Phosphating – The Process

Published by admin | Filed under Engineering

Phosphating relates to a precision engineering process of applying a phosphate coating to metal substrates, more particularly to lessen the harmful effect of sludge formation in such a process. The invention also relates to new phosphating solutions.

Phosphate coatings may be applied to metal substrates, particularly ferrous substrates, by reaction of the substrate with an aqueous acidic solution of certain metal phosphates, e.g. phosphates of iron, manganese and zinc. There are certain by-products of the reaction with the substrate some of which are precipitated from the phosphating solution as coating proceeds. In a precision engineering company the processes usually include an insoluble phosphate salt of the substrate metal, for example ferric phosphate in the case of a ferrous substrate.

The impulsive is a barrier to efficient coating since it may form a crust on the walls of the coating bath and its associated equipment, in particular any heat-transfer surfaces. Also, the precipitate build up a layer of sludge in the bottom of the coating bath or in any reservoir for the working coating solution and this sludge may be difficult to remove when its removal is desirable. A further possibility is that the phosphated work pieces may become impure. The formation of an insulating crust on the heat transfer surfaces located in the coating bath and, in the case of a spray process, on the spray nozzles, requires frequent scraping of these and other parts of the equipment in order to maintain the efficiency of the process; for example good heat transfer and temperature control.

The present process is applicable to a number of precision engineering services especially to all conventional phosphating processes, for example to spray and dip processes. Preferably it is applied to ferrous substrates, but may be applied to zinc, aluminium or mixed metal surfaces.