Galvanic corrosion coupled anodic surface

Corrosion is due to electrochemical potential differences (galvanic corrosion) between the HAZ/fusion line and the parent material, attributed to the unstable MnS inclusions produced during the welding cycle. It was observed that enhanced corrosion of the weld metal was due to electrochemical potential differences between the weld metal and the base metal, such that the weld metal is anodic in the galvanic couple. The potential difference may only be of the order of perhaps 30-70 mV, but the low surface area ratio of anode to cathode results in high corrosion rates (1-10 mm). (Bond)5... 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

Case Study 6.1—Effect of the Ratio of the Surface Area of the Cathode to the Surface Area of the Sacrificial Anode on Galvanic Corrosion of the Tin-Platinum Galvanic Couple... 

Fig. 6.6 Effect of the cathode-sacrificial anode surface area on galvanic corrosion of a tin-platinum galvanic couple.

Fig. 6. 27 SEM analysis of anode coating top surfaces after the 20 h galvanic corrosion test (a) Ni8o-Al2o (b) Nigs-Ahs (c) Nigo-Aho (d) Ni/graphite coupled to Nigs-Alos [80].

E6.6. Sn and Pt are immersed in an acidic solution with unit hydrogen ion activity. Using the electrochemical parameters listed below, construct the Evans diagram and evaluate the effect of the cathode-sacrificial anode electrode surface area ratio on galvanic corrosion of a tin-platinum galvanic couple (see Case Study 6.1). 

Galvanic corrosion can occur in a polycrystaUine alloys, such as pearMc steels, due to differences in microstmctural phases. This leads to galvanic-phase coupling or galvanic microceUs between ferrite (a-Fe) and cementite FesC) since each phase has different electrode potentials and atomic stmcture. Therefore, distinct localized anodic and cathodic microstmctural areas develop due to microstmctural inhomogeneities, which act as micro-electrochemical cells in the presence of a corrosive medium (electrolyte). This is an electrochemical action known as galvanic corrosion, which is mainly a metallic surface deterioration. 

The surface area of the electrode in the corrosive medium is known as the relative area, which influences the rate of galvanic corrosion. For instance, the experimental data on brass/steel couple in 20%NaCl at room temperature reported by Jones [19] can be used to determine that increasing the surface area ratio increases the the galvanic corrosion potential. Therefore, the corrosion potential of galvanic couplings is strongly dependent on the cathode-to-anode surface area ratio. 

However, the situation can lead to the corrosion of steel when the surface area of the cathode (copper) approaches that of the anode (steel) such as shown in Fig. 7.20 or when the surrounding environment is more conductive or corrosive in the region where the galvanic coupling exists (Fig. 7.21). In the example shown in Fig. 7.20, the anodic corrosion of the anchor is partly due to the galvanic corrosion cell formed by the... 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

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