Das elektrochemische verhalten von Schweißverbindungen

The electrochemical behaviour of weld seams On the basis of reproducible electrochemical tests in macro and micro ranges a correlation is derived between electrochemical data and the corrosion behaviour of weld seams of steels of different compositions and aluminium welded with dissimilar fillers. In austenitic welds differences of the rest potential may give rise to the formation of local elements, and in passive welds there may exist active zones showing pitting and intercrystalline corrosion. The area and location of the active and passive region can be defined by current density potential curves. The susceptibility to selective corrosion is established by measurements on micro regions.     

Localised Corrosion of Stainless Steels During Food Processing

Abstract

The austenitic stainless steels are widely used in the food industry as constructional materials for processing plant. One of their drawbacks is a susceptibility to localised attack, particularly in the presence of the chloride anion. The various forms of localised attack which stainless steels can suffer, including pitting, crevice attack, deposit attack, stress corrosion cracking and corrosion fatigue, are illustrated with case histories drawn from the food processing industry.     

Stromdichte-Spannung-Messungen an nichtrostenden Stählen in aktivierenden Medien

Current density-potential measurements on stainless steels in active media In order to elucidate the susceptibility to selective corrosion of weld seams in stainless steels potentio-kinetic current density-potential measurements have been made. When the polarisation curve for the weld seam is more anodic in the vicinity of the rest potential than the curve corresponding to the base material the weld seam is selectively attacked; when, however, the weld seam is moer cathodic than the base metal there is almost no corrosion in the adjacent areas because of the large area ratio (large anode, small cathode). It is only in extreme cases (very high potential difference between cathodic weld seam and anodic plate) that local corrosion is found in the base metal. Tests with Hastelloy B in 20% hydrochloric acid show that this material cannot be welded successfully with identical material; it might, however, be feasible to weld it with cathodically alloyed weld fillers (alloyed with Pt or Pd).     

Electrochemical investigations of pitting corrosion

By using chronopotentiostatic and stepwise potential change experiments with potentiokinetic and galvanostatic testing, the following types of pitting corrosion of stainless steels in chloride-containing solutions have been investigated: sulphate inhibition of pitting; 35%Cr-Fe alloy; Cr-Ni-Mo stainless steels. The circuit resistance was found to be of fundamental importance. The pit passivation potential depends on the intensity of a corrosion attack before potential change only in the case of inhibited solutions. In uninhibited solutions passivation and formation potentials are nearly equal only in the case of potentiostatic circuit conditions. The Cr-Fe alloy and the Cr-Ni-Mo stainless steels show a potential range of repassivating pitting. Of practical interest is the critical potential of stable pitting which decreases with increasing circuit resistance. The beneficial effect of Mo is only valid for the pitting potential obtained potentiostatically and not at higher circuit resistances. Considering the practical meaning of the addition of Mo it may be concluded that this element is probably essentially connected with repassivation of pits and conditioning effects of the passive layer.     

Effect of Friction Stir Welding on Pitting and Stress Corrosion Cracking Behavior of AFNOR7020-T6 Aluminium Alloy

Present work mainly focused on the pitting and stress corrosion cracking behavior of AFNOR7020 aluminium alloy friction stir welds and compared those results with the base material. Initially, microstructural studies, TEM, microhardness tests, tensile tests and general corrosion properties of the welds were investigated. The elongated grains present in the base material have been transformed into superfine grains in the weld nugget. Transmission electron micrographs obtained from various regions of the weld indicated that almost all strengthening precipitates dissolved in the nugget region while partial dissolution of precipitates occurred in the thermo-mechanically affected zone and coarsening occurred in heat affected zone. Hardness in the weld nugget was found to be very nearer to the base material without considerable difference. The welds showed superior joint efficiency of 85% in terms of the yield strength and 95% in terms of ultimate tensile strength. General corrosion resistance of the welds was better than that of the base material at different pH value and spraying time. Pitting corrosion studies revealed that less significant difference in pitting corrosion resistance has been observed between the weld nugget and the base material. It was found that the susceptibility towards stress corrosion cracking is relatively more in base metal compared to welded joints. It has been concluded that friction stir welding plays a very important role in corrosion properties of the AFNOR7020 aluminium alloy.     

Effect of metallurgical development on the corrosion of carbon and low-alloy construction steels for ship building and harbour construction

The corrosion behaviour of continuous cast low-alloy steels is discussed and compared with that of ingot cast steels. There is no evidence for any influence of the casting technique on the resistance of these steels to either general attack or pitting corrosion in neutral, aqueous solutions. Disadvantages and failures of high-strength low-alloy steels described in the literature, e.g. localized corrosion at welds, pitting corrosion, stress corrosion cracking and difficulties in hot-dip galvanizing, which are considered to be related to the casting technique, are discussed and explained in terms of the actual knowledge and principle rules of the corrosion behaviour of steels.     

Evaluation of techniques for measuring corrosion activity of carbon steel welds

Abstract

Three test methods, namely immersion tests, galvanostatic anodic dissolution tests and the scanning vibrating probe technique, have been evaluated as candidate methods for ascertaining the susceptibility of welds in carbon steels to preferential corrosion. The use of hardness measurements by profiling and mapping was also evaluated. Three welds were tested for which the production had been controlled to produce different types of corrosion behaviour. In addition, a weld that had failed 'in service' was used to validate the findings of this assessment. In the immersion tests, lasting 9 months, the three controlled weld specimens ex posed to ASTM D1141 sea water behaved as expected, in terms of which region of the weld was most susceptible to corrosion. The results of the rapid galvanostatic anodic dissolution tests compared well with those of the immersion tests, with the same regions of the controlled welds showing susceptibility to corrosion. The galvanostatic anodic dissolution test on the failed weld produced corrosion in the regions that had been susceptible to corrosion in service. However, at very high current densities the differential corrosion loss between different regions of the weld was not maintained and careful control of this parameter is therefore necessary in testing. The scanning vibrating probe technique successfully identified the areas susceptible to preferential corrosion in both the controlled weld specimens and the in service failure. The electrolyte used was ASTM D1141 sea water diluted 10 : 1 with distilled water. However, these trends were not apparent when tests were conducted in distilled water owing to the lower solution conductivity. Hardness mapping produced a two-dimensional visual representation of the specimen hardness. This was very effective in locating small regions of high hardness, in contrast to conventional hardness profiling, and would be effective in locating hot spots for hydrogen embrittlement. However, no correlation between hardness and corrosion susceptibility was observed in the three weld specimens that were examined. The galvanic anodic dissolution test is recommended as a rapid and reliable test to determine the relative corrosion susceptibility of carbon steel welds. The scanning vibrating probe technique can also be effective but at considerably greater expense.     

CO2环境下油酸咪唑啉对X65钢异种金属焊缝电偶腐蚀的抑制作用研究

研究了X65管线钢与316L不锈钢、Inconel 625双金属复合管的异种金属焊缝在CO2环境下的电偶腐蚀行为,以及油酸咪唑啉的缓蚀作用。结果表明,随着电偶电位差的增大,异种金属焊缝的腐蚀速率明显升高,并且都显著高于母材。添加油酸基咪唑啉缓蚀剂能降低异种金属焊缝在CO2环境下的均匀腐蚀速率。但是,当缓蚀剂浓度添加较低时,异种金属焊接试样的碳钢一侧出现了严重的沟槽腐蚀或密集的点蚀坑;进一步增加缓蚀剂浓度才能消除沟槽腐蚀现象。讨论了缓蚀剂对异种金属焊缝电偶腐蚀的抑制机理,该项研究可为异金属焊接接头处的腐蚀防护提供借鉴。     

Korrosionsprognose für den Bereich der Schweißnaht

Forecasting the corrosion susceptibility in the weld zone Forecasting the corrosion susceptibility of a weld seam requires the knowledge of as much data as possible of the material-electrolyte system. Important data are structural changes in the material as a consequence of heat flow during welding. It is therefore important to know the precipitation conditions in materials intended for use in welded structures. A particularly useful tool in corrosion forecasting is the potential probe curve the basic shapes of which and conclusions to be drawn therefrom are described. Its validity is checked using ship steels welded by different methods and being exposed to seawater. It is shown that less noble portions in the potential probe curve are a necessary but not a sufficient condition for selective corrosion. The curves agree with practical experience only when the welded specimens are exposed to flowing seawater. On the other hand, regular shapes of the curve may be taken for a necessary and also sufficient condition for the impossiblility of selective corrosion in the weld zone. As to steels the variety of possible material/electrode combinations may be divided into two large groups: unalloyed steels in aqueous, aerated and near neutral, and stainless steels in neutral and acid media.     

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.     

Untersuchung der lokalen Korrosionserscheinungen an den Schweißnähten in Apparaturen aus nichtrostendem Stahl

Investigation into local corrosion phenomena at the welds in chemical apparatus of stainless steel Corrosion phenomena at the welds of apparatus consisting of stainless steel can only be investigated in an active medium. The active medium used for this purpose was a 5 pC hydrochloric acid where virtually all the incidental electro–chemical phenomena disappear, and where the corrosion rate is exclusively determined by the transit potential at the boundary between plate and weld. The paper describes the preparation of the specimens, the design of the measuring cell, and the testing procedure adopted. In this way, a “hydrochloric acid test” was developed which permits a rapid distinction between “good” and “poor” welds of specimens. The reliability of the hydrochloric acid test is corroborated by recording the current density/potential characteristics in 5 pC hydrochloric acid. The technique of recording these characteristics with the aid of the potentio-kinetic method is described, and illustrated by photographs and diagrams.     

点焊热镀锌双相高强度钢的电极磨损规律

热镀锌双相高强度钢是适应汽车轻量化与安全性需要而刚刚发展起来的一种新型板材,强度高,抗腐蚀性能好,但点焊时电极磨损严重,不确定性大,对焊点质量造成很大影响.首先确定了点焊强度为600 MPa的热镀锌双相钢的焊接性范围,然后根据焊接性范围确定焊接工艺参数进行电极磨损试验.研究了电极磨损时的电极表面形貌、轴向磨损、端面直径磨损的变化规律,分析了不同电极磨损阶段的轴向磨损对端面直径磨损的贡献率.结果表明初始焊接阶段电极磨损速率较大,点蚀加剧了电极磨损与失效.     

Enhancing the localised corrosion resistance of 316L stainless steel via FBR-CVD chromising treatment

The resistance of stainless steels to localised corrosion can be adversely affected by environmental and metallurgical heterogeneities existed in complex industrial infrastructures such as seawater desalination plants exposed to aggressive evnironments. It is therefore critical to enhance the localised corrosion resistance and understand the corrosion behaviour of stainless steels in complex and aggressive industrial environmental conditions. In this work, the localised corrosion resistance of chromised stainless steel 316L (SS316L) in simulated seawater desalination systems has been investigated by electrochemical and surface analytical techniques. It has been found that chromising processes have improved the localised corrosion resistance of SS316L by reducing its susceptibility to pitting, crevice, and welding zone corrosion in simulated seawater desalination environments. This increased corrosion resistance has been explained by electrochemical polarisation studies and surface analysis showing that the chromising treatment at 1050°C resulted in a continuous and stable chromium-enriched layer on the SS316L surface.     

The influence of Mn compared to that of Cr,Mo and S on the resistance to initiation of pitting and crevice corrosion in austenitic stainless steels

A large number of production and laboratory heats in grades AISI 304 and 316 with normal and extremely low managanese and sulphur contents and a number of production heats in more highly alloyed austenitic stainless steels have been studied with regard to their resistance to initiation of pitting and crevice corrosion at various temperatures. The criteria for resistance to initiation was the potentiodynamic pitting potential in 0.1 M NaCl and synthetic seawater and the time to attack initiation for crevice corrosion in 0.5 and 5% NaCl solutions. A large number of production and laboratory heats in grades AISI 304 and 316 with normal and extremely low managanese and sulphur contents and a number of production heats in more highly alloyed austenitic stainless steels have been studied with regard to their resistance to initiation of pitting and crevice corrosion at various temperatures. The critieria for resistance to initiation was the potentiodynamic pitting potential in 0.1 M NaCl and synthetic seawater and the time to attack initiation for crevice corrosion in 0.5 and 5% NaCl solutions. The main aims of the study were to examine both the effect of manganese relative to that of chromium, molybdenum and sulphur and the effect of heat treatment and sulphide composition on steels with low manganese contents. Mathematical models for calculation of the pitting potentials have been constructed and multiple linear regression analysis gave the equations and their reliabilities. Lowering of the Mn content in austenitic stainless steels to 0.2% gives rise to a material of interest for constructions where pitting or crevice corrosion are judged to be the only potential types of attack, where operational disturbances leading to greatly increased corrosivity do not occur, where attack can not be tolerated, and where steel with normal managanese content has not exhibited fully satisfactory corrosion resistance. If the above conditions are fulfilled the low manganese content can be said to correspond to the same positive effect as is obtained by an addition of the least 1.5% Mo.     

Widerstand der Schweißverbindungen kohlenstoffarmer und mit Molybdän legierter korrosionsbeständiger Stähle gegen interkristalline und Messerlinienkorrosion

Resistance to intercrystalline and knifeline corrosion of welds in low carbon stainless steels containing molybdenum Welds in steels containing (%) 0,019–0,080 C, 1,00–1,56 Mn, 0,03–0,92 Si, 0–0,026 P, 0–0,018 S, 10–13,60 Ni, 16–20,50 Cr, 0–3 Mo, 0–0,057 Ti, 0–0,87 Nb, prepared by electroslag and automatic submerged arc welding are as a general rule not susceptible to intercrystalline and knifeline corrosion; a certain susceptibility encountered with the Mo containing types can be largely eliminated by sensibilizing at 650 °C. Differences in the corrosion behaviour between base metal and weld seam could not be encountered; in some cases the weld metal turned out to be even more resistant. The test solutions used were: sulfuric acid + Cu sulfate + Cu, boiling, 48 hrs, and 65% nitric acid, boiling, 3 · 48 hrs.     

Elektrochemische Untersuchung der Lochkorrosion chemisch beständiger Stähle

Electrochemical investigation into the pitting corrosion of corrosion resistant steels In view of the fact that the characteristics termed pitting potential depend from the measuring methods and nonstationary defects and hysteresis phenomena produced thereby these condition have been investigated on three different systems. In sulphate inhibited chloride solution an actual hysteresis is found with two critical boundary potentials for pit formation and pit passivation. The first of these potentials may be found either by potentiocinetic or by potentiostatic methods, the second only galvanostatically. In the case of 35% Cr steel no hysteresis is found; in this case there is no connection between pitting potential and the current and potential variations found during the investigation. With this material pitting corrosion may occur below the pitting potential while the pit passivation potential is a function of the properties of the measuring circuit. The pitting behaviour of austenitic CrNi steels containing various amounts of molybdenum may be characterized potentiostatically or potentiocinetically, but at low voltage feed rates only.     

Der Einfluß von Martensit auf das Korrosionsverhalten von 18 Cr-10 Ni-Stahl

The effect of martensite on the corrosion properties of 18 Cr-10 Ni steel The corrosion properties of the martensitic phases formed in an austenitic matrix by (i) cooling to low temperature (-196 and ?269°C resp. ?320 and ?452°F) and (ii) cold working at room temperature was investigated for two chromium-nickel stainless steels of the 18 Cr10 Ni type. Austenite and martensite have the same chemical composition but different lattice structures. In sulfuric acid, both martensitic phases formed at low temperature and by cold working arc preferentially attacked in the active range of corrosion. The effect of potential on the corrosion attack was elucidated by potentiostatically controlled experiments with subsequent light-optical and SEM-investigation of the specimens. Selective corrosion attack of the martensitic phase was found down to a potential being 0.8 V more negative than the cathodic limiting potential of the active corrosion range of a fully austenitic steel, irrespective of the way of the martensite transformation. The preferential attack of martensite may be explained by the supposition that the deposition of cathodically protecting layers possibly consisting of nickel is rendered more difficult at the martensitic surface areas. In sulfuric acid, no differences in the corrosion properties between austenite and martensite were found in the passive and transpassive range. In chloride containing aqueous solutions, the resistance to pitting is not effected by martensite formed at low temperature. From this it is concluded that also martensite formed by cold working does not effect the pitting resistance.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

碳钢及低合金钢点蚀性能的研究

选择三种典型的碳钢、低合金钢,通过极化试验比较了它们之间的点蚀诱发敏感性和模拟孔蚀的“闭塞腐蚀电池”,试验研究了它们之间的孔蚀扩展行为。结果表明,B钢的点蚀诱发能力小于A钢和C钢;在同样的阴极极化电位下,B钢的阳极溶解电流也明显小于后两者。电子探针分析了不同夹杂物在诱发点蚀过程的腐蚀特征,夹杂物是钢中主要的点蚀诱发源。显微分析“闭塞腐蚀电池”腐蚀形貌,发现腐蚀形貌具有平行沟槽状。初步分析原因是沿轧向延伸的磷偏析带及夹杂物所导致的。扫描电镜和能谱仪对腐蚀产物的形貌、成分分析结果表明：B钢的锈层均匀致密,而A钢的锈层呈网状、疏松且有大量裂纹和孔洞。     

Zur Entwicklung der elektrochemischen potentiokinetischen Reaktivierungsmethode – des EPR‐Tests

On the development of the electrochemical potentiokinetic reactivation method – the EPR‐test Electrochemical potentiokinetic reactivation methods, but also other polarisation methods, are suitable as quasi non destructive testing methods to describe the corrosion resistance of steels and alloys without needing a very pretentious surface preparation. They may be used also to check finished components or even on site such in service, especially welding areas or weld overlays. Compared to standard tests they are quicker, more sensitive, more objective and quasi quantitative. They may be applied not only with austenitic standard steels but also with high alloyed ferritic, martensitic or duplex steels as well as with nickel based alloys. After any case of heat treatment the sensitivity to intergranular corrosion but also to pitting or selective attack can be determined by these testing methods. They may inform on the general corrosion resistance or the microstructural changes which may occur with materials, too. Based on the experience of many years with such methods which, starting from the early developments, have now been applied to very different materials, one may attend new fields of application in research and also as a nationally and internationally testing standard.