The state of aggregation of the metal, crystal size, presence of different phases within the matrix and their distribution are factors known to influence corrosion (Matsushima 2011a,b, Wang et.al. 2014, Davis 2006). Between the weld metal, which during the welding process reached and usually exceeded the metal melting point and the relatively much cooler parent metal, zones exist within which all intermediate temperatures are reached during welding. Each of these different temperatures and the subsequent cooling of the metal promote localized changes in the microstructure. This may include changes also to micro-alloys and micro-constituents. Such changes in nature, form, size and distribution of the micro-constituents and alloys also can be obtained by conventional heat treatment techniques. As is well-known, heat treatment is an operation involving the heating of the solid metal to defined temperatures, followed by cooling at suitable rates in order to achieve the desired results. Examples include Annealing (controlled slow-cooling), normalizing (naturally air cooled) and quenching (fast cooled by water immersion) (Palmer & King 2004, Braestrup 2005).