Why passivation is required over the welding joint of SS-Plate.?
Answer:
*Passivation improves the corrosion resistance properties of components made of precipitation-hardened, austenitic, ferritic and martensitic steels. Passivation treatments improve the surface condition of stainless steel by dissolving iron that has been im- bedded in the surface during forming or machining. If allowed to remain, the iron can corrode and give the appearance of rust spots on the stainless steel.
*Passivation, which consists of immersing stainless steel components in a solution of nitric acid without oxidizing salts, will dissolve the imbedded iron and restore the original corrosion-resistant surface by forming a thin, transparent oxide film.
*Features:
• Maximum stainless steel corrosion protection
without plating.
• Superior clean surface.
• Eliminates iron contamination reactions with
other materials.
• No rust discoloration.
• Surface prepared for priming and painting.
Specifications
AMS-QQ-P-35 (Replaced QQ-P-35)
AMS 2700
ASTM A-380
ASTM A-967
ASTM B-254
MIL-S-5002
*Please visit:
http://www.imagineering-nc.com/white_pap...
Passivation is the process of making a material "passive" in relation to another material prior to using the materials together. For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and peroxide alternating with deionized water. The nitric acid and peroxide oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities. Another typical passivation process of cleaning stainless steel tanks involves cleaning with NaOH and citric acid followed by nitric acid (up to 20% at 120F) and a complete water rinse. This process will restore the film, remove metal particles, dirt, and welding generated componds (ex. oxides).
In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few atoms thick.
Under normal conditions of pH and oxygen concentration, passivation is seen in such materials as aluminium, iron, magnesium, zinc, magnesium copper, stainless steel, titanium, and silicon. Ordinary steel can form a passivating layer in alkali environments, as rebar does in concrete. The conditions necessary for passivation are recorded in Pourbaix diagrams.
Some corrosion inhibitors help the formation of a passivation layer on the surface of the metals to which they are applied.
The reason that SS does not rust is due to a "passive" layer that normally forms on the surface. This layer is actually a form of rust or corrosion, but has an attractive appearance and fully coats the surface, blocking further progression of the corrosion. When you weld or grind stainless, you can destroy the passive layer, and under the right conditions, it may not automatically re-form, leaving a ss structure that will rust.
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