Stainless steel passivation is a multi-stage process that begins with the exacting specs as published by ASTM International (American Society of Testing and Materials).
A typical RAMCO Citric stainless steel passivation process 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 combined with citric acid. Thorough removal is crucial to avoid producing missed spots that may be vulnerable to corrosion.
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: Passivation 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. A second cascading rinse is used to ensure all the citric acid residue is fully removed prior to the drying process.
Step 5: Drying
The Hot Air Knife Blow-Off Dryer (pictured above) from RAMCO operates up to 250 degrees Fahrenheit. The streamlined drying process uses an oscillating elevator in conjunction with a high volume blower and heating system to create a dynamic hot air knife that can rapidly dry parts. Recirculating the air and moving the parts up and down in front of the air knife allows the system to effectively dry parts at lower temperatures.
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.
ASTM’s documentation procedural directives detail the established guidelines for stainless steel passivation.
Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts
Passivation of Corrosion Resistant Steels
Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems
Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants
Standard Practice for Permanent Marking of Orthopoedic Implant Components
Standard Guide for Descaling and Cleaning Titanium and Titanium Alloy Surfaces
Corrosion-Resistant Steel Parts: Sampling, Inspection and Testing for Surface Passivation(STABILIZED Apr 2017)
For information about RAMCO’s passivation systems, please contact our factory at 800-553-3650.