Cathode:anode ratio galvanic corrosion

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... 

Area effects in galvanic corrosion are very important. An unfavorable area ratio is a large cathode and a small anode. Corrosion of the anode may be 100 to 1,000 times greater than if the two areas were the same. This is the reason why stainless steels are susceptible to rapid pitting in some environments. Steel rivets in a copper plate will corrode much more severely than a steel plate with copper rivets. 

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

For galvanic corrosion tests it is important to maintain the same ratio of anode to cathode in the test sample as in the service environment. 

Experience shows that increasing the cathode-to-anode area ratio increases the rate of consumption of the anode and decreases the corrosion rate of the cathode, but the galvanic series alone would not allow a quantitative analysis of these effects. Inspection of Fig. 32 reveals that the abscissa has been changed to current from current density. When dealing with unequal areas, such a transfor-... 

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... 

Steel socket welds is a good example of rapid local corrosion. The potential difference between the anodic and the cathodic states drives the corrosion cells (this is an example of galvanic corrosion). Corrosion due to adjacent active-passive sites can be particnlarly rapid if the corrosion cell has an unfavorable anode/cathode area ratio. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Corrosion testing for galvanic corrosion may be predicted by ASTM standards in the form of potential measurements. The driving force for galvanic corrosion is the potential difference between the anode and cathode. The galvanic currents between two dissimilar metals are measured using a zero resistance ammeter (ZRA) for a chosen length of time. The ratio of anode to cathode areas is 1 1. 

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). 

When more and less noble materials are placed in contact, the more noble material offers an extra area for the cathodic reaction. Therefore flie total rate of the cathodie reaction is increased, and this is balanced with an increased anodic reaction, i.e. increased dissolution of the less noble material (galvanic corrosion. Section 7.3). If the more noble material (the cathodic material) has a large surface area and the less noble metal (the anodic metal) has a relatively small area, a large cathodic reaction must be balanced by a correspondingly large anodic reaction concentrated in a small area. The intensity of the anodic reaction, i.e. the corrosion rate (material loss per area unit and time unit) becomes high. Thus, the area ratio between the cathodic and the anodic materials is very important and should be kept as low as possible. It should be mentioned that in a galvanic corrosion process, the more noble material is more or less protected. This is an example of cathodic protection, by which the less noble material acts as a sacrificial anode (see next section). 

A metallic structure secured with rivets or screws (Figure 7.11) offers a particularly good illustration of how the surface ratio between anode and cathode affects the corrosion rate galvanic corrosion is much more damaging if the rivets, with their small surface area, are anodic relative to the sheet metal than vice versa. For the same reason, a weld requires solder whose corrosion potential is equal to or exceeds that of the structure. 

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