In stainless steel, passivation means removing the free iron from the surface of the metal using an acid solution to prevent rust. When the surface iron is removed, the other components of the alloy (primarily chromium, often nickel as well) are left behind as a surface layer over the underlying steel.
Standard recommendations and precautions for cleaning, descaling, and passivating of new stainless-steel parts, assemblies, equipment, and installed systems.
Consideration shall be given in the design of parts, equipment, and systems that will require cleaning to minimize the presence of areas in which dirt, or cleaning solutions might become trapped, and to provide for effective circulation and removal of cleaning solutions.
Materials shall be precleaned. Scales shall be removed through chemical descaling, acid pickling, and mechanical descaling. Degreasing and general cleaning shall be accomplished by immersion in, swabbing with, or spraying with alkaline, emulsion, chelate, acid, solvent, or detergent cleaners or a combination of these; by vapor degreasing; by ultrasonics using various cleaners; by steam, with or without a cleaner; or by high-pressure water-jetting.
Recommended cleaning practices shall be followed for welds and weld-joint areas, specially critical applications, installed systems and post-erections.
The following shall be used as the basis for cleanness acceptability: Visual inspection; wipe tests; residual pattern; water-break test; free iron test such as water-wetting or drying; high-humidity test; copper sulfate test and ferroxyl test
Precaution shall always be practiced to minimize iron contamination, in reuse of cleaning and pickling solution, in water rinsing, in circulation of cleaning solutions and rinse water, in protection of cleaned surfaces, for safety, and disposal of used solutions and water.
At CMC we do Nitric acid passivation and Citric acid passivation.
Nitric Acid Passivation
Nitric acid passivation is the traditional method of removing free iron from the surface of stainless steel, titanium and other metals for corrosion prevention.
As the legacy method of passivation, nitric acid on stainless steel offers the advantage of a proven, established process. For those committed to using nitric passivation, a variety of options exist to optimize performance with the particular grade of stainless steel.
Less corrosion-resistant grades of stainless steels have a greater risk of etching, also known as flash attack, during passivation in nitric acid. To reduce that risk, the following approaches may be used:
- Addition of sodium dichromate to the nitric acid
- Higher nitric acid concentration
- Higher temperature of nitric acid.
Note that even though these options reduce the risk of etching, they do increase the risks to worker safety associated with hazardous chemicals. Sodium dichromate is a hexavalent chromium compound, and a known carcinogen.
Options for Nitric Acid Passivation in ASTM A967
| Type | Chemistry | Temperature | Time | Best For |
| Nitric 1 | 20 – 25% nitric acid by volume, 2.5% +/- 0.5% sodium dichromate by weight | 120 – 130 °F | 20 min. minimum | Precipitation hardenedMartensiticFree machiningFerritic, except types 430 and 446 |
| Nitric 2 | 20 – 45% nitric acid by volume | 70 – 90 °F | 30 min. minimum | AusteniticDuplex (type 329) |
| Nitric 3 | 20 – 25% nitric acid by volume | 120 – 140 °F | 20 min. minimum | Austenitic, except high-carbon grades (304H, 316H, 321H, 347H)Duplex (type 329)Ferritic, except lower chromium grades (405, 409) |
| Nitric 4 | 45 – 55% nitric acid by volume | 120 – 130 °F | 30 min. minimum | Precipitation hardenedMartensitic, except types 416 and 420Ferritic lower chromium grades (405, 409, 429)Austenitic high-carbon grades (304H, 316H, 321H, 347H) |
| Nitric 5 | Other combinations of temperature, time, nitric acid concentration and other chemicals that produce parts that pass test requirements in ASTM A967. | |||
Nitric acid itself is a hazardous chemical that emits toxic and corrosive fumes, and therefore requires special attention to ventilation and safe chemical handling. Though not a known carcinogen, nitric acid can cause severe chemical burns rapidly. When released in the atmosphere, nitric acid emissions contribute to acid rain and smog, and can affect the ozone layer. All of this explains why the industrial use of nitric acid is highly regulated, with stringent requirements for worker safety and environmental protection.
Citric Acid Passivation
Citric acid passivation offers a safer, more environmentally friendly alternative to nitric acid passivation. Citric acid is an organic acid that comes from citrus fruits like oranges, and as such is safe for employees to handle. Fumes from citric acid are not toxic and not harmful to the atmosphere. Citric passivation meets all current industry standards, and is effective with nearly all grades of stainless steel.
In the past, some manufacturers avoided citric acid due to potential organic growth and molding issues. Today, citric acid has improved with new formulations for biocides which prevent organic growth in solution. These new advancements have allowed smaller manufacturers which had little to no experience with chemical handling to bring their passivation process in-house.
Citric passivation vs. nitric passivation also offers shorter cycle times. Parts can be processed in as little as 4 minutes when using citric acid, compared to 20 minutes minimum with nitric acid.
As with nitric acid, it is important to maintain a balance between temperature, immersion time and acid concentration with citric acid passivation, to avoid the challenges of runaway corrosion known as flash attack.
Options for Citric Acid Passivation in ASTM A967
| Type | Chemistry | Temperature | Time |
| Citric 1 | 4 – 10% citric acid by weight* | 140 – 160 °F | 4 min. minimum |
| Citric 2 | 4 – 10% citric acid by weight | 120 – 140 °F | 10 min. minimum |
| Citric 3 | 4 – 10% citric acid by weight | 70 – 120 °F | 20 min. minimum |
| Citric 4 | Other combinations of temperature, time, citric acid concentration and other chemicals that produce parts that pass test requirements in ASTM A967. | ||
| Citric 5 | Same as Citric 4, except the pH of the immersion bath must be maintained in the range of 1.8 – 2.2. | ||
*Note that the citric acid percentage is measured by weight, while the nitric acid percentage is measured by volume.
The following chart provides a summary of the key differences between nitric vs citric passivation.
Comparison Summary: Nitric vs Citric Passivation
| Nitric Acid | Citric Acid | |
| Safety | Hazardous handling required | Very safe to use as directed |
| Air breathing | Emits toxic and corrosive gases | No toxic and corrosive gases emitted |
| Ease of use | Chemical handling safety equipment and extreme care required for most use | Minimum chemical handling safety equipment required |
| Passivation | Excellent passivation of most grades of stainless steel | Excellent passivation of nearly all grades of stainless steel |
| Environment | Environmentally hazardous | Environmentally friendly |
| Cost | Low cost raw materialHigh cost maintenance and disposalHigh cost ventilation system for safe use | Higher cost chemistryLower maintenance, safety and waste disposal costsLower concentration of chemicals requiredLower overall cost to use |
| Passivation process duration | 20 minutes to several hours | 5-20 minutes typical |
| Process temperature | Elevated temperature required for many gradesNitric acid very dangerous at elevated temperatures | Room temperature use for many gradesElevated temperature expedites the processSafe without ventilation |
| Chemical process maintenance | Regular solution monitoring required (titration) | Regular solution monitoring required (specific gravity) |
| Iron oxide removal | Slowly removes iron oxides | Readily removes iron oxides |
| Equipment | Long-term corrosive degradation of non-stainless steel metal or polymer-based equipment or components | No corrosive degradation of equipment |
| Process stability | Must control time and temperature closely as danger of nitric oxide gas exists – proper ventilation required | Less prone to time and temperature variation, no hazardous vapors |
| ASTM A967 | Meets requirements | Meets requirements |
| AMS 2700 | Meets requirements | Meets requirements |
| ASTM A380 | Meets requirements as referenced in ASTM A967 | Meets requirements as referenced in ASTM A967 |
| AMS QQ-P-35 | Meets requirements as referenced to AMS 2700 and ASTM A967 | Now referenced to AMS 2700 and ASTM A967 |
Questions & Answers
- Before putting stainless steel parts into use
- After mechanical machining operations
- After welding
- When new components are joined to existing components
- Passivation must be done after contamination
- As a form of preventive maintenance
What Is Passivation?
Passivation is a post-fabrication process that makes a material passive or inert to chemical reactions that can change its composition and ultimately lead to failure. In the industry, the passivation process is typically carried out to make a metal surface more resistant to corrosion or oxidation by building a protective film over it.
This thin film, also known as a passivation layer or passivation film, covers the material’s surface but does not make any changes to the base metal. The passive film acts as a barrier to reduce the chemical reactivity of the material making it more resistant to corrosion and contamination. While surface passivation can be done for many ferrous materials, it is mostly associated with stainless steels.
Passivation vs Pickling
Pickling and passivation are often misunderstood to mean the same. However, the two are separate processes with different natures and working principles.
Fabrication processes such as welding often result in a heat-affected zone. This heat-affected zone introduces contaminants as well as destroys the high chromium content layer in the part. This reduces the chromium content at the surface.
Pickling is used to remove oxide scale from a material’s surface to diminish the heat-affected zone and the low chromium content layer. The part is immersed in an acid tank for a set duration. The process also removes any embedded iron particles and contaminated carbon steel. However, if the contamination is high, the part will require cleaning with an alkaline solution before the pickling bath.
Passivation, on the other hand, is a more complex process, and in many instances, consists of pickling as the first step. The passivation process uses a different set of acidic solutions to not only clean the surface but also prompt the development of a passive oxide layer.
Thus, pickling is used for cleaning metal parts and is often a pretreatment process whereas passivation not only cleans the surface but forms a chemically non-reactive layer. While pickled parts obtain a dull, matte grey look, passivated products do not undergo any change in appearance.
When Is Passivating Used?
Passivation is a rather quick and automatable surface treatment process. As a result, it has many use cases. Some of the situations where passivating proves to be a feasible and effective solution are as follows: