I am told that applying zinc rich coatings to TP 321 Stainless Steel piping operating in higher temperature ranges is not an acceptable practice. If this is true, does anyone know the reasons? If it it true, is there an acceptable method of removing the zinc coating?

KS Harris Zinc Rich Coating - Inorganic coating with 92% Zn content - is known to embrittle Austenitic S.S -( namely: Liquid Metal Embrtillement i.e, Zinc combines with Nickel and this results in nickel depleted zone near austenite grain boundaries) - only if temperature exceeds 570 deg cent (although melting point of Zn is 420 deg cent). Zn diffusion is slow in S.S with phase transformation and diffusion limitation in autstenite S.S grain bundaries in an already stressed material. This is not an instantaneous process but occurs under conditions of fire or explosion or when welding or heat treatment is done without removal of zinc rich coarting properly before welding. Isolated failures are reported when welding is done on Zinc rich paint or heat treatment is done on zinc rich paint coatings on S.S pipe or structurals. Otherwise, Zn coating do not normally affect S.S. In the case of paints - only inorganic Zn rich paints (Zn rich powder) can cause embrittlement (Liquid Metal embrittlement) on Austenitic S.S. Paints containing Zinc Oxide or Zinc Chromate (Organic coating) are known not to cause Zinc embrittlement (Liquid metal embrittlemnt). Barring accidental fire or welding or heat treatment without removing Zinc rich coating, Zinc rich coating - inorganic or organic - will not affect S.S materials upto temperature < 400 deg cent. Zinc coating can be removed by sand blasting, mechanical wire brush or by grinding. In extreme cases, dilute acid pickling will remove any tightly adherent Zinc rich (metallic dust) coating. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Nov 22,2005 E-mail:nishi@vsnl.com >I am told that applying zinc rich coatings to TP 321 >Stainless Steel piping operating in higher temperature >ranges is not an acceptable practice. If this is true, does >anyone know the reasons? > >If it it true, is there an acceptable method of removing the >zinc coating?