Chromate-free etch process

An important chromate-free acid etching technique is also available, it is designated the P2 [24] process. Here, as ferrous sulphate replaces the dichromate salts used in the more conventional chromic-sulphuric acid pickling, it is seen as being more friendly to the environment. Although not currently specified as the principle etching process, specifications for pretreatments such as phosphoric acid... 

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