Galvanized steel heat-treatment

The electrolysis protection process using impressed current aluminum anodes allows uncoated and hot-dipped galvanized ferrous materials in domestic installations to be protected from corrosion. If impressed current aluminum anodes are installed in water tanks, the pipework is protected by the formation of a film without affecting the potability of the water. With domestic galvanized steel pipes, a marked retardation of the cathodic partial reaction occurs [15]. Electrolytic treatment alters the electrolytic characteristics of the water, as well as internal cathodic protection of the tank and its inserts (e.g., heating elements). The pipe protection relies on colloidal chemical processes and is applied only to new installations and not to old ones already attacked by corrosion. 

In rare cases, a relatively small area near the weld will be an anode to the relatively large cathodic surface area of the parent metal. In moderately corrosive media, this zone may corrode much faster than either the weld metal or the parent metal. Postweld heat treatment is usually helpful. In some instances, normalizing (or even solution annealing in the case of an austenitic stainless steel) the weldment is necessary, a measure that can cause significant distortion problems. In most cases, the weld metal, HAZ, and parent metal do not have significant galvanic differences. 

Consequently, if for high strength steels the heat treatments recommended after pickling and after plating (BS 1706 electroplated steels) are adopted, it is clear that galvanizers are adopting an ultrasafe policy, which would reassure users. 

Sherardizing is a diffusion process in which the steel parts are heated in the presence of zinc dust or powder in inert medium. Aluminium oxide or sand in amount of 20 % is added to the zinc powder as inert filler and 1 to 2 % halide salts are used as activator. The thermochemical treatment can be carried out in retort, rotated drum or as a pack-cementation process at 350 to 500 °C for three to twelve hours. The structure of the obtained layer is the same as the structure on steel surface after hot-dip galvanizing with a thickness about 50 - 400 pm... 

---