eekpod,
I believe the best way to understand this requirement is to understand that the objective is to clean or prepare (such as roughning) the surface for coatings application. As defined in NACE SP-10 "A near-white metal blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter, except for staining as noted in Section 2.2. 2.3 of SP-10 gives acceptable variations in appearance that do not affect surface cleanliness, and without going into all of them, the following are some of them, weld metal, heat treating, heat affective zones.
So with that being said, I believe the answer is yes, color change from heating after a SP-10 blast still meets the intent of SP-10.
Hope that helps
Jim
If the intent is to remove surface contamination, including oxides, I would be forced to say leaving the "color" intact would be a violation of the requirement.
The colors observed are simple oxides, sometimes referred to as temper colors. The hot metal surface oxidize when exposed to air or should I say exposed to the oxygen contained in air. The temper colors change as the temperature of the base metal changes. At roughly 400 degrees F the color of the oxide film is light straw. As the temperature increases the colors are dark straw (500 degrees), brown, purple blue, and finally black oxide when the base metal is at roughly 800 degrees and exposed to air.
The oxide is a film. Like any film, it can affect the adhesion of the paint to the metal's surface.
Jim says the discoloration is acceptable. Since he has quoted the standard I have to say the standard appears to accept the temper colors, but you and your customer should review the specific text and draw your own conclusions. You could also contact the paint manufacturer and ask them for their recommendations.
Best regards - Al
thanks guys for the input, I need to meet whatever my customer requires.
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