Stainless steel thread forming screws and their susceptibility to stress corrosion cracking

Abstract

Thread forming fasteners incorporate both drilling and hole tapping features and are commonly used in the construction industry to fix together steel sheets of different material types. With this practical application in mind, fasteners manufactured from martensitic and austenitic stainless steels have been subjected to alternating corrosion conditions in accordance with test standards DIN 50021-SS and DIN 50018-K WF 2·0. The torque applied to the screws during these tests was controlled to place the fasteners under equal tensile loads, independent of their tensile strength. Thus, the results provided information on their relative susceptibilities to stress corrosion cracking. At the end of the tests, up to 80% of the martensitic stainless steel drilling and tapping screws had failed due to hydrogen induced stress corrosion cracking. The fasteners manufactured from austenitic materials withstood identical test conditions without any evidence of cracking or crack initiation. It is concluded that fasteners manufactured from modified martensitic stainless steel are more susceptible to stress corrosion cracking under the conditions of test than those made from cold worked austenitic stainless steels. This suggests that in practical applications the potential for catastrophic failure due to stress corrosion cracking could be considerably higher in modified martensitic fasteners in comparison with austenitic stainless steel fasteners, including those with hardened carbon steel drill points.     

Untersuchungen zur Spannungsrißkorrosionsbeständigkeit nichtrostender Stähle mit unterschiedlichem Verformungsgrad

Investigations on stress corrosion cracking resistivity of stainless steels with different degree of cold straining Within the context of the elaboration of a new approval for stainless steels for civil engineering applications the question arised whether cold straining could adversely affect the resistance against stress corrosion cracking. By means of experimental tests with different stainless steels it should be clarified whether cold straining increase the stress corrosion cracking susceptibility under high chloride loads like e.g. in swimming hall atmospheres. The investigations were carried out using the high-alloyed steels 1.4462 and 1.4529. As reference material with known susceptibility against stress corrosion cracking the steel 1.4301 was used. The tests were performed using U-bent and tension specimens with saturated MgCl₂ salt spots. Indications for an increase in stress corrosion cracking susceptibility of the cold drawn steel were not found.     

Potentiodynamische und potentiostatische Messungen an gespannten Drahtelektroden in gesättigten Ca(OH)2-Lösungen mit verschiedenen Zusätzen

Potentio-dynamic and potentio-static measurements on tensioned wire electrodes in saturated Ca(OH), solutions with different admixtures The influence of mechanical tensile stresses on the corrosion behaviour of unalloyed steel wires has been examined in saturated calcium hydroxide solutions with and without additions of chloride or chromate ions to simulate the practical conditions encountered with steel reinforcement in concrete. In order to account for the influence of oxygen dissolved in the corrosion medium, the tests were carried out in an air-saturated solution as well as in a solution free from oxygen. The tests revealed a considerable increase in the dissolution rate of iron in the active potential range as well as an increase in the passive current density with increasing tensile stress. Also with increasing tensile stress the stress corrosion cracking potential is shifted towards the negative side. The test results indicate that the thickness of the passive film increases with the tensile stress. All these effects are even more marked in air-saturated solution.     

Spannungsrißkorrosion an Spannstählen

Stress corrosion cracking of prestressing steels During the investigation of a post-tensioned bridge structure incipient cracks of the prestressing steels of the transverse prestressed members were observed. Defects related to non-injected ducts or the presence of corrosion inducing substances could not be detected. The prestressing steel used is a quenched and tempered steel, strength class St 140/160, which was produced in the former GDR. The cause for the cracks is the susceptibility of this type of steel to hydrogen-induced stress corrosion cracking as could be shown in laboratory tests. Under unfavourable conditions cracks can be initiated before grouting. Additional magnetic particle tests at selected areas of the longitudinal prestressed members did not indicate any signs for incipient cracks.     

Assessment of test methods for evaluation stress corrosion cracking susceptibility of prestressing steels

Prestressing steels can be exposed in prestressed concrete structures in construction stage (ungrouted duct) to environmental conditions causing formation and growth of hydrogen induced cracks with brittle fracture of the steel. The risk of this hydrogen induced stress corrosion cracking can be minimized by appropriate treatment, but there is a requirement for construction to approve only prestressing steel having no enhanced susceptibility to stress corrosion cracking. The paper describes the historical development of the different test methods proposed in the past and gives an overview on the general requirements for a suitable test method to assess the hydrogen‐induced stress corrosion susceptibility of prestressing steels. Since 1982, based on investigations on construction sites and transfer of the results into laboratory tests, a test method is available enabling realistic estimation of suitable application possibilities of prestressing steels. This so called DIBt‐test is approved to distinguish between prestressing steels susceptible to hydrogen induced stress corrosion cracking and those suitable and therefore approvable steels at practical environmental conditions. Furthermore the test enables estimation of corrosion risk for newly developed prestressing steels with higher strength.     

Untersuchungen zur Rißkorrosion hochfester Baustähle in Meerwasser

Studies of stress corrosion of high-strength structural steels exposed to seawater Seawater is liable to cause stress corrosion cracking. The risk of cracking may increase as the strength of the steel rises. The report describes the results of a number of test series with welded highstrength structural steels. Long-time exposure tests were performed in natural seawater of the North Sea near Helgoland. In constant deflection tests the influence of the maximum hardness in the heat affected zone on the corrosion behaviour and the effect of chromium as an alloying element were investigated. The potential risk attributable to welded joints containing undiscovered cracks was determined by fracture mechanics tests. Proof was given that under static stress conditions, flawless welded joints on high-strength structural steels up to S890Q grade are not susceptible to hydrogen-induced stress corrosion cracking. Maximum hardness of 400 HV in the heat affected zone also do not have a negative effect on steels containing up to 1% chromium. Steels with more than 1.5% chromium are endangered if the maximum hardness exceeds values of 350 HV. A susceptibility to stress corrosion under slow strain and creep conditions can be excluded from the potential of free corrosion up to the potential of cathodic protection with zinc. High-strength tensile structural steels do not differ in this regard. Limitations on the use of high-strength structural steels in the submerged area of offshore structures are shown by the fracture mechanics tests with welded joints. The results of the tests show that under free corrosion conditions in seawater high-strength steels up to S500Q do not react more sensitively in terms of corrosion crack growth than the proven steel grade S355N. The choice of steel grade may therefore be guided by other criteria such as the fracture mechanics or fatigue characteristics and processing behaviour.     

The predictability of stress corrosion cracking susceptibility of steels in acetate solutions from potentiodynamic polarization curves

Stress corrosion cracking susceptibilities of mild and low alloy steels in acetate solutions, assessed using slow strain-rate testing techniques, correlate well with those predicted on the basis of data from fast and slow sweep rate potentiodynamic polarization curves. Accurate prediction of cracking susceptibility necessitates allowances being made for changes of solution pH which occur during stress corrosion and polarization tests. A predictive technique which allows for these pH changes has been developed, and when implemented yields polarization data that indicates the influence of potential, steel composition and solution temperature on cracking susceptibility. The stress corrosion cracking of steels in acetate solutions is intergranular and occurs only when the electrochemical conditions correspond to those of a predicted potential-pH domain that is associated with the Fe²⁺/Fe₃O₄ transition.     

Anodische Spannungsrißkorrosion eines Stahls in flüssigem Ammoniak

Anodic stress corrosion cracking of steel in liquid ammonia Steel rods W.No. 1.8907 were tested for stress corrosion cracking in liquid ammonia at about 20 °C under controlled electrochemical conditions at constant rate of elongation. Stress corrosion cracking was not observed in pure ammonia at the free corrosion potential of the steel corroding in the active state, but only upon polarization to the passive state. The susceptibility to stress corrosion cracking grew with the temperature and was negligible at temperatures below about 10°C. In presence of oxygen at pressures exceeding about 0.1 bar, stress corrosion cracking occurred at the free corrosion potential in the passive range at more negative electrode potentials than in absence of oxygen. Additions increasing the specific conductivity of ammonia and 0.15% water inhibited stress corrosion cracking. The small danger of hydrogen embrittlement was enhanced by cathodic polarization and by addition of acids.     

稀土铒和铬离子注入对电机护环钢SCC性能的影响

研究了解和铬离子高、中能量重迭注入对５０Ｍｎ１８Ｃｒ４电机护环钢ＳＣＣ性能的影响．ＳＣＣ对比试验结果表明：（１）在ＱＨＪ－７９标准硝酸盐介质中，离子注入试样的ＳＣＣ出现时间（ｔｆ）比不经注入试样的延长了６倍以上，致钝和维钝电流密度下降了一个数量级；（２）在阴极充氢条件下，两种试样均对氢致开裂（ＨＩＣ）不敏感，但离子注入可抑制氢诱发腐蚀．用ＡＥＳ－ＰＲＯ、ＲＢＳ、ＸＰＳ－ＰＲＯ、ＥＤＡＸ、金相及电化学方法分析讨论了护环钢在ＱＨＪ－７９标准介质中的ＳＣＣ机理和离子注入改善ＳＣＣ抗力及抗氢锈发腐蚀性能的机制．     

Inhibition von Niedertemperaturkreisläufen in Kraftwerken mit Lithiumhydroxid

Inhibition of low temperature circulating systems with lithium hydroxide in power stations Corrosion current density measurements of mild steel St 37 in deionized, air-saturated water between 20 and 60°C showed that addition of small quantities of lithium hydroxide (ca. 25 ppm) protects against corrosion attacks similar to pitting corrosion and is found adequate even up to 5 ppm chloride ion concentration. Constant strain rate tests of mild steel St 37 showed no indication of stress corrosion cracking (SCC) at 70°C, up to 500 ppm lithium hydroxide, and 100 ppm chloride ion concentration. Metallographic examination of specimens indicated intergranular stress cracking with cracks of 1–30 m?m depth in environments containing 2%, 4%, and saturated lithium hydroxide (with solid excess salt) at 70°C. Accumulation of lithium hydroxide should therefore be avoided. No SCC was observed in austenitic stainless steel X 5 CrNi 18 9 specimens in the above environments.     

Zum Spannungsrißkorrosionsverhalten rekristallisierter Bleche der AlLi-Legierung 8090-T81

On the stress corrosion cracking behaviour of recrystallized 8090-T81 Sheets The stress corrosion cracking behaviour of a recrystallized sheet of the Al-Li-Cu-Mg-Zr alloy 8090-T81 was studied performing accelerated tests under constant deformation, constant load, and slow strain rate conditions. The used electrolytes were an aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH = 3, and synthetic seawater according to ASTM D1141. Alternately immersed in 3.5% NaCl solution according to ASTM G44 the investigated alloy was found to be susceptible to stress corrosion cracking was not promoted by continuous immersion in aerated 3.5% NaCl solution, 3.5% NaCl solution saturated with carbon dioxide, and in acid chromate inhibited 2% NaCl solution. Using the slow strain rate technique with continuously immersed flat tensile specimens stress corrosion cracking was only observed in synthetic seawater. Under specific environmental conditions hydrogen embrittlement can occur in the investigated material.     

Anfälligkeit gewindeformender Schrauben aus korrosionsbeständigen Stählen gegenüber Spannungsrisskorrosion

Susceptibility of thread‐forming screws made from stainless steels with relation to stress corrosion cracking Thread‐forming screws, manufactured from martensitic and austenitic stainless steels, were tested concerning their susceptibility with relation to stress corrosion cracking. Based as closely on application usage as possible, testing comprised using the fasteners to clamp steel sheets of different material types at changing corrosion conditions according to DIN 50021‐SS and DIN 50018‐KWF 2,0S. During the exposure until 90% of the martensitic drilling screws had failed due to hydrogen‐induced stress corrosion cracking. Such fasteners represent a safety risk in building practice, even if corrosion resistance requirements are minimal. The screws made of austenitic materials bore the identical test conditions without cracking.     

2507双相不锈钢钾盐蒸发罐腐蚀的原因分析

以某院设计的两个项目中的蒸发罐实际使用情况为切入点,从2507双相不锈钢设备的使用环境和制作工艺两方面分析了钾盐蒸发罐发生腐蚀开裂现象的原因,进而讨论了如何在设计、制造及使用过程中避免双相不锈钢设备发生腐蚀。结果表明:双相不锈钢蒸发罐的使用环境和制造工艺均对其耐蚀性有很大的影响,尤其是当有焊接瑕疵和组装残余应力造成的应力集中,或在强酸、高温、高Cl~-含量介质溶液的环境中运行时,设备极易发生应力腐蚀裂纹。     

CORROSION FATIGUE BEHAVIOR OF STAINLESS STEEL 3RE60 IN 3.5% SODIUM CHLORIDE SOLUTION

The corrosion fatigue behavior of stainless steel 3RE60 in 3.5%NaCl solution was studied at different cyclic stress levels. The results showed that both intergranular corrosion cracking and transgranular corrosion cracking initiated at the bottom of pits. The corrosion fatigue behavior of 3RE60 may be related to complex electrochemical and mechanical coupling effects between the three phases (austenite, ferrite and martensite), where martensite and ferrite were anodic in the corrosion cell and could be prone to crack under certain conditions.     

On the corrosion behaviour of 8090-T6 sheet material

The exfoliation corrosion and stress corrosion cracking behaviour of an unrecrystallized sheet of the Al-Li-Cu-Mg-Zr alloy 8090-T6 was investigated. The material was highly susceptible to exfoliation corrosion in the 96 h EXCO test, whereas the intermittent salt spray test (MASTMAASIS test) revealed only a slight degree of exfoliation. The stress corrosion cracking behaviour was investigated in 3.5% NaCl solution under constant load, constant deformation, and slow strain rate conditions. Tests under constant load were also performed in an inhibited and acidified 2% NaCl solution according to the German standard LN 65666. In these accelerated tests the investigated alloy was found to be not susceptible to stress corrosion cracking in the rolling direction. The slow strain rate tests revealed a great scattering of the relative fracture energy with decreasing strain rates. This scatter is probably caused by pits acting as crack nuclei thus giving rise to scatter of the ductility.     

Electrochemical investigation on carbon steel behaviour in CO-CO2-H2O environment for the interpretation of the SCC mechanism

Some failures of carbon steel equipment which have occurred in industrial plants since 1963 were attributed to stress corrosion cracking (SCC) phenomena produced by carbon monoxide-carbon dioxide-water environments. In spite of the fact that this problem has been known for many years, its characterization from the electrochemical point of view has not been throughly investigated. In this paper, the electrochemical characterization of SCC phenomena experienced in industrial plants was performed on a laboratory scale by means of polarization curves; the tests were performed keeping as reference conditions those encountered in a gas tank sealed by water, located in a steel plant in the South of Italy, which was strongly affected by SCC. The polarization curves registered at high and low scan rates did not allow to identify the potential range for SCC probability. The potential range in which the carbon steel shows a tendency to passivation was identified by chronopotentiostatic tests. Several tests were performed in a carbon dioxide-water environment which is not able to cause localized corrosion. The electrochemical tests were accompanied by Scanning Electron Microscope (SEM) observations which confirmed the obtained results.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Die Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

Determination of threshold stress intensity factor for stress corrosion cracking (KISCC) of steel heat affected zone

Threshold stress intensity factor for stress corrosion cracking of heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. HAZ microstructure produced during manual metal arc welding of grade 250 steel was simulated over a length of 35 mm of CNT specimens, using a thermo-mechanical simulator. Inter-granular stress corrosion cracking has been confirmed with scanning electron microscope. The results presented here validate the ability of CNT technique for determination of K_ISCC of HAZ and base metal.     

The intergranular corrosion and stress corrosion cracking of mild steel in clarke's solution

Measurements have been made of the corrosion rates and stress corrosion crack velocities for mild steel exposed to hydrochloric acid containing antimony additions at open circuit and various applied potentials. Cracking occurs within a sharply defined range of potentials, within which markedly slower intergranular corrosion occurs, while general corrosion occurs at potentials above or below those that promote cracking. It is suggested that the role of Sb in the cracking mechanism is essentially the same as the role of oxide films in other environments, of restricting lateral dissolution at the crack sides.