Authors: W. Roth, K. Stritch
Passivating stainless steel parts to maximize corrosion resistance
Passivation is a critical post-fabrication step that maximizes the inherent corrosion resistance of stainless steel alloys. It can make the difference between satisfactory performance and premature failure of machined components. Incorrectly performed, passivation can actually induce corrosion.
Passivation of stainless steel is achieved through the controlled growth of an oxide film (also referred to as a passive film) that protects the base metal from corrosive attack. It is not intended to be used as a cleaning process to remove scale from heat treating, paint/coatings, or any rust that may have formed on the part surface. A clean, polished, or pickled stainless steel part automatically acquires this oxide film from exposure to oxygen in the atmosphere. Under ideal conditions, this naturally acquired film would completely cover and protect the base metal from corrosive attack.
In practice, however, contaminants such as shop dirt or iron particles from cutting tools may be transferred to the surface of the stainless steel parts during machining. If not removed, these foreign particles can reduce the effectiveness of the original protective film and corrosive attack may begin. Although the metal may appear shiny in the as-machined condition, the invisible particles of free iron can lead to rusting on the surface after exposure to the atmosphere.
A two-step passivating procedure can provide the best possible corrosion resistance: 1) cleaning, a fundamental but sometimes overlooked procedure, and 2) an acid bath, or passivating treatment.
A common misconception is that the following passivation step will act as the cleaning step since parts are soaked in an acid bath. This is not the case. Care must be taken during the cleaning step to ensure that the acid solution comes into full contact with the surface of the part to achieve optimal corrosion performance. Thus, a thorough cleaning process should always come first. Grease, coolant, or other shop debris must be thoroughly cleaned from the surface to obtain the best possible corrosion resistance. Machining chips or other shop dirt are carefully wiped off, and machining oils or coolants are removed using common, commercially available degreasers. An ultrasonic tank may also be helpful in removing foreign debris from the surface.
After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. In current practice, any one of three approaches can be used: nitric acid passivation, nitric acid with sodium dichromate passivation, and citric acid passivation. Which bath and composition to use depends on the grade of stainless steel and/or the prescribed acceptance criteria for a given application.
For more details on which passivation bath should be used for a given stainless steel grade, consult industry specifications such as AMS 2700 or reach out to a Carpenter Technology corrosion expert.
Case Study: Tips for Maximizing the Corrosion Resistance of Custom 465®
Custom 465 is a high-strength stainless steel that offers good corrosion resistance, high strength, and excellent fracture toughness. As a sustainable alternative to cadmium or chromium-coated high-strength steels, Custom 465 can be employed in various structural applications where high strength, fracture toughness, and innate corrosion protection are required. To best ensure corrosion resistance, passivation of the final component is recommended.
A robust passivation process for Custom 465 consists of proper surface preparation, cleaning/degreasing, and the appropriate passivation procedure in accordance with the component’s specifications or the AMS 2700 standard. Some general tips to properly process and passivate Custom 465, or by extension, any PH grade of stainless steels, are as follows:
Proper surface preparation: Ensure any scale, heat tint, or oxide layer from upstream processing is removed through mechanical grinding or chemical methods.
- Custom 465 will develop a thin, nanometric “heat tint” at the low-temperature aging temperatures in inert atmospheres and especially in air. This oxide must be removed prior to passivation or good general corrosion resistance will not be achieved.
- “Heat-tint” may be imperceptible at low thickness, or sometimes has a gold or straw yellow coloration for slight oxidation. Thicker oxides may be bluer or purple in appearance. These oxides must be removed until shiny base metal is revealed.
- Chemical removal, for example by pickling, exposes Custom 465 to hydrogen and must be baked out to avoid problems with hydrogen embrittlement (HE), as is common with all high-strength steels.
- Fundamentally, exposing clean base metal is required for the formation of a dense, uniform passive film during the passivation process that will maximize the corrosion resistance of the part.
Cleaning: Make sure the component or part is rid of any remnant grease, machining debris, preservation oil, or coolant. While passivation may remove any free Fe contamination, ensuring the minimal number of contaminants from the start is the best way to optimize corrosion resistance of Custom 465.
- Cleaning can be done with standard degreasing solutions, ultrasonic baths, or by vapor degreasing.
- Care and maintenance of vapor degreasing solutions to prevent the deposition of organic solvent on the part should be achieved. Organic solvents can form HCl and cause severe damage to the material if allowed to deposit on Custom 465.
- Cleaning solutions should be cleaned and replaced regularly to prevent the deposition of debris on the Custom 465 component.
Passivation: Carpenter recommends an alkaline-acid-alkaline (A-A-A) method of passivation. The additional alkaline bath in the A-A-A method ensures neutralization of any remnant acid from the passivation bath and has been found effective at achieving optimal corrosion resistance on Carpenter Technology’s stainless steel offerings.
Overall, ensuring maximum corrosion resistance of Custom 465 requires well-maintained, clean, and robust processes prior to and during passivation. Care should be taken in ensuring quality surface preparation, successful cleaning, and finally passivation to reach the maximal performance for your application.