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With steel sheet containing very low levels of silicon, differing behaviour on the two sides has often been observed. While, on one side, a normal zinccoating made up of compact layers of alloy is formed, on the other side the coating can be thick, uneven and made up of structurally weak deposits. Inpipes made of this type of steel, the phenomenon takes the form of a perfectly satisfactory coating on the inside while, on the outside, the deposit isthick and irregular owing to the uneven structure of the alloy layer. Phenomena of a similar character have already been described and attention hasbeen drawn to the fact that the formation of these uneven alloy layers can sometimes be prevented by grinding and polishing the samples.

Other investigations have indicated that, by annealing steels with a very low silicon content around 0.05%, this abnormal reaction, i.e. formation ofstructurally uncohesive layers, can be overcome. This result can be achieved more easily and quickly by normalizing treatment than by annealingbelow 850 deg.c.

These two special aspects of the reaction may possibly be explained by the fact that the silicon is locally converted into oxide by internal oxidation, sothat it is prevented from influencing the reaction. It has recently been clearly demonstrated that such internal oxidation does indeed have this kind ofeffect on the reaction pattern. Grinding off the surface layer which has been affected by the oxidation has the effect that the original behaviour pattern,with the formation of structurally unsound deposits, is restored.

In the case of steels containing about 0.05% silicon, the formation of unsound deposits can sometimes be avoided by blasting the surface. Blastingwith glass shot or a coarse blasting medium has no effect, but blasting with fine corundum leads to the formation of compact deposits. It is not yetknown to what extent the reaction pattern is affected by the work-hardening of the surface due to the blasting or whether the change should be putdown to the nature of the microstructure at the surface.

`EFFECT OF SILICON ON HOT DIP GALVANIZING REACTION

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11. J J Sebisty and G E Ruddle: Proceedings Seminar on Galvanizing of Silicon Containing Steels, Liege, May 21-22, 1975. pp. 219-241. ILZRO,

1975.12. N.C. Jain, Influence of Silicon Content of Iron and Steel on the Hot Dip Galvanizing Reaction, Seminar on Galvanizing of Concast Silicon

Containing Steels, 4-5 August, 1989, New Delhi, Organized by Indian Hot Dip Galvanizers Association Delhi