Anhydrous citric acid provides an easy and safe way to effectively passivate stainless steel surfaces. This naturally derived citric acid powder provides a safer and environmentally friendly way to properly protect your metal components from corrosion and rust.
What is Metal Passivation?
Metal passivation is a chemical process crucial for ensuring the longevity and durability of metals, particularly stainless steel, by enhancing the natural protective surface layer that inhibits corrosion. This is achieved by removing free iron and other contaminants from the metal surface, thus forming a passive oxide layer that significantly reduces the potential for rust formation.
For metalworking and manufacturing, passivation serves two primary purposes; strengthening the surface integrity and enhances the aesthetic appeal of finished components. It also allows simple compliance with industry standards, such as those set by ASTM A967, which provides guidelines for effective passivation, descaling, and cleaning processes tailored for stainless steel.
Why should I use Citric Acid over Nitric Acid for Passivation?
Effective metal passivation is achievable with both nitric and citric acid bath systems, but how do you know which is right for you? The short answer is citric acid is safer to handle, but the full breakdown is listed below.
Nitric Acid
Requires special handling
Lower pKa value than citric, meaning its a much stronger acid
Toxic and highly corrosive
Ventilation needed
Difficult disposal
Longer processing time required for passivation >20 minutes
Negative impacts on non-stainless steel metal or polymer-based systems and components
Lower cost for chemistry, but higher disposal & maintenance costs
Citric Acid
No special handling required
Higher pKa value than nitric, meaning a much weaker acid
Non-toxic (derived from natural sugar fermentation)
Easy to dispose and biodegradable
Ventilation suggested, but not required
Faster processing time required for passivation (5-20 minutes)
Medium cost for chemistry, but low disposal & maintenance costs
What is ASTM A967 and does Citric Acid meet the industry requirements?
The American Society for Testing and Materials (ASTM) is an industry association that defines the specific methods, practices and guidelines for a broad range of specifications and end applications. The ASTM A967 is a widely recognized standard for the chemical passivation of stainless steel in the effort of strengthening the corrosion resistance. This helps set a standard for properly protected metal for manufacturers across the country and peace of mind for consumers knowing their equipment or purchased metal pieces won't prematurely rust.
The main purpose of ASTM A967 is to outline a passivation process that effectively removes the free iron and contaminant particles from the metal surface and ensuring the passive oxide-layer is properly formed. There are a 6 tests in the specification to measure the effectiveness of the passivated part. Sections 1.4 to 1.4.6 include a tests against water immersion, high humidity, salt spray, copper sulfate, potassium fericyanide-nitric acid, and a free iron test.
The anhydrous citric acid from Sucroal does indeed meet the requirements of ASTM A967 under section 7 and requires a solution that contains 4 to 10 wt% (weight percent) and exposure times from 4 to 20 minutes depending on the temperature of the immersion bath.
For more specific recommendations, please reach out to your ChemPoint representative.
