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Stainless Steel Passivation Services
We are a noteworthy firm, engrossed in rendering Stainless Steel Passivation Services in varied manufacturing industries. The provided services are highly praised for their timely execution and hassle-free management. These services aim at maximizing the essential corrosion resistance of components, structures and parts made from stainless steel. We use modish machines and tools to provide a protective oxide film on stainless steel. Also, our Stainless Steel Passivation Services are well known for providing immense clientsï¿½ satisfaction.
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- Appreciated for hassle-free management
PASSIVATION: AMS 2700, SAE-AMS-QQ-P-35, ASTM A967 and A380
Passivation is a critical step in maximizing theÂ essential corrosionÂ resistance of parts and components
machined from stainless steels.
Passivation is not a scale removalÂ treatment, nor is it like a coat of paint.
Passivation provides a protective oxideÂ film on the surface of the stainless steel. This invisible film is considered to be extremely thin, less than 0.0000001 inch thick, which is about 1/100,000 the thickness of a human hair.
A clean, freshly machined, polished or pickled stainless steel part automatically acquires this oxide film from exposure to oxygen in the atmosphere. Under ideal conditions, this protective oxide film completely covers all surfaces of the part. However, contaminants such as shop dirt or particles of iron from cutting tools may be transferred to the surface of the stainless steel parts during machining or transporting. These foreign particles can reduce effectiveness of the protective film. During the machining process, a microscopic amount of free iron may be worn off the cutting tool and transferred to the surface of the stainless steel part.
This may cause a thin coating of rust to appear on the part. This is actually corrosion of the steel from the tool and not the corrosion from the substrate. The embedded particle of steel from the cutting tool or its corrosion products may cause an attack of the part itself.
Similarly, small particles of iron-containing shop dirt may adhere to the part surface. Although the metal may appear shiny in the as-machined condition, the invisible particles of free iron can lead to rusting on the surface after exposure to air.
Exposed sulfides also can be a problem. They come from the addition of sulfur to stainless steels to improve mach inability. Sulfides improve the alloyÃÂ¢Ã¢âÂ¬Ã¢âÂ¢s ability to form chips that break away cleanly from the cutting tool during the machining process. Unless the part is properly passivated, sulfides can act as initiation sites for corrosion on the surface of the fabricated product.
In all cases, passivation is used to maximize the natural corrosion resistance of the stainless steel. It can remove surface contamination, such as particles of iron-containing shop dirt and iron particles from cutting tools that can form rust or act as initiation sites for corrosion. Passivation also can remove sulfides exposed on the surface of free-machining stainless alloys.
Types of Passivation :
After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. Any one of three approaches can be usedÃÂ¢Ã¢âÂ¬Ã¢â¬Ânitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation. Which approach to use depends on the grade of stainless steel and prescribed acceptance criteria.
More resistant chromium-nickel grades can be passivated in a 20 percent-by-volume nitric acid bath. Less resistant stainless grades can be passivated by adding sodium dichromate to the nitric acid bath to make the solution more oxidizing and capable of forming a passive film on the surface. Another option, used in place of nitric acid plus sodium dichromate, is to increase the concentration of nitric acid to 50 percent-by-volume. The sodium dichromate addition and the higher nitric acid concentration both reduce the chance of undesirable flash attack.
The procedure for passivating free-machining stainless steels is somewhat different from that used with the non-free-machining stainless grades. That is because the sulfides of sulfur-containing free-machining grades are partially or totally removed during passivationÂ in a typical nitric acid bath, creating microscopic voids in the surface of the machined part. Even normally efficient water rinses can leave residual acid trapped in these voids after passivation. This acid can then attack the surface of the part unless it is neutralized or removed.
Citric acid passivation has become increasingly popular with manufacturers who want to avoid the use of mineral acids or solutions containing sodium dichromate, along with the disposal problems and greater safety concerns associated with their use. Citric acid is considered environmentally friendly in every respect.
Passivation treatment in citric acid baths has been found useful for a large number of stainless steel families, including several individual stainless grades.
The ultimate choice of passivation method will depend on the acceptance criteria of the end user.
Specifications for passivation treatments for stainless steels
ASTM A380 - Practice for Cleaning, Decaling and Passivating of Stainless Steel Parts, Equipment and Systems
ASTM A967 - Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C