Evaluation of corrosion rate AK80 and ZK60 Mg alloys - Essay Example

(R.K. Sinnott, 2005) (2) Galvanic Corrosion- When two dissimilar metals are in contact in an atmosphere conducive to corrosion, then by its position in the electrochemical series one forms the anode and the other forms the cathode. The anode thus formed readily corrodes. This principle is used to protect the outside hull of ships by using sacrificial zinc anodes. By its position in the electrochemical series zinc forms the anode and undergoes material erosion thereby protecting the steel hull.

(Oliver M. Siebert, B.et al; 1997) (3) Pitting- This is a highly localized form of corrosion. Usually materials are given a good surface finish to prevent its occurrence. The rate of corrosion in such cases is difficult to predict because once the corroding point breaks through the surface finish the material degradation progresses in a rapid manner. This proceeds by forming a pit and in an oxygenated atmosphere its activity is increased. Slag inclusion and entrapment of air during welding and formation of bubbles in pipes and valves during liquid flow all lead to pitting corrosion. (4) Inter-granular Corrosion-This is a form of corrosion which happens in alloying materials.

Since alloys are a mix of different metals, the impurities tend to accumulate at the grain boundaries during heat treatment. This leads to propagation of corrosion although the amount of material corroded is small. (Oliver M. Siebert, B.et al; 1997) (5) Stress induced corrosion- This form of corrosion proceeds when materials are subjected to highly stressed environments. After a welding process, there will always be residual stresses that need to be relieved by post weld heat treatment. Failure to do so will lead to these stresses creating cracks and fissures in the welds.

Materials that are subjected to repeated cyclic loads may fail due to fatigue stresses that are generated with time. Under such conditions the