Selektive Korrosion an Verbindungsschweißen von Stählen

Selective corrosion at welded assemblies of steels The paper deals with the possible selective corrosion phenomena at the most important types of weld assemblies of steels, viz. those of unalloyed ferritic ship building steels, austenitic manganesechromium steals, aluminium-killed boiler steels an well as chemically resistant ferritic and austenitic chrom-nickel steels. With unalloyed ship building steels, a strong corrosion element is liable to occur between not fully descaled plates and scaleless welding material, where the latter is particularly liable to be attacked. But even where scalefree plates are welded, a local element may be set up, depending on the killing intensity and on the filler metals, between plate and welding material in such a way that, again, the welding material is liable to be dissolved first. In the case of austenitic manganese-chromium steels, exposed to sea water, intercrystalline corrosion takes place at a certain distance from the weld as a result of chrome-carbide segregation. In the case of welded assemblies of aluminium-killed steels, intercrystalline fissuration is particularly liable to occure in a more or less marked decarbonised zone at the transition. With ferritic chromium steals and austenitic chrom-nickel steels, the welds are decisively influenced by the segregation of chrome-carbides at the grain boundaries which results in a proneness to intercrystalline grain decomposition. Measures are discussed by which the segregation of chrom-carbides during welding can be avoided. Finally, mention is made of the effect of delta-ferrite on the corrosion behaviour of austenitic welding material.     

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).     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.     

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.     

Korrosionsuntersuchung an geschweißten Proben aus AlMg3 unter Einwirkung von 2%iger Ammoniaklösung

Corrosion tests on welded specimens of AlMg 3, exposed to a 2% ammonia solution The welds of specimens of AlMg 3, welded with AlSi 5, AlMg 3 and AlMg 3, were found to be highly corrosion-prone. In the weld deposit from AlSi 5, there is a strong corrosion effect in the form of notches in the transition zone. This is also accompanied by an abrupt change in the potential of about 1200mV. In the weld itself, the non-homogeneity of the structure gives rise to strong pitting corrosion. With AlMg 3 as weld material, corrosion is less marked but there are numerous small pits along the transition zone. Here again, the difference in potential between base metal and weld deposit (approx. 200mV) is sufficient to account for the corrosion proneness. The potentials of the specimens welded with AlMg 5 showed virtually the same behaviour in the structure as those welded with AlMg 3.     

Korrosionseinflüsse auf Schweißverbindungen

Corrosion effects on welds The author stresses the pecularities of the weld zone as compared to the properties of the base material and shows the impact of these pecularities on susceptibility to intercrystalline corrosion, stress corrosion cracking, crevice corrosion fatigue. On the basis of comprehensive data from literature weld ability and problems related therewith are discussed with reference to the following material: unalloyed and low alloy steels, stainless steels (ferritic and austenitic), Al and its alloys, Ti, Zr and Ta. Finally the influence of galvanizing on the weldability of constructional steels is discussed and the importance of design is pointed out.     

Das Verhalten von Unter-Pulverschweißnähten in Meerwasser

Behaviour of submerged arc welds in seawater Some examples are used to demonstrate the importance of the correct selection of base metal and the corresponding filler material. In the case of a high strenght, pearlite free fine grain steel welded with the filler metal S2Mo under basic powder the base metal is preferentially attacked; in view of the low corrosion loss however, this material can be considered safe for use in seawater containing oxygen. In welded systems of fine grain steel St 38 with increased Mn content, however, the potential of the weld seam obtained under identical welding conditions is clearly lower than that of the base metal, so that such welds must be considered unsuitable for use in sea water. The corrosion susceptibility was determined in a solution of 1% artificial seawater in 99% methanol.     

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.     

In-situ corrosion investigation at Masnedø CHP plant – a straw-fired power plant

Various austenitic and ferritic steels were exposed on a water-cooled probe in the superheater area of a straw-fired CHP plant. The temperature of the probe ranged from 450–600°C and the period of exposure was 1400 hours. The rate of corrosion was assessed based on unattacked metal remaining. The corrosion products and course of corrosion for the various steel types were investigated using light optical and scanning electron microscopy. The ferritic steels suffered from corrosion mainly via material loss. The austenitic steels suffered from predominantly selective corrosion which resulted in depletion of chromium from the alloy. A clear trend was observed that selective corrosion increased with respect to the chromium content of the alloy.     

Inter- und transkristalline Spannungsrißkorrosion austenitischer Mn- und CrNi-Stähle in Meerwasser

Intercrystalline and transcrystalline stress corrosion cracking of austenitic Mn and CrNi steels in seawater The MnCr steels which were originally used for the construction of special ships turned out to be susceptible to intercrystalline stress corrosion cracking in seawater; later on, transcrystalline corrosion susceptibility was found, too; this latter type of corrosion appears in cold seawater and is not due to a sensilizing annealing. This type of corrosion was also found with austenitic CrNi steels in chloride solutions of higher temperature, not, however, at room temperature. The author has made an effort at defining the susceptibility regions of the particular austenitic steels. According to the results obtained it is necessary always to take account of the possibility of stress corrosion cracking when Mn based austenitic steels are used, while austenitic CrNi steels can be considered to be resistant to this type of corrosion. Sensilizing annealing, too, has a bearing on the Mn containing steels only, while the influence of temperature, potential, specimen diameter and stress does not reveal any difference between the two types of steels.     

Untersuchungen über den interkristallinen Kongrenzenangriff von austenitischen Mangan-Chrom-Stählen durch Wasser und wäßrige Lösungen

Research on intercrystalline grain boundary corrosion of austenitic manganesechrome steels caused by water and aqueous salt solutions Stressed bar specimens of welded and tempered austenitic manganesechrome steel such as as X 40 MnCr 18 and X 22 MnCrNi 885, exposed to sea water, tap water or destilled water, are liable to suffer intercrystalline stress corrosion cracking. The occurrence of intercrystalline corrosion is not necessarily dependent on mechanical tensil stress. In particular, even unstressed specimens exposed to tap or distilled water are liable to show marked intercrystalline corrosion. Additional tensile stress merely has the effect of speeding up the inter-crystalline corrosion. The cause for the intercrystalline corrosion must be seen in the segregation of chrome-rich carbids at the grain boundaries and the consequent reduction in the chrome content within the grain boundary zone. It is possible to prove the segregation of a chrome-rich carbide of the structure M₇C₃ at the grain boundaries of steel X 40 MnCr 18, and the aggregation of a chrome-rich carbide of the structure M₂₃C₆ at the grain boundaries of steel X 22 MnCrNi 885. Through contact with zinc, it is possible to obtain a complete corrosion protection with the manganese-chrome steels investigated. A short circuit with passive austenitic chrome-nickel steels increases the general or selective corrosion rate through anodic polarisation. By adding chromate to the sea water, the corrosion rate is retarded. In tap water, an addition of chromate result in complete corrosion protection.     

Oberflächenpotentialprofile von metallen

Surface potential profiles of metals Using a microcapillary method in connection with simultaneous metallographic observation surface potential profiles have been determined with the potential probe in alcoholic acid solutions on cast iron, CrNi aciers steels 188, Cr steel with 17% Cr (welded) and an AlCu alloy. It was possible to distinguish cementite from ledeburite, not however, from austenite. The potential of 188 steel under pitting conditions decreases as a linear function of pit depth. The appearance of potential minima in a weld of ferrite steels is evidence of a intercrystalline corrosion in that particular medium. The precipitation of CuAl₂ in AlCu alloys at 150 °C at the grain boundaries forms an element with the grain boundary as the anode giving rise to intercrystalline corrosion.     

CERT (Constant Extension Rate Testing) als Spannungsrißkorrosionsprüfmethode für Schweißverbindungen. I. Mechanisch-chemische Untersuchungsergebnisse

CERT (Constant Extension Rate Testing) as a test method for stress corrosion cracking at weld joints. I. Mechano-chemical test results Investigations by means of CERT on the corrosion system structural steel/nitrate solution have been performed with special consideration of the influence of weld joints on stress corrosion cracking (SCC) susceptibility. Specimen with real weld joints and specimen with simulated weld joint structures (base material, course grain region, fine grain region) have been used. Results expressed as relative fracture energy show a significant influence of the heat affected zone of the simulated weld joints but not at the real weld joints. As expected, the SCC resistance decreases in the sequence of the steels 15 Mo 3 - StE 355 - H II. All experiments exhibit increasing SCC resistance with increasing heat input. Part II of this work contains metallographic results and discussion of the applicability of CERT for weld joints.     

X80钢焊接结构海水腐蚀及温度的影响规律

以X80钢焊接结构为研究对象，采用SEM、EDS及化学分析表征X80钢焊接接头的组织及成分分布，采用电化学法测量焊接接头不同部位在模拟海水环境中的极化曲线和电化学阻抗谱，研究温度对其腐蚀行为的影响规律。结果表明：海水介质中焊接热影响区的腐蚀倾向最大，焊缝较母材具有更好的耐蚀性；温度升高加速物质扩散及放电过程，因阳极去极化而加速腐蚀，但焊缝处因生成的腐蚀产物致密且附着性优于母材与热影响区，表现出更好的耐蚀性。分析认为：焊缝因低C、Mo、Nb等元素导致组织较母材粗大，晶界数量明显减少，又因Ni、Cr、Al耐蚀元素的增多，因而表现出较好的耐蚀性；热影响区组织复杂、活化能较高，具有较大的腐蚀倾向。     

The HAZ transformations in the welding of duplex steels

The increasing use of duplex steels in plant and welded constructions has resulted in the formulation of quality criteria for such steels and the joints welded in them. These cover the areas of macro-and microstructures, mechanical properties, and resistance to corrosion. The criteria rely, primarily, on the value of the ratio of the ferrite to austenite, the existence of carbides on grain boundaries, and on the precipitations of intermetallic phases in both the weld and the heat affected zone. To meet these criteria, it is necessary to properly control the sequence of the phase changes, and of the precipitation processes which are decisively affected by the character of the welding cycle of the steel.     

两种船体钢焊接接头耐海水腐蚀性能对比

选择了两种碳素船体钢（经稀土变性处理和未经稀土变性处理的3C钢）和3种常用的焊接,加工成6种焊接接头试样,通过室内全浸搅拌挂片试验、间浸挂片试验和恒电位阳极溶解试验,对同一焊接接头不同区域以及不同焊接接头之间腐蚀行为的差异进行了比较,并结合冶金检测结果分析探讨了造成该腐蚀特点的原因。结果表明,对3C钢常用焊接接头而言,均为焊缝区耐海水腐蚀性能最差,其次为母材,热影响区耐蚀性最好;经稀土变性处理后,3C钢不仅钢种本身的耐蚀性得到了提高,而且以该钢为母材的焊接接头的耐蚀性也得到了相应的改善（同种焊接材料相比）;无论是普通3C钢还是稀土变性处理3C钢均以J422为焊接材料的接头的耐海水腐蚀性能最好。     

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.     

Difference of fatigue strength of cruciform welded joints due to specimen configuration

Abstract

The increasing use of duplex steels in plant and welded constructions has resulted in the formulation of quality criteria for such steels and the joints welded in them. These cover the areas of macro–and microstructures, mechanical properties, and resistance to corrosion. The criteria rely, primarily, on the value of the ratio of the ferrite to austenite, the existence of carbides on grain boundaries, and on the precipitations of intermetallic phases in both the weld and the heat affected zone. To meet these criteria, it is necessary to properly control the sequence of the phase changes, and of the precipitation processes which are decisively affected by the character of the welding cycle of the steel.     

合金元素对钢在海水飞溅区腐蚀的影响

讨论了合金元素对钢在海水飞溅区腐蚀的影响。合金元素在青岛、厦门和榆林海域对钢在海水飞溅区的腐蚀有基本相同的影响效果。Mn、P、Si、Cr、Mo和Ni能减轻钢在飞溅区的腐蚀,其影响大小的顺序为：P＞Si＞Cr和Mo＞Ni和Mn;S、Al、V对钢的飞溅区腐蚀有害。Cu-P、Mn-Mo、Ni-Cr-Mo复合对减轻钢在飞溅区的腐蚀有好的效果。     

不锈钢海水潮汐区16年腐蚀行为

在青岛、厦门和榆林3个试验站的潮汐区对5种不锈钢暴露16年，总结其腐蚀行为和规律。在潮汐区暴露的不锈钢受点蚀和缝隙腐蚀破坏。不锈钢在潮汐区暴露1至4年的点蚀速度较大，以后点蚀速度减慢。耐点蚀性能较好的不锈钢，耐缝隙腐蚀性能也较好。不锈钢在潮汐区的腐蚀随暴露地点的海水温度升高而加重。增加Cr含量、添加Mo能明显提高不锈钢在潮汐区的耐蚀性。Ni对提高的耐蚀性有效，但影响效果较小。海生物污损能引起不锈钢的局部腐蚀，它对不锈钢在潮汐区的腐蚀有显著影响。