Galvanic anodes testing

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

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. 

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].

Duration of exposure is extremely important in galvanic corrosion testing. In many galvanic couples, the amount of corrosion is highly variable with time. Initial high corrosion rates may fall dramatically, initial low rates may suddenly increase, and the relationship between anodic and cathodic materials can even reverse with time. For these reasons, the duration of the test exposure should be of sufficient length to ensure that the corrosion processes have reached steady... 

Figure 5.17 Typical galvanic anode installation layout and test points. (By kind permission of Cheveron Corp., USA)...

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

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. 

---