Chapter C.13 - Galvanic corrosion

Laboratory tests were carried out to compare the electrochemical characteristics of galvanic couples made of aluminum alloys (1050, 5086, 2024, and 7075) with other metals (plain steel, Type 316 stainless steel, titanium) in tap water and in artificial sea water. It was found that: One must first choose materials which individually resist corrosion. Only when this requirement is fulfilled can the risk of galvanic corrosion be evaluated. In aluminum-steel assemblies, taking into account only the risk of galvanic corrosion leads to the following bad choices: (1) the choice of copper containing aluminum alloys which are less anodic than copper free alloys (poor choice except when justified by mechanical factors), and (2) the choice of plain steel which is less noble than stainless steel. Although they generate weaker galvanic currents, field tests show that those assemblies made of corrosive materials have shorter life than stainless steel assembled with a copper free aluminum alloy. The galvanic current produced by the couple decreases rapidly with time, then stabilizes within a few days. The comparison of these stabilized currents seems to be the best way to compare the risks of galvanic corrosion. For a given couple of different metals, the life span of the assembly depends upon the conductivity and the aggressiveness of the environment; in other words the complexity and the cost of the protection to be used, for a given couple increase with the aggressiveness of the environment.

Galvanic corrosion In couples of AISI 304 stainless steel (SS), Ferralium(1) Alloy 255, Hastelloy(1) Alloys B-2 and G-3, grade 2 titanium, and graphite was examined in boiling 10% sulfuric and ambient-temperature hydrochloric acids. The galvanic corrosion rates were determined by measurement of the galvanic current using (1) a zero resistance ammeter technique and (2) weight loss measurements. Prediction of galvanic corrosion was attempted by determining the potentiodynamic polarization curves of the individual alloys and combining these with an iterative numerical solution for the zero potential difference between the alloys. In some cases, a reasonably good correlation was obtained between the predicted and measured galvanic corrosion rates. No correlation was found between the measured open circuit potentials (OCPs) (galvanic series) and galvanic corrosion rates. The effect of changing the area ratios was also examined.

The Mauritian textile industry is, nowadays, facing serious competition on the international market. With the aim of improving its productivity, corrosion has been identified as a major cost for the industry due to the huge amount of chemicals used in the dyehouses. The present study was therefore performed at one of the major dyehouses in Mauritius so as to identify the different types of corrosion occurring, the effect of the chemicals employed on the common metals used in the dyehouse and the cost of corrosion in the plant. The different types of corrosion occurring were assessed through visual inspection on the pipes, accessories and machines in the dyehouse. The effect of the chemicals on the common types of metals used was investigated through electrochemical tests using the Tafel technique. Three different types of metal and seven different types of chemicals were tested. It was found that low carbon steel had the highest corrosion rate among the metals tested and stainless steel UNS 31603 was the more corrosion resistant. Hydrochloric acid was the most corrosive fluid. The cost of corrosion over a yearly period was estimated to be $450,000 and this was 75% of the maintenance cost.

Corrosion, Galvanic

Corrosion impacts the value, safety, and readiness of many assets in almost all industries. The cost of maintaining and repairing runs into astronomical numbers. The current approach to evaluate susceptibility to galvanic corrosion is by determining the relative position of metals in the galvanic series table valid for a given environment. However the potential difference between metals provides no information on the kinetics of galvanic or bimetallic corrosion. Additionally, there is a wide range of factors which affect the corrosion severity like surface area ratio, flow rate, temperature, dry-wetting cycle, inhibiting effects, etc. Moreover, in most mechanical assemblies the overall effect of multiple material components and the complexity of the geometry make corrosion prediction difficult. In this article a unique 3D simulation technology to predict galvanic corrosion will be presented. Potential distributions and corrosion rates are calculated based on electrochemical characterization of the different materials in lab conditions. Corroded areas are visualized by the software tool and are compared with real-life observations on assemblies. The examples show cases where engineers select different material combinations of complex assemblies in a virtual environment during the design process and assess the corrosion behavior of the complete system under various environmental conditions. The technology provides quantitative information on the corrosion risk as well as the corrosion rate. Thus, ensuring target performance and safe operation.

In this study, the galvanic effect of non-ferrous fasteners materials with similar base plate were investigated. Five candidate nickel based fastener materials were immersed in natural flowing seawater at ambient temperature and pressure. These materials were electrically attached to Alloy 625 (N06625) with greater than ten times the surface area. Corrosion currents and open circuit potential were measured for twenty-eight to one hundred and five days. Corrosion rates were calculated from nominal current densities and are presented. Results indicated that nickel base fasteners alloys may suffer from galvanic corrosion even in cases where open circuit potentials were similar to the structural material. INTRODUCTION Traditional use of fasteners in the marine environment encompasses abroad range of conditions. These conditions include full immersion and atmospheric as well as the environs created at the air/water and mud/water interfaces. This is complicated by the dynamics of low or stagnant flow and seawater velocities. Each of these situations would require thorough investigation prior to selection of fastener and structural materials. Typically, the mechanical requirements of the design would limit the fastener choices. The corrosion analysis would evaluate the material for known environmental effects and compatibility with the adjoining structure. As a general rule, the fastener should be cathodic when compared to the structure to avoid an unfavorable anode to cathode surface area ratio.l This means that if corrosion does occur, the structure would be the anode and would corrode. As corrosion resistant materials find broader use in structural applications, the choice of fastener materials are limited. The fastener material should be cathodic to the base metal and therefore should not corrode. In the case of steel structures, corrosion resistant materials such as the nickel base alloys maybe acceptable choices for fasteners. Strength requirements and resistance to corrosion may indicate the use of Monel K-500 (N0550) as a fastener material. The fastener would probably not corrode due to the cathodic protection it receives from the steel structure. When the structure requires an alloy with higher corrosion resistance, selection of the fastener material will have to account for any change in anode/cathode relationship. The fastener must be compatible with the structure so that the threads or clearance hole would not deteriorate to the point of failure. Comparison of alloys using the galvanic series in seawater may not be a good indicator of performance. This study is part of a larger investigation to select materials for fastening nickel base alloys. Mechanical properties and other environmental effects are important aspects of the application, but this work was primarily concerned with galvanic compatibility. EXPERIMENT Objective Our primary objective was to identify a fastener material which was galvanically compatible with an Alloy 625 (N06625) structure. Measurement of corrosion current between the fastener materials and the base plate was to provide the essential data. At the same time, other forms of corrosion would be monitored. Seawater immersion Seawater tanks constructed of polyethylene and measuring ten inches by fourteen inches with a depth of nine inches were used for exposure of specimens.

Crude oil is an energy resource that mostly composed with hydrocarbons and trace corrosive compounds as well found some cruxes in the petroleum refining industry because of the vast applications of the different types of metals. There were expected to investigate the effect of two different types of crude oils on the corrosion of seven types of selected ferrous metals in the pertinent research. The concerned corrosive properties of both crude oils were tested by standard methods and instruments also the chemical compositions of selected ferrous metals were tested. The corrosion rates of a series of similar size prepared metal coupons were tested after limited immersion time periods as three trials in crude oils by the weight loss method since aiding the microscopic analysis to identify the corrosion compounds. Apart from those speculations the decayed metallic elemental mounts from the metals into crude oils and the reductions of the initial hardness of metal coupons were tested. The obligatory results showed the relatively higher corrosion rates from carbon steels, relatively lower corrosion rates from stainless steels in both crude oils, formation of FeS in most of observations, decay of ferrous and copper into crude oils from some metals and slight reductions of the initial hardness of metal coupons due to the formation of the corrosion on the metal surfaces.

A Type 1 niobium (Nb) part and a grade 5 titanium (Ti6Al4V) part were joined by laser weld, to form a component to be used in implantable medical devices for electrical stimulation therapies. This component could be exposed to body fluid or particularly water which could penetrate through the medical adhesive seal over the time of implantation. To meet the device reliability, performance, and patient safety goals, electrochemical immersion testing was conducted to study corrosion of the base metals and their galvanic couples and to evaluate corrosion resistance of the weld joints. Samples of as-manufactured Ti6Al4V parts, Nb parts, and weld joints were prepared, cleaned and inspected for testing (Figure 1, Table 1). A wire made of the same material and mostly sealed by medical adhesive, was welded to each sample for electrical connection. The surface areas of samples were calculated. They were divided into groups and tested at 37°C in glass jars containing de-ionized (DI) water or phosphate buffered saline (PBS), respectively. Electrochemical tests including open circuit potential (OCP); cathodic polarization (CP) or anodic polarization (AP); galvanic corrosion or zero resistance ammeter (ZRA) testing; anodic cyclic polarization (ACP) and potentiostatic polarization (PP) were performed at different times during immersion of the samples. The potentiodynamic scan rate for CP, AP and ACP scans was 0.2 mV/s. The tests used three-electrode configuration consisting of a working electrode (WE) which was the testing sample; a reference electrode (Ref.) which was a Ag/AgCl electrode in 3 M NaCl solution; and a counter electrode (CE) which was a piece of platinum sheet, or for galvanic corrosion testing, which was the secondary working electrode of the Nb/Ti6Al4V galvanic couple. Visual and stereomicroscopic examinations were made during and after the testing with a total of immersion time of about 100-130 days. After 3-4 days of immersion, OCP was stabilizing with an average of about 0.08, 0.05 and 0.07 V vs. Ref. in DI water and 0.11, -0.02 and 0.06 V vs. Ref. in PBS, for the Ti6Al4V, Nb and joint samples, respectively. Though the OCP difference was small, it indicated Nb would act as anode when joined or coupled to Ti6Al4V. Galvanic corrosion could occur in Nb of the joint, especially considering the anode (Nb) to cathode (Ti6Al4V) area ratio was as small as 0.01. A galvanic current was estimated to be about 9-13 nA/cm2 in DI water and 24-42 nA/cm2 in PBS based on anodic polarization of Nb and cathodic polarization of Ti6Al4V in potentiodynamic scans (Figure 2a). Further, galvanic corrosion potential and current (Figures 2b and 2c) of the Nb/Ti6Al4V couples was measured by ZRA testing. After ~48 days of testing, the galvanic current was stabilized with an average of about 1 nA/cm2 in both DI water and PBS, indicating a stable passive film formed on Nb surface. The average galvanic corrosion rate of Nb of the couples was about 0.008 μm/year in both DI water and PBS, calculated from the measured galvanic current which was considered more accurate (as obtained in a more steady state) and much lower than the estimation from potentiodynamic scans. ACP curves of the joint and base metal samples showed similar polarization behavior in both DI water and PBS (Figure 2d). All samples displayed stable passivation with low current from OCP to a potential of Ep ≅0.8 V vs. OCP. Beyond Ep, current increased markedly, and small hysteresis formed in ACP scans for the joint and Nb samples. Additional potentiostatic polarization was thus performed on the joint and Nb samples in PBS for ~4 days at potentials of 1.2 V and 1.4 V vs. Ref. which were above Ep and in the hysteresis range. The current would stabilize at the increased levels for both potentials. Neither localized pitting or crevice corrosion attack nor active gas bubbling was observed on the joints and parts. Corrosion rate estimated from the increased current at 1.4 V vs. Ref. for Nb could be as high as 66 μm/year. Depending on the type of device design, an electrochemical potential possibly applied to the joint component was assessed under simulated electrical signals for stimulation, which was measured to be under 0.3 V vs. Ref. and smaller than Ep. Therefore, the joint materials of Nb and Ti6Al4V are stable and suitable for the device application with no concern of corrosion. Figure 1

Galvanic corrosion between two different kinds of steel rebars is usually the case in practical engineering. Open circuit potential (OCP), linear polarization resistance (LPR), Tafel polarization, scanning vibrating electrode technique (SVET), scanning electron microscopy (SEM) and reflection digital holographic microscopy (DHM) were used to study the galvanic corrosion of a novel corrosion-resistant steel bar (CR) and low-carbon steel bar (LC) in simulated concrete pore solutions with different pH values and a chloride ion concentration of 5 mol L−1. The pH of the simulated concrete pore solution had a significant impact on the corrosion behaviour of CR and LC when they were in contact and were attacked by chloride ions. As the pH increased, the potential between CR and LC decreased and the driving force for the galvanic corrosion decreased. When the pH was 9.0, galvanic corrosion occurred on CR and LC at a high rate. CR developed local pitting corrosion, while LC mainly developed uniform corrosion, each with an apparent accumulation of corrosion products on the sample's surfaces. When the pH was 11.3, galvanic corrosion occurred when CR and LC were in contact. CR showed a relatively smooth surface, with only a small amount of pitting corrosion. In contrast, LC developed both pitting corrosion and uniform corrosion, and both apparent pitting corrosion and an accumulation of corrosion products on the sample surface were observed. When the pH was 13.6, there was no galvanic corrosion when CR and LC were in contact; the corrosion of CR and LC was mainly pitting corrosion. Therefore, for regions with chloride ion corrosion and severe carbonization, the galvanic corrosion between CR and LC cannot be ignored.

Chapter 2.1.4 - Electrometallurgical Principles

Welding has a very important role in engineering and metal production repairs. This is very reasonable because every construction can hardly be separated from metal which involves welding elements. Metal is widely used because it has several advantages, including being strong, rust-resistant, resistant to wear and tear and the raw materials are very economical. The development of technology and science has also had an impact on the increasing demand for metal connections of different types. Several industries that use joining by welding different types of metal include the transportation industry, car manufacturing, shipping and airplanes. One thing that can be done is connecting different types of metal which can be applied in the shipping industry. This research aims to determine the impact of variations in welding FCAW Grade A material 16 mm thick on ship deck insert plates. The use of Flux Core Arc Wellding (FCAW) is often found in the shipping industry. FCAW or what is usually called CO2 arc welding actually includes Metal Inert Gas (MIG) welding, only the protective gas is not only a noble gas such as Argon (Ar) but also CO2 gas or it could be a mixture of the two gases. The FCAW welding process is often used because the protective gas used is CO2 so it is suitable for use in welding steel construction. CO2 decomposes into CO and O2 when in contact with high temperatures. As a result of the high freezing speed, the CO gas produced is trapped in the metal which then forms cavities. Efforts are made to prevent this by providing a mixture of Si and Mn to the electrode so that no gas is trapped and no reaction occurs.

Galvanic corrosion between dissimilar materials was investigated using laboratory simulations of oil/gas production environments. Galvanic corrosion of materials used in production equipment (4130, 9 Cr, 13 Cr, 2205, 718, 825, NIC 32, NIC 42, SM 2550, Beta-C Ti, C-276, 925) was studied in corrosive environments, which included sweet well produced fluids, sour well produced fluids, heavy brine packer fluids, and acidizing fluids. Corrosion coupons of various geometries were used to measure corrosion rates and morphologies. Electrochemical measurements were performed to determine potentials and current densities. The experimental study found that the severity of galvanic attack is a strong function of the type of corrosion products that form on metal surfaces. Galvanic interactions are mitigated in produced fluids where carbonate and/or sulfide scales dominate the corrosion morphology. Carbonate scales tend to block long-range galvanic currents and sulfide scales tend to short-circuit them. As a result, coupling of dissimilar materials in produced fluids may be less of a problem than suspected. In more aggressive fluids, such as acidizing or packer fluids where protective scales do not form, the severity of galvanic corrosion is much more pronounced. In these situations, however, many chromium-containing materials actively corrode, and their chromium content provides short-range galvanic assistance to dissolution, thus reducing long-range effects provided by coupling to dissimilar materials. A galvanic series was constructed based on coupon data and electrochemical measurements for each of the four environments examined.

Galvanic corrosion is an electrochemical phenomenon that arises due to the coupling of two different metals in an electrolytic environment, resulting in the deterioration of the less noble metal at an accelerated rate. This phenomenon poses a significant challenge in the economy of mixed-metal assemblies in many industrial applications due to the high maintenance and replacement expenditures that such systems incur. In this study, a stainless steel tube was galvanically coupled with a carbon steel fitting, and both were immersed in a chloride solution to study the galvanic interactions. The electrochemical processes associated with galvanic corrosion were simulated using a finite element multiphysics modeling approach (COMSOL Multiphysics). The simulations reproduced the metal–electrolyte interface potential and current density as well as the preferential anodic dissolution of carbon steel over stainless steel, which was observed during the coupled polarization. The numerical results matched the results predicted using assumptions for the steels’ electrochemical behavior. The results of the study confirmed that finite element simulation is an effective means of modeling galvanic corrosion and optimizing the design and life of metal component assemblies that are subjected to highly aggressive environments such as high-chloride environments. The numerical results matched the trends observed from experimentation and those previously reported in the literature and serve to provide qualitative and semi-quantitative insight regarding galvanic corrosion mechanisms instead of complete corrosion predictions regarding long-term corrosion behavior.

Recently the use of dissimilar metals in spine instrumentation has increased, especially in the case of adult deformities, where rods made from Cobalt Chrome alloys (CoCr) are used with Titanium (Ti) screws. The use of dissimilar metals increases the risk of galvanic corrosion and patients have required revision spine surgery due to severe metallosis that may have been caused by corrosion. We aimed to assess the presence of corrosion in spine implant retrievals from constructs with two types of material combinations: similar (Ti/Ti) and dissimilar (CoCr/Ti). First, we devised a grading score for corrosion of the rod-fixture junctions. Then, we applied this score to a collection of retrieved spine implants. Our proposed corrosion grading score was proven reliable (kappa > 0.7). We found no significant difference in the scores between 4 CoCr and 11 Ti rods (p = 0.0642). There was no indication that time of implantation had an effect on the corrosion score (p = 0.9361). We recommend surgeons avoid using implants designs with dissimilar metals to reduce the risk of corrosion whilst a larger scale study of retrieved spine implants is conducted. Future studies can now use our scoring system for spine implant corrosion. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 632-638, 2018.

The galvanic corrosion of GW103 and AZ91D coupled to an Al alloy (Al 7005 [UNS A97005]) in an ethylene glycol (EG [C2H6O2]) solution at ambient (25°C) and elevated temperatures (90°C) was investigated using open-circuit potential (OCP), coupling potential (CP), galvanic current density, potentiodynamic polarization curve, and electrochemical impedance spectroscopy (EIS). Moreover, weight-loss measurement and scanning electron microscopy (SEM) also were used to investigate the galvanic corrosion damage of the coupled Mg alloys. The galvanic corrosion behaviors of the couples were compared in the EG solution before and after a combination of inorganic inhibitor sodium phosphate (Na3PO4) and organic inhibitor sodium dodecylbenzenesulfonate (C18H29NaO3S) was added to the solution. It was found that both the general corrosion and galvanic corrosion of the Mg alloys were mitigated by the organic-inorganic inhibitor package. The most interesting finding was that the anode and cathode of the Mg alloy-Al alloy couples were reversed by adding inorganic and organic inhibitors into the blank EG solution. The anode-cathode reversal can be attributed to the OCP changes of the anode and cathode after inhibitor addition. The measured galvanic behavior was in good agreement with the mixed-potential theory predictions.