Bigpond Zn rich primer paint coating on C.S / Low alloy steel - if applied for your high temp. piping - may not cause any harm to C.S / Low alloy steel at 400 deg cent. Paint removal has to be done by shot (grit blasting) or sand blast cleaning to SA 2.5 finish only. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 29,2009 E-mail: nishi@vsnl.com >Dear All, >We have few issues with high temperatures pipes and >equipments which are made in CS and Alloy steel and have Zn >coating. I would lilke to know how detrimental are these >coating for operating temp of more than 400 Deg C. And also >what should be the criterion for removing such coating from >existing pipe work and equipment in terms of operating >tempature and material of construction. >I have read the similar question asked in the past dated 24 >Sept 2004 >(http://hghouston.com/cgi-bin/dcforum/dcboard.cgi?az=show_thread&om=118&forum=DCForumID2&viewmode=all) >regards > > >regards

Bigpond This depends. Usually in hot dip galvanized steel - "Dross inclusions" plays a part (i.e., the loose iron particles during galvanizing process plays a part - namely dross inclusions). Dross inclusions - i.e., loose iron particles in molten zinc forms a metallurgical reaction at temp > 400 deg cent. This may be applicable for the case of Zinc -Hot Dip Galvanizing process where temp of > 400-480 deg cent plays a part due to "dross inclusions" and the metallurgical reactions. For surface protection of piping, usually 92% metallic Zn rich epoxy primer coating is applied at ambient temp after sand blast or abrasive blast cleaning. So chances of loose iron particles playing a part (dross effect) may not arise as this method of application is totally different from normal hot dip galvanizing effect where steel part is dipped in molten zinc kettle. Zinc plays a crucial part in the case of austenitic S.S alloys and creating LME effect in austenitic S.S when zn coating is applied purely as a cathodic protective paint application against SCC. When S.S is re-heated > 450 deg cent due to welding or process line temp is higher than 450 deg cent , then possibility of LME due to Zn penetration in aus s.s lattice structure exists. For C.his may not be the case when Zn is applied as a surface protection technique. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Feb 2, 2009 E-mail: nishi@vsnl.com >Thanks, this was really helful. As you said its safe up to >400 Deg C, is it harmful after 400 Deg c, given that Zn >melts at 427 Deg C, cna you please elaborate little more. >regards