In tin plate production, cold rolled strip undergoes alkaline electrodegreasing. After batch annealing, coloured staining of strip edges may be observed. These differences in surface aspect between axis and edges of the strip can be very detrimental to the end product as they remain noticeable even after tinning or after chromium electrocoating (ECCS).
By measuring the chemical composition of the gaseous phase present between the wraps of the coil during batch annealing, a clear distinction is made between annealing of electrodegreased black plate and nonelectrodegreased cold rolled strip with respect to the water vapour evolution which is very intense during the heating and soaking parts of the annealing of black plate.
This water vapour evolution is induced by the dehydration of the silica film and is responsible for the selective oxidation of alloying elements and for superficial decarburization detected by the carbon monoxide and carbon dioxide evolution at high temperature.
It is suggested that coloured staining of the strip edges is related to the more pronounced selective oxidation of alloying elements at the edges, due to the higher temperature reached in these parts of the coils during batch annealing.
Before tinning or chromium electrocoating, the annealed black plate is pickled in a sulphuric acid solution. Pickling simulation on the laboratory scale shows that pickling cannot remove at all the difference in appearance between axis and edge due to the existence of a difference in electrochemical behaviour between these 2 parts of the strip.
Consequently, the best practice is to insure surface chemistry homogeneity across the whole width of the strip. This maybe achieved by performing batch annealing according to the new technology using high convection and pure hydrogen as a protective atmosphere which could decrease the temperature gradients between axis and edges during annealing.
