Stainless Steel Passivation: A Closer Look

RAMCO Parts Washers > Stainless Steel Passivation: A Closer Look

Stainless Steel Passivation: A Closer Look

Stainless steel passivation (or the passivation of other metals) is a multi-stage process that begins with the exacting specs as published by ASTM International (American Society of Testing and Materials).* ASTM’s documentation procedural directives detail the established guidelines for stainless steel passivation.

ASTM A967/A967M-17:

Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts

AMS2700F:

Passivation of Corrosion Resistant Steels

ASTM A380/A380M-17:

Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems

ASTM F86-21:

Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants

ASTM F983-86(2018):

Standard Practice for Permanent Marking of Orthopoedic Implant Components

ASTM B600-11(2017):

Standard Guide for Descaling and Cleaning Titanium and Titanium Alloy Surfaces

AMSSTD753B:

Corrosion-Resistant Steel Parts: Sampling, Inspection and Testing for Surface Passivation(STABILIZED Apr 2017)

All stainless steel passivation consists of four basic steps.

Step 1: Washing

This eradicates any existing corrosives such as dirts, grease, oil, etc. from the surface of the parts to be washed. Parts are sorted according to their characteristics, relative size, etc., and deposited into stainless steel baskets. They are then positioned onto a transport elevator. Agitation of the parts while submerged in a heated, high alkaline cleaning solution loosens layer of contaminants.

Step 2: Rinsing

Each of the baskets is transported and carried along rollers, then all parts are rinsed. Vertical agitation in warm water (temperature may vary) helps separate all remaining contaminants from the parts’ surfaces, leaving them prepped for passivation. The industry standard is to rinse parts in water that has undergone either one of two processes:

Deionization – deionized water is water from which all mineral ions (anions and cation) have been filtered out. Sulphate and chloride are among the anions that are removed. Cations to be removed include sodium, calcium, copper and iron.

Reverse osmosis  – water is forced under pressure through a membrane through which the water’s molecules create a hydrogen bond. These molecules are sifted through, leaving purified water. Any materials with a molecular weight greater than one hundred such as oils, bacteria, microorganisms, etc., are sifted out.

Step 3: Passivation

The length of time for the passivation step and precise temperature of the acid bath varies depending upon factors like size of parts, level of corrosion, etc. Baskets containing the parts are conveyed across rollers to another heated immersion bath of deionized water that most commonly combined with either nitric acid (sometimes augmented with sodium dichromate) or citric acid. Either one will react with the iron and eliminate it off the surface. Thorough removal is crucial to avoid producing missed spots that may be vulnerable to corrosion.

Care must be taken at this stage to avoid a phenomena referred to as “flash attack,” caused by lack of accurate control of the attributes of the bath. Flash attack will cause the metal’s surface to darken and degrade, becoming copiously etched. Flash attack can be prevented by cycling the acid bath out at regular, predetermined intervals and replacing it with new solution. This will obviate the accumulation of contaminants for the duration of the passivation process.

Nitric acid, in addition to reacting with the iron, also acts on the chromium, thereby creating the passive layer of chromium oxide that shields the iron underneath from corrosives. Since nitric acid is a particularly volatile substance, stringent protocols are necessary for handling, exposure, safety and environmentally acceptable means of disposal.

Citric acid itself does not actually passivate stainless steel parts. Rather, it prepares the parts’ surfaces which, exposed to ambient air, passivate automatically. Citric acid is fast on the way to becoming the industry standard for passivation. It’s far less volatile and it’s biodegradable after use. Citric acid is able to passivate a wider variety of stainless steel alloys compared with nitric acid. Furthermore, the use of citric acid significantly lowers the risk of stripping some of the nickel, iron and chromium, resulting in a passive layer that is too thin.

Step 4: Second Rinse

Again, using deionized water, the parts are immersed and agitation provides the motion necessary to ensure that all parts are free of any remaining contaminants as well as traces of the acid bath solution.

Step 5: Drying

The Hot Air Knife Blow-Off drying process from RAMCO operates at 180 Farenheit, yielding streamlined drying and evaporation at a relatively low heat.

Step 6: Testing and Quality Control

Meticulous inspection and testing of the passivated parts will confirm the results of the procedure.

Following passivation, care must be taken to avoid any damage that may compromise the passivated layer. Scraping the parts can certainly damage them. So too, excessive heat and cold can cause expansion and contraction that ruins the passivated surface. However, if the passivation process was done correctly, the passive layer can actually restore itself if there is enough oxygen exists to merge with the chromium.

For information about RAMCO’s passivation systems, please contact our factory at 800-553-3650.

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