance of AISI Type 301LN Stainless Steels

The pitting and intergranular corrosion (IGC) resistance of AISI type 301LN stainless steels were evaluated using ASTM methods, anodic polarization, and electrochemical impedance techniques. The IGC results indicated that the microstructure of the samples after sensitization heat treatment at 675 °C for 1 h shows step or dual structure for both imported and indigenous materials indicating insignificant Cr₂₃C₆ precipitation. The results of immersion tests in boiling 6% copper sulfate + 16% sulfuric acid + copper solution for 24 h followed by the bend test (ASTM A262 Practice-E method) indicated no crack formation in any of the tested specimens. Pitting corrosion resistance carried out in 6% FeCl₃ solution at different temperatures of 22 ± 2 and 50 ± 2 °C (ASTM G 48) up to the period of 72 h revealed pitting corrosion attack in all the investigated alloys. The potentiodynamic anodic polarization results in 0.5 M NaCl revealed variation in passive current density and pitting potential depending on the alloy chemistry and metallurgical condition. The passive film properties studied by electrochemical impedance spectroscopy (EIS) correlated well with the polarization results. The x-ray diffraction (XRD) results revealed the presence of austenite (γ) and martensite (α′) phases depending on the material condition. The suitability of three indigenously developed AISI type 301LN stainless steels were compared with imported type 301LN stainless steel and the results are highlighted in this article.     

Pitting corrosion behaviour of austenitic stainless steels with Cu and Sn additions

The influence of Cu and Sn on the pitting corrosion resistance of AISI 304 and 316 stainless steels in chloride-containing media has been investigated. The corrosion behaviour was evaluated by cyclic polarization, potentiostatic CPT measurements and electrochemical impedance spectroscopy in 3.5 wt% NaCl. The corrosion resistance was also studied in FeCl₃ under Standard ASTM G-48. According to the results, Cu addition favours pit nucleation but inhibits its growth, whereas Sn exerts the opposite effect, favouring pit growth and inhibiting its nucleation. Studies by SEM, X-ray mapping and EDS analysis showed Cu-, Cl- and O-rich corrosion products that reduce the extent of corrosion damage.     

Pitting corrosion behavior of superferritic stainless steel in waters containing chloride

On a cost-performance basis, superferritic stainless steel (SFSS) grades can now be considered competitive choices for brackish and sea water cooling systems. The pitting corrosion of a SFSS (X2CrNiMoTi 25 3.6 3.5) was tested in a chloride/sulfate solution similar to sea water, as a function of temperature, pH, and salinity. Potentiodynamic polarization techniques and experimental stability diagrams were used to evaluate the pitting conditions. The critical pitting temperature (CPT) was found to be above 50°C, and the re-passivation tendency under critical temperatures was satisfactory even when the pH was less than the depassivation pH, a condition which simulates a propagating occluded cell. The SFSS behavior in the presence of chlorine was evaluated by means of potentiodynamic tests and by studying the cathodic reduction kinetics of Cl₂ and O₂ (air).     

The nature of crevice corrosion of aluminum in chloride solutions

To study crevice corrosion of pure aluminum, polished specimens partly covered with a glass foil were polarized potentiostatically in 1 N NaCl-solution at potentials negative to the critical potential for stable pitting (pitting potential). For comparison, non-crevice experiments were performed on polycrystalline and singlecrystalline material in neutral as well as acidified 1 N NaCl-solution and in AlCl₃-solutions. Corrosion morphology was examined by scanning electron microscopy. In current-time plots recorded during experiments on crevice corrosion, both an incubation and a propagation stage are discernible. If experiments were interrupted during the induction period, micropits were found inside the crevice. This unstable micropitting is detectable down to 0.30 V below the pitting potential. In contrast, during crevice corrosion propagation, the aluminum surface undergoes general attack. In a range of 0.2 V below the pitting potential, dimpled surfaces are produced. At more negative potentials, metal dissolution occurs crystallographically oriented. An identical behaviour was detected on unshielded samples polarized in the same potential range in both 1 N AlCl₃- and acidified 1 N NaCl-solution. Hence, the build-up of an acidic electrolyte is considered the sufficient requirement for crevice corrosion initiation.     

Optical spectroscopic and electrochemical characterization of oxide films on a ferritic stainless steel

Colored oxide films that form on ferritic stainless steel in a high-temperature, oxidizing environment and correspond to different chemical compositions can cause a deterioration of pitting resistance and corrosion performance. Herein, optical spectroscopic and electrochemical techniques have been used to reveal the relationship between color, chemical composition, and corrosion resistance of oxide films formed in the temperature range from 400°C to 800°C for 30 min and at 800°C for 10, 20, 30, and 60 min. The substrate with a thin and dense passivation film leads to a low pitting potential but high corrosion resistance. Oxide films of yellowish or brownish color formed below 600°C are mainly iron oxides, which correspond to low corrosion resistance. No passivation characteristics can be observed for polarization curves of oxide films formed at 500°C and 600°C. The color of oxide films varies from blue to dark gray with the increase of oxidation time at 800°C. Corrosion resistance changes with different proportions of Fe₃O₄, Cr₂O₃, and FeCr₂O₄. The gray oxide films formed at 800°C for 30 min exhibit the lowest pitting susceptibility and the highest corrosion resistance.     

Interpretation of electrochemically and chemically investigated corrosion behaviors of 316L SS in acid chloride environments

The electrochemical behavior of 316L stainless steel was investigated in acid chloride environments, and pitting potentials were determined electrochemically and chemically. An increase in the anodic maximum current density was observed upon decreasing the cathodic potential from which the scan was initiated to determine the polarization curve. To determine the critical pitting potential through the chemical method, the potential was increased by increasing the concentration of ferric ions in ferric chloride while holding the chloride ion concentration constant with sodium chloride. When 316L stainless steel was immersed in 15 g/1 of FeCl₃6H₂O containing the same chloride ion concentration as 5% NaCl with pH=2 at 57°C, the corrosion potential increased to 0.47 V (SHE) within two minutes due to initial passivation. Immediately after reaching 0.47 V (SHE), which was just above the pitting potential of 0.45 V (SHE) determined electrochemically in 5% NaCl (pH=2, 57°C), the corrosion potential continuously decreased, indicating the onset and propagation of pitting corrosion. A correlation between the electrochemical and chemical methods can be verified if the proper measurements are made and the observations are properly interpreted.