Surface finishing for stainless steel fasteners -Pickling and passivation

Surface finishing for stainless steel fasteners -Pickling and passivation

Pickling and passivation:

Passivation is the generic term applied to procedures developed to remove the presence of metallic (free) iron from the surface of stainless steel products. The presence of free iron particles on the surface of stainless steel products can cause the product to show rust spots which should not occur with a clean or “passive” stainless steel surface. A PASSIVE stainless steel surface would have NO free iron particles which could potentially corrode or “rust.”

During the handling and processing operations such as forming, machining and tumbling, particles of iron and tool steel can be embedded or smeared on the surface of the fastener. If allowed to remain, these free iron particles will corrode and show as rust spots.

The passivation process involves submerging the stainless steel fastener into a solution of nitric acid (20-25% by volume), sodium bicarbonate (2-3% by weight) and water or 35% Nitric by volume and water for some period of time. Twenty to thirty minutes is typical. The nitric acid dissolves the embedded or smeared iron and restores the original corrosion-resistant surface to the part.

*Above description is cited from PENN ENGINEERING

Improved corrosion protection for screws

If a screw requires increased corrosion protection, this can be provided by giving it a surface coating. Example: Hanger bolts are only available in the stainless-steel grade A2. If users require corrosion protection that corresponds to A4, this can be achieved with a coating.

The standard for this is zinc flake coating. The zinc flake applied in liquid form guarantees excellent corrosion protection. Screws protected with a zinc flake coating are also resistant to moisture and cleaning agents as well as fuel, engine oil, brake fluid and antifreeze. This makes zinc flake coating a preferred surface technology in the automotive industry. Coated screws like these are also excellent for use on outdoor applications, such as decks or facades.