Passivation: An Important Factor in Improving Stainless Steel Corrosion Resistance

Maximizing the Oxide Layer through Chemical Treatments

Outside of basic maintenance, there are advanced treatments and procedures available to enhance the corrosion resistance of stainless steel—particularly in machining and fabrication environments.

In this guide, we’ll be covering one of the most popular methods—passivation treatments.

Most often, these treatments are performed after parts are fabricated or machined and before assembly or installation.

This is because obtaining the best passivation results often requires submersion in treatment solutions and full coverage of stainless surfaces.

We’ll start with a basic overview and dig in deeper as we go.

When you’re done reading, you should have a solid concept of how passivation works and if it is something you should consider adding to your maintenance and upkeep routines.

What is Passivation and How Do Treatments Help?

Before we can talk about treatments, it’s important to understand how passivation works.

A thin chromium oxide level provides the characteristic stain and corrosion resistance for which stainless steel is known. This layer forms when the surface of the steel is exposed to oxygen. However, if the surface of the steel is dirty or damaged, this can inhibit the natural formation of an oxide layer.

While passivation treatments help to encourage natural formation of this layer, they also help boost corrosion resistance through other means.

The first is by reducing free iron introduced during machining or from the environment. If your stainless steel contains sulfides—common in grades typically used for machining—passivation can help to reduce any exposed sulfides on the surface of your steel.

This is important as exposed sulfides provide opportunities for corrosion to start and spread.

Passivation Methods and Products

Passivation treatments use acids—predominantly nitric and citric acids—at different temperatures and concentrations.

For less resistant grades of steel, sodium dichromate can promote faster oxidation while reducing risk of degrading the steel.

For free-machining grades of stainless steel, methods differ.

Instead of a single bath and rinse, passivation is achieved by alternating between alkaline baths, such as sodium hydroxide solutions, and acid baths to ensure neutralization of acids used in passivation.

This is of particular concern in these steels as the surface sulfides removed can create tiny discontinuities that hold acids even after thorough rinsing. If not removed, these acids can continue to attack the surface of the steel after treatment.

Submerging parts in racks, baskets, or other separating containers will help to ensure full, even submersion of parts and avoid the risk of creating pockets of acid or concentrating corrosion products near components within the solution.

Exact recommendations will vary between steel grades and even manufacturers. However, the American Society for Testing and Materials (ASTM) offers several standards related to passivation:

• A380 / A830M - Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems

• ASTM A967 / A967M - Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts

• ASTM B912 - Standard Specification for Passivation of Stainless Steels Using Electropolishing

The total time you must submerge parts, the temperatures used, and even the chemicals used will vary. However, these baselines provide a general idea of what to expect.

As recommended above, be sure to follow all safety precautions and obtain the help of a professional for exact steps and requirements if you are unsure.

High-Chromium Steels (Grades with 17% or More Chromium Content)

• Thoroughly clean and rinse the steel using degreaser or cleaner and warm water.

• Submerge the steel in a 20% by volume nitric acid bath held between 120F and 140F (49C and 60C)

• Thoroughly rinse the steel using warm water

• Dry

Straight Chromium (12-14%), High Carbon/High Chromium, and Precipitation Hardening Steels

• Thoroughly clean and rinse the steel using degreaser or cleaner and warm water.

• Submerge the steel in a 20% by volume nitric acid bath with 3 ounces per gallon (22 grams per liter) of sodium dichromate held between 120F and 140F (49C and 60C)

• Alternatively, a stronger nitric acid solution may be used at 50% by volume for 30 minutes.

• Thoroughly rinse the steel using warm water

• Dry

Free Machining and High-Sulfur Steel Grades

• Thoroughly clean and degrease using a solution containing 5% by weight of sodium hydroxide between 160F and 180F (71 and 82C) for 30 minutes

• Rinse the steel using warm water

• Passivate using 20% nitric acid at temperatures between 110F and 200F (43C and 93C) with Sodium Dichromate added at 3 ounces per gallon (22 grams per liter) for low chromium grades

• Alternatively, citric acid may be used at 10% by weight concentrations

• Rinse the steel again using warm water

• Use a 5% by weight sodium hydroxide bath at 160F to 180F (71C to 82C) for 30 minutes to neutralize the previous acid bath

• Thoroughly rinse the steel with warm water a final time

• Dry

These different methods outline the concise nature of passivation and the range of variables to consider before attempting passivation.

Failure to follow procedures and safety requirement may lead to personal injury or further degradation of the steel.

Care must also be taken to keep the passivation solutions free from chlorides. Passivating in solutions with an increased chloride level can cause an opposite reaction in the oxide layer.

Instead of speeding up oxide layer creation and a shiny, smooth surface, chlorides create what is known as a flash attack. The result is a darkened, etched surface with a degraded oxide layer.

This makes managing baths and chemical solutions an essential part of passivation operations.

Disclaimer: These treatments involve caustic chemicals. This means there are personal safety risks, risks of damaging your steel, and disposal concerns to consider. This guide is meant as a generalized informational source. Always consult an engineer or maintenance professional, review any MSDS data available for the products used in passivation, and account for specific requirements related to your exact stainless steel grades, usage environments, and other variables before proceeding with passivation treatments.

A Quick Summary

Passivation is an excellent option for improving the corrosion resistance and oxide layer of stainless steel parts and components.

However, if not approached with care and precision, it might cause more harm than benefits.

Be sure to:

• Always thoroughly clean steel before passivation

• Monitor acid baths using titration to ensure chemicals are at proper concentrations and contaminants are within tolerance

• Avoid chlorides in passivation baths

• Maintain baths through a replacement routine

• Avoid mixing grades of steel in passivation processes

• Avoid metal-to-metal contact using racks

• Check with a professional if you have questions regarding chemistry and treatment before proceeding

• Always follow safety protocols for both use and disposal of chemicals used in passivation