Comparison between open circuit and imposed potential measurements to evaluate the effect of temperature on galvanic corrosion of the pair alloy 31–welded alloy 31 in LiBr solutions

Galvanic corrosion between alloy 31 and welded alloy 31 has been studied under open circuit conditions using a zero-resistance ammeter (ZRA) in highly concentrated LiBr solutions at different temperatures. These results have been compared with results estimated from the polarization curves according to the mixed potential theory (imposed potential measurements). Results showed that the imposed potential measurements provided lower galvanic potentials and higher galvanic current densities than those obtained using open circuit measurements. The effect of temperature on the galvanic corrosion estimated using both techniques showed good agreement, since temperature increased the galvanic current density of the pair in both cases.     

Effect of temperature on the galvanic corrosion of a high alloyed austenitic stainless steel in its welded and non-welded condition in LiBr solutions

Galvanic corrosion generated by the coupling between the austenitic stainless steel Alloy 926 (UNS N08926) and the welded Alloy 926 has been studied by means of electrochemical methods. The materials have been tested in highly concentrated LiBr solutions at different temperatures. The effect of Li₂CrO₄ as corrosion inhibitor has also been evaluated. Galvanic corrosion has been studied under open circuit conditions using a zero-resistance ammeter (ZRA). Results have demonstrated the poor severity of the coupling between the Alloy 926 and the welded metal in the studied conditions. The probability of localized corrosion increased with temperature and concentration, as the galvanic current density and galvanic potential data demonstrated.     

Galvanic corrosion of titanium coupled to welded titanium in LiBr solutions at different temperatures

The galvanic corrosion generated between the titanium-welded titanium pair has been studied in heavy brine LiBr solutions at 25, 50 and 100 °C under open circuit conditions using a zero-resistance ammeter (ZRA). The results showed that welded titanium was the anode of the pair, so that its corrosion resistance decreases due to the galvanic effect. However, the extremely low galvanic current densities registered by the pair reveal the poor severity of the coupling under the studied conditions. Furthermore, it was observed that the electrodes were in the passive state, increasing the probability of localized corrosion with temperature.     

Corrosion behaviour and galvanic coupling of titanium and welded titanium in LiBr solutions

Corrosion resistance and galvanic coupling of Grade 2 commercially pure titanium in its welded and non-welded condition were systematically analyzed in LiBr solutions. Galvanic corrosion was evaluated through two different methods: anodic polarization (according to the Mixed Potential Theory) and electrochemical noise (using a zero-resistance ammeter). Samples have been etched to study the microstructure. The action of lithium chromate as corrosion inhibitor has been evaluated. Titanium and welded titanium showed extremely low corrosion current densities and elevated pitting potential values (higher than 1 V). The results of both methods, anodic polarization and electrochemical noise, showed that the welded titanium was always the anodic element of the pair titanium-welded titanium, so that its corrosion resistance decreases due to the galvanic effect.     

Effects of microplasma arc AISI 316L welds on the corrosion behaviour of pipelines in LiBr cooling systems

The effect of microplasma arc welding (MPAW) on the electrochemical and corrosion behaviour of AISI 316L stainless steel tubes has been studied. Scanning electrochemical measurements were performed in sodium chloride to evaluate the difference in the electrochemical activity of base (non-welded) and welded samples. Oxygen reduction rates increase in AISI 316L due to the heat treatment effect induced by welding, indicating a higher electrochemical activity in the welded samples. Additionally, the use of MPA weldments in lithium bromide (LiBr) absorption machines was also analysed at typical operating temperatures and Reynolds numbers. The welding process increases corrosion rates, hinders passivation and increases the susceptibility to pitting attack in LiBr. However, zero-resistance ammeter and localization index measurements show that the galvanic pair generated between the base and welded alloys is weak, both electrodes being in their passive state. Temperature greatly affects the corrosion process.     

The effect of temperature on the galvanic corrosion of the copper/AISI 304 pair in LiBr solutions under hydrodynamic conditions

The corrosion behaviour of copper and AISI 304 stainless steel and the galvanic corrosion generated by the copper/AISI 304 pair, have been studied by electrochemical methods. These materials have been tested in an 850 g/L LiBr solution at different temperatures (25-75 °C) and at different Reynolds numbers (1456-5066) in order to study their performance in absorption machines. Results show that copper was always the anodic element of the pair and its corrosion resistance decreases due to the AISI 304 stainless steel galvanic effect. Galvanic corrosion increases with temperature and Reynolds number. However, it was proved that the effect of temperature on galvanic corrosion is more influential than the Reynolds number effect. This fact is also certain for corrosion of uncoupled copper and for corrosion of AISI 304 stainless steel. Experimental values of the corrosion current densities fit well the Arrhenius plot at all the Reynolds numbers analysed and a potential relation between the corrosion current densities and the Reynolds number has been found.     

Effect of different micro-plasma arc welding (MPAW) processes on the corrosion of AISI 316L SS tubes in LiBr and H3PO4 solutions under flowing conditions

This work studies the corrosion under flowing conditions of four kinds of AISI 316L materials welded by the micro-plasma arc welding technique in different media: a basic (LiBr) and an acidic (H₃PO₄) solution by means of polarization measurements. Sensitization tests, galvanic corrosion evaluation, microstructural analyses and microhardness tests have been also carried out. Corrosion parameters revealed that, among the materials welded with backing gas, the alloy which presented better corrosion behaviour was the one welded without filler alloy. However, this kind of material could undergo several corrosion problems if a crack is formed or due to an inadequate joint penetration.     

Imposed potential measurements to evaluate the pitting corrosion resistance and the galvanic behaviour of a highly alloyed austenitic stainless steel and its weldment in a LiBr solution at temperatures up to 150 °C

Pitting corrosion resistance and galvanic behaviour of Alloy 31, a highly alloyed austenitic stainless steel (UNS N08031), and its weldment were studied in a heavy brine LiBr solution 1080 g/l at different temperatures (75–150 °C) using electrochemical techniques. The Mixed Potential Theory was used to evaluate the galvanic corrosion between the base and welded metals. Cyclic potentiodynamic curves indicate that high temperatures make passivation and repassivation of pits difficult, because the whole passivation range and the repassivation potential values decrease with temperature. The critical pitting transition occurs between 100 and 125 °C.     

Effect of aqueous LiBr solutions on the corrosion resistance and galvanic corrosion of an austenitic stainless steel in its welded and non-welded condition

The corrosion resistance and galvanic behaviour of a highly alloyed austenitic stainless steel (UNS N08031) and a nickel-base alloy (UNS N06059) in its welded (GTAW) and non-welded condition were analysed in LiBr solutions by means of electrochemical measurements. Samples microstructure was studied by SEM and EDX analysis. The alloys considered showed passive behaviour and they were able to repassivate after the breakdown of the passive film. The pitting resistance of the materials increased as the LiBr concentration decreased. With regard to galvanic corrosion, the welded metal was always the anodic element of the UNS N08031-welded metal pair.     

Corrosion behaviour of micro-plasma arc welded stainless steels in H3PO4 under flowing conditions at different temperatures

This paper studies the general corrosion behaviour of the micro-plasma arc welded AISI 316L stainless steel in phosphoric acid at different temperatures (25–60 °C) and at a Reynolds number of 1456. Galvanic corrosion has been studied using zero-resistance ammeter (ZRA) measurements and polarization curves (by the mixed potential theory). Results show that the microstructure of the stainless steel is modified due to the micro-plasma arc welding procedure. Coupled current density values obtained from polarization curves increase with temperature. ZRA tests present the highest i_G values at 60 °C; however, the values are very close to zero for all the temperatures studied. This is in agreement with the low value of the compatibility limit and of the parameter which evaluates the importance of the galvanic phenomenon. Both techniques present the most positive potentials at the highest temperature. This study reveals that micro-plasma arc welded AISI 316L stainless steels are appropriated working in the studied H₃PO₄ media from a corrosion point of view for all the temperatures analysed.     

Stress corrosion cracking of gas-tungsten arc welds in continuous-cast AZ31 Mg alloy sheet

AZ31 Mg alloy sheet was welded using a gas-tungsten arc (GTA) process over inserts containing 2.3–9.3 wt.% Al. The welded specimens were susceptible to SCC in distilled water, with susceptibility increasing with decreasing weld metal Al (or β particle) concentration. Primary stress corrosion cracks initiated at the weld metal–HAZ interface by stress-assisted localised dissolution and propagated through the weld and base metals by transgranular and intergranular H-assisted fracture (TG-HAF and IG-HAF) respectively. The IG fracture mode may be intrinsic to the texture imparted upon the base metal by rolling. The increase in SCC susceptibility with decreasing weld metal Al concentration is contrary to the purported roles of β particles in promoting localised corrosion and as crack nucleation sites, but corresponds with increases in weld – base metal galvanic current density and weld metal localised corrosion susceptibility.     

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.     

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 pair in aqueous LiBr solutions

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 stainless steel pair in a concentrated lithium bromide solution was investigated according to the mixed potential theory. A hydraulic circuit was designed to study dynamic corrosion processes in situ. A potential relation between corrosion current density (i_corr) and Reynolds number (Re) was found for copper, showing a mixed control of a chemical step and mass transport through the corrosion products film with the predominance of the former. No dependence of i_corr on Re could be established for AISI 304, showing a chemical step control. Moreover, under stagnant conditions, partial passivation may occur in AISI 304; however, under flowing conditions passivation is not possible. Copper is the anodic element of the pair under all flowing conditions analysed. The galvanic phenomenon is more important as Re increases, but the results show compatibility of both materials at all Re values analysed. Similarly, a potential relation between galvanic current density (i_G) and Re was found, showing a mixed control of a chemical step and mass transport with the predominance of the latter. Copper corrosion resistance decreases more rapidly as Re increases due to the AISI 304 galvanic effect: there is a synergy between the galvanic effect and the hydrodynamic conditions. Under stagnant conditions, the galvanic behaviour of the materials is close to the compatibility limit and an inversion of the anodic element of the galvanic pair takes place.     

Corrosion behaviour of Mg-Cu and Mg-Mo composites in 3.5% NaCl

The corrosion behaviour of pure magnesium, Mg-Cu (0.3, 0.6, and 1 vol.%) and Mg-Mo (0.1, 0.3, and 0.6 vol.%) composites has been studied in 3.5% NaCl solution by weight loss and polarisation methods. Corrosion rates determined by weight loss method were considerably higher than that determined by polarisation method. The corrosion rate increased with increasing volume fraction of reinforcement in Mg-Cu and Mg-Mo composites. At the same volume fraction of reinforcement, molybdenum reinforced composite corroded faster than copper reinforced composite. The galvanic current density between Mg-Cu and Mg-Mo couples has been experimentally measured using zero resistance ammeter technique. The experimentally observed galvanic current densities were in close agreement with those obtained using mixed potential theory analysis. SEM observation of corroded samples confirmed microgalvanic activity at the matrix/reinforcement interfaces. The poor corrosion resistance of composites has been attributed to microgalvanic effects between the matrix and reinforcements and inferior quality of surface films.     

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.     

核电汽轮机焊接转子接头在氯离子环境中的电偶腐蚀行为

为了研究焊接接头的尺寸效应对焊接接头的电偶腐蚀行为的影响,采用宏观电化学、微区电化学（SVET）、浸泡试验等试验研究了25Cr2Ni2MoV转子钢焊接接头在氯离子溶液中的电偶腐蚀行为.结果表明,焊缝的腐蚀电位最低,腐蚀电流密度最大,焊缝区作为阳极优先发生腐蚀,而母材和热影响区作为阴极区被保护.进一步运用三维形貌仪（IFM）和扫描电镜（SEM）观察腐蚀表面形貌,发现随着焊接接头尺寸的增加电偶腐蚀效应更显著,但是当接头尺寸增大到一定程度后,电偶腐蚀效应又有所减弱.     

Investigating corrosion processes in the micrometric range: A SVET study of the galvanic corrosion of zinc coupled with iron

The galvanic corrosion of an iron/zinc pair immersed in aqueous 0.1 M Na₂SO₄ solution has been investigated by using the scanning vibrating electrode technique (SVET). In this way, investigations in the micrometer range of the progress of the electrochemical reactions involved in galvanic process were performed. The anodic oxidation process is observed to be initiated on the zinc sample in a localized manner, whereas the cathodic reaction involving the electroreduction of dissolved oxygen is homogeneously distributed over the iron sample. This later process is the rate determining step in the overall corrosion process, as demonstrated by the changes in the ionic and galvanic currents measured in the system when the area of the iron specimen is varied relative to that of zinc. The occurrence of coupled chemical reactions in the solution phase involving the products of the corrosion reactions could also be deduced from the integration of the ionic currents measured for each half-reaction during a SVET scan. Thus, the corrosion processes involved in the galvanic coupling of iron and zinc have been further understood by using this microelectrochemical technique appropriately, helping to better interpret large scale measurements.     

E550钢埋弧焊接接头在模拟海洋大气环境中的腐蚀行为

采用增重测量、锈层截面观察、物相定量分析和电化学实验等方法研究了低合金耐候钢E550埋弧焊焊接接头在室内干湿循环加速腐蚀中的行为。结果表明:E550钢焊接接头的腐蚀速率低于母材。E550母材与焊缝的成分差异导致焊接接头不同部位的电位不同,并引起电偶腐蚀。电偶腐蚀对常规的大气腐蚀产生了抑制作用,造成焊接接头腐蚀前沿平整,锈层缺陷减少。随着表面锈层的生长,焊接接头表面不同部位的电位发生不同程度变化,并造成反复发生极性逆转,使腐蚀变得均匀,减弱了电偶腐蚀的局部腐蚀特征。     

Corrosion behavior of SiC reinforced magnesium composites

The corrosion behavior of two SiC reinforced Mg-based metal matrix composites, Mg-6SiC and Mg-16SiC (in volume percent), has been studied in freely aerated 1 M NaCl solution and compared with that of pure Mg. The presence of SiC particles deteriorated the corrosion resistance of magnesium. Corrosion resistance decreased with increasing SiC volume fraction. The galvanic corrosion current density between pure SiC and pure Mg has been experimentally measured using zero resistance ammeter technique and theoretically determined using mixed potential theory. Galvanic corrosion between Mg matrix and SiC reinforcement in the composites did not contribute significantly to the overall corrosion rate. Electrochemical impedance spectroscopy indicated that the higher corrosion rates for the composites could be related to the defective nature of surface film.     

Über den Einfluß des Flächenverhältnisses auf die Stromdichte in galvanischen Zellen und Belüftungszellen

Influence of the area ratio on the current density in galvanic and aeration cells With a view to corrosion phenomena on conventional metal combinations the corrosion current density has been studied as a function of the cathode: anode area ratio in aeration cells (Fe/Zn, Fe/Fe, Zn/Zn, with Na-sulfate and NaCl as electrolytes) and in galvanic cells (Cu/Fe, Cu/Zn, with Cu? , ammonium and zinc sulfate as electrolytes). It was the aim of the investigation to find out the extent to which the theoretical ideas of Mansfeld and Akimov concerning film rupture and diffusion control may be applicable. It is found that mansfeld's concept must be extended to include the effective electrode potentials in order to allow an interpretation of corrosion current density in aeration cells. In the case of galvanic cells, however, there is a limitation attributable to the cell resistance which is determined by independent resistance measurements. The corrosion behaviour of the metals studied is interpreted in terms of the measured current densities.