Identification of the selective corrosion existing at the seam weld of electric resistance-welded pipes

The selective corrosion existing at the seam weld of high frequency electric resistance welded pipes of carbon steel with low sulfur content in electrolyte solutions is revealed by localized electrochemical measurements. The seam weld, mainly consisted of ferrite, has more negative open circuit potential and higher anodic dissolution current density than the base metal consisting ferrite and pearlite. Between the seam weld and the base metal, there is a galvanic coupling effect accelerating the dissolution kinetics of the seam weld such that V-shaped corrosion groove preferentially occurs at the seam weld.     

Potential-Sonden-Messungen und Potential-Profile über Schweißverbindungen korrosionsbeständiger Nickel-Basis-Werkstoffe und Sonderstähle

Potential probe measurements and potential profiles across weldments of corrosion resistant nickel-base alloys and special stainless steels A great number of potential profiles was measured across weldments (SMAW, GTAM, GMAW, Plasma) of the highly corrosion-resistant nickel-chromium-molybdenum alloys 22, C-276, C-4, of the nickel-chromium-iron-molybdenum-copper alloy G-3 as well as of the highly corrosion-resistant austenitic steel Cronifer 1925 hMo. After several tests had been made in a high-ohmic electrolyte solution of 90 vol.-% of methanol + 10 vol.-% of 10^?3 M HCl ( χ = 18 to 20 μs/cm) the tests were continued, for reasons of easier handling, in deionized water whose conductivity was adjusted by using 0.1 M HCl to χ = 10 to 20 μs/cm. It clearly proved that the potential profiles of various weld joints were dependent on the electrolyte. The potential profiles measured across weldments of the NiCrMo alloys which were made with matching filler metal revealed only insignificant or no differences between the potentials of the base material and the weldment. Consequently, in chloride-containing electrolytes it is very unlikely that matching NiCrMo weldments are subject to a preferred corrosion attack. This corresponds to practical experience. The potential profiles in the aqueous HCl electrolyte across weldments of the 6% Mo stainless steel Cronifer 1925 hMo and of the NiCrFeMoCu alloy G-3 welded with a NiCrMo-filler metal showed a potential drop of up to 420 mV over the weldment when compared with the base material. These potential differences can be explained by means of current density-potential curves, showing the electrochemical reactions of base material and weldment with the electrolyte to be different. Nevertheless, the corrosion resistance of the weldment is not inferior to that of the base material.     

Zu einem Phänomen der Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo-Legierungen in Chloridlösungen

On a phenomenon of the limitation of pitting corrosion at high alloyed special stainless steels and NiCrMo-alloys in chloride solutions Testing the pitting corrosion resistance of high alloyed special stainless steels and NiCrMo-alloys in chloride solutions there was observed a limitation of the pitting corrosion range toward more positive potentials. Above this limitation, the so-called pitting corrosion limitation potential, the pit initiation by all means is prevented, but the growth of pits which had been initiated before in the pitting corrosion range not necessarily comes to a stop. Therefore current density-potential curves which are obtained by downward polarization after an initial potential jump into the transpassive region and chronopotentiostatic tests are more suitable to investigate this phenomenon than cyclic polarization measurements and potentiostatic alteration tests. There is indicated a dependence of this phenomenon on temperature. The phenomenon of a limitation of the pitting corrosion range toward more positive potentials has been found until now at alloy 926, alloy 31, alloy 28, alloy 59 and alloy C-276.     

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.     

Galvanic corrosion of gas tungsten arc repair welds in 17-4PH stainless steel in 3·5% NaCl solution

Abstract

In this paper, the galvanic corrosion of individual components of a 17-4PH repair welded stainless steel in 3·5% NaCl solution was investigated using various dc electrochemical measurements and microscopy. Open circuit potential measurement of the regions in the vicinity of a repair weld [i.e. parent metal, weld metal and heat affected zone (HAZ)] in 17-4PH stainless steel in 3·5% NaCl solution indicated that the most likely galvanic couple was between HAZ and weld with the HAZ acting as the anode and weld metal as the cathode. Slow scan rate potentiodynamic polarisation measurement of pitting potentials revealed a lower passive current density and a higher pitting potential in the weld region, while the HAZ showed the highest passive current density and the lowest pitting potential. Observation of the material after applying an anodic potential close to the pitting potential of the individual weld parts also confirmed the formation of several stable pits in the HAZ but only a few metastable pits in the weld and parent metal zones. Galvanic coupling using a zero resistance ammeter also showed a higher current density in the weld metal/HAZ as compared with the parent metal/HAZ and parent metal/weld galvanic couples. Although, the current densities in all measurements were in the range of a few to tenths of nanoampere per square centimetre, it can still be concluded that the weld metal/HAZ couple has the highest risk of galvanic corrosion among the three individual galvanic couples.     

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。     

16MnR 钢在催化裂化再生环境中的应力腐蚀裂研究简

通过U形试样应力腐蚀实验、电化学极化曲线等方法,在模拟催化裂化再生器环境条件下,研究了HNO3-H2SO4-H2O体系中16MnR钢及其焊缝的应力腐蚀行为。结果表明:16MnR钢焊接接头在不同实验条件下均比基材更易产生硝酸盐应力腐蚀开裂,其机理主要是阳极溶解;引入硫酸根和降低pH值均能破坏16MnR钢的保护膜,增大其焊接接头的应力腐蚀敏感性;当溶液的pH值降低至2以下时,材料处于活化状态,发生严重的均匀腐蚀。     

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.     

Electrochemical characteristics of stainless steel using impressed current cathodic protection in seawater

Stainless steels such as STS 304, 316 and 630 are frequently used as shaft materials in small fiber reinforced polymer (FRP) fishing boats. If the shaft material is exposed to a severely corrosive environment such as seawater, it should be protected using appropriate methods. The impressed current cathodic protection was used to inhibit corrosion in shaft materials. In anodic polarization, passivity was remarkably more evident in STS 316 stainless steel than in STS 304 and STS 630. The pitting potentials of STS 304, 316, and 630 stainless steels were 0.30, 0.323, and 0.260 V, respectively. The concentration polarization due to oxygen reduction and activation polarization due to hydrogen generation were evident in the cathodic polarization trends of all three stainless steeds. STS 316 had the lowest current densities in all potential ranges, and STS 630 had the highest. Tafel analysis showed that STS 316 was the most noble in the three. In addition, the corrosion current density was the lowest for STS 316.     

Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Abstract

Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information was used to determine the partition coefficients k of the elements in each dissimilar weld. The dilution level was found to decrease as the ratio of volumetric filler metal feedrate to net arc power increased. Reasons for this behaviour are discussed in terms of the distribution of power required to melt the filler metal and base metal. In addition, the segregation potential of Mo and Nb was observed to increase (i.e. their k values decreased) as the Fe content of the weld increased. This effect is attributed to the decreased solubility of Mo and Nb in austenite with increasing Fe additions. Since the Fe content of the weld is controlled by dilution, which in turn is controlled by the welding parameters, the welding parameters have an indirect influence on the segregation potential of Mo and Nb. The results of the present work provide practical insight for corrosion control of welds in super austenitic stainless steels.     

钛合金焊缝金属在酸性硫酸铜溶液中的阳极极化行为

针对钛焊区优先腐蚀问题,研究了电子束扫描焊接对钛合金焊缝金属在酸性硫酸铜溶液中阳极极化行为的影响,结果表明,通过改变电子束扫描频率控制焊缝金属凝固组织形态及尺寸,从而控制的焊缝金属的阳极极化行为,找出了焊缝金属与母材相同的阳极极化曲线的最佳匹配,提出了通过控制焊接凝固结晶,使钛合金焊缝金属与母材均等耐腐蚀的新技术。     

The corrosion behavior of stainless steels and copper alloys exposed to natural seawater

The corrosion behavior of stainless steels, titanium and copper alloys exposed to flowing Pacific Ocean water was characterized using surface analytical and electrochemical techniques. Biofilm formation on stainless steels and titanium resulted in thin films of bacteria and diatoms that did not cause significant changes of the corrosion potential (E_corr) or surface properties. Rotating cylinder experiments indicated that both E_corr and corrosion rates for stainless steels and titanium were independent of mass transport. Four surface layers were identified on copper-containing materials: substratum metal; an inorganic chloride corrosion layer that contained alloying elements; a biofilm; and crystalline, spherical phosphate-rich deposits. All copper surfaces were colonized by bacteria independent of alloy composition. The complexity of the impedance spectra for copper-containing materials was attributed to formation of surface layers and contributions of charge transfer and mass transport controlled reactions mediated by the layers. Both anodic and cathodic reactions for copper-based materials were affected by mass transport.     

Grooving corrosion in electric resistance welded steel pipe in sea water

The mechanism of grooving corrosion in electric resistance welded (ERW) carbon steel pipe in sea water was investigated. Grooving corrosion is a selective localized corrosion of the weld. Scanning electron microscopic observation revealed that corrosion pits initiated immediately on the MnS inclusions and developed into grooving corrosion with the aid of the macro-cell formed between the anodic narrow weld and cathodic base metal of the pipe. High localized corrosion susceptibility of MnS inclusions in the weld is due to their surrounding S-enriched portions caused by the rapid heating and cooling in welding.     

Galvanic corrosion of laser weldments of AA6061 aluminium alloy

Galvanic corrosion of laser welded AA6061 aluminium alloy, arising from the varying rest potentials of the various weldment regions, was examined. The weld fusion zone is found to be the most cathodic region of the weldment while the base material is the most anodic region. The rate of galvanic corrosion, controlled by the cathodic process at the weld fusion zone, increases with time until a steady state maximum is reached. On galvanic corrosion the corrosion potential of the weld fusion zone shifts in the positive direction and the free corrosion current increases. It is proposed that the cathodic process at the weld fusion zone causes a local increase in pH that in turn causes dissolution of the surface film resulting in the loss of Al to solution and the increase of intermetallic phases. The increase in galvanic corrosion may result from either the build up of the intermetallic phases in the surface layer and/or significant increase in surface area of the weld fusion zone due to the porous nature of the surface layer.     

双相不锈复合钢板埋弧自动焊

研究了Ｈ１１／Ｘ２ＣｒＮｉＭｏＮ２２．５双相不锈复合钢板埋弧自动焊工艺，焊接材料的选择及其焊接性。结果表明，复层焊缝的δ铁素体含量在３５％～４５％内，其焊接性优良。过渡层的焊接采用较弱焊接规范和单道焊工艺，以控制较小的稀释率和良好的焊缝成形，防止在与基层焊缝的熔合线附近产生大量马氏体组织和其它硬化相。Ｈ１１／Ｘ２ＣｒＮｉＭｏＮ２２．５双相不锈复合钢板埋弧自动焊焊接接头具有良好的力学性能，复层焊缝具有极为优良的抗晶间腐蚀和应力腐蚀的能力。本研究为煤气工程中汽化炉的制造选定了合适的焊接材料并制定了最佳埋弧自动焊工艺。     

基于镀锡银钎料钎焊304不锈钢接头的腐蚀行为

为揭示304不锈钢钎焊接头的腐蚀行为,以BAg50CuZn钎料为基材,采用电镀扩散工艺制备AgCuZnSn钎料,对304不锈钢进行感应钎焊,在60 ℃,3.5% NaCl溶液中评价不锈钢接头的局部腐蚀性,借助扫描电镜对其腐蚀形貌进行分析. 结果表明,经NaCl溶液腐蚀后,钎缝与不锈钢界面出现较长的腐蚀沟;304不锈钢表面腐蚀较严重,存在大范围坑洞、裂纹等缺欠,而钎缝区几乎无腐蚀缺欠,优先被腐蚀的是富铜相. 随着腐蚀时间延长,钎缝和304不锈钢的腐蚀速率均呈现先升高后降低的趋势,钎缝腐蚀速率略低于母材;腐蚀2.5 h后,钎缝区和304不锈钢的平均腐蚀速率分别为0.098和0.104 g/(m2·h).     

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.     

30 Jahre anodischer Korrosionsschutz

30 years anodic corrosion protection The technical use of the anodic corrosion protection is entering the fourth decade. A summary of the development in the past thirty years verifies the efficiency of this kind of protection. Damage to un- and low alloyed steel construction, specially on the aluminium producing industry, by intercrystalline stress corrossion cracking in alkaline solution are stopped by anodic corrossion protection. The anodic corrosion protection allows the application of low cost materials in a “passive” state, e.g. stainless steels (1.4301, 1.4401) in acid equipment, especially sulfuric acid units, under high corrosive conditions, and no damage will occur. The anodic corrosion protection enables the use of equipment at a very high level of safety and economy.     

改善0Cr17Mn14Mo2N钢抗稀硫酸腐蚀性能的电化学研究

常用的铬镍奥氏体不锈钢含有贵缺的元素镍,这类钢代镍的问题,早就引起人们的注意。从1958年起,中国科学院金属研究所开展了以锰和氮代镍的铬锰氮奥氏体—铁素体复相不锈钢的研究。经过生产和使用部门的共同努力,已将一种0Cr17Mn14Mo2N钢作成设备和部件,应用到腐蚀性的尿素、醋酸、合成纤维和印染等工业介质。实践证明其耐蚀性和使用性良好。实验室的研究得出,这种钢在稀硝酸、磷酸、草酸和柠檬酸中具有与铬镍不锈钢相近或更高的耐蚀性。但在稀硫酸中,耐蚀性很差,不能使用。因此,有必要改善它的耐蚀性能。本文研究了合金元素在提高铬氮锰不锈钢腐蚀中的作用。     

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.