When quality stainless steel is produced, it typically leaves the mill with an equal concentration (1:1) or less of chromium (Cr) and iron (Fe) atoms on its surface. When formed, the chromium will interact with the atmospheric oxygen to create a chemically inert, passive layer. It is this passive layer that helps the stainless steel resist corrosion. However, this naturally occurring layer is only 1-3nm (0.000001 – 0.000003mm) thick and is not consistent across the surface. Additionally, contact with water or other substances can cause the iron atoms to oxidize, forming rust that can spread onto the metal and degrade it. For this reason, stainless steel is chemically passivated to remove the free iron, along with any surface contaminants, allowing for increased chromium and a more consistent passive layer. The goal is to achieve a higher ratio of chromium atoms to iron on the surface of the metal.
It should be made clear that passivation is not a process for removing scale or discoloration, nor does it change the color of the metal’s surface. A surface painted, plated, or coated cannot be passivated once the surface is so covered.
Chemistry is Key
There are three chemicals broadly used for passivating stainless steel; phosphoric acid, nitric acid, and citric acid. Each has its relative strengths compared to the others making them more suitable to certain applications over others. Regardless of which chemical is used, the surface or object must be cleaned before the passivation treatment to remove contaminants, including grease, oils, or any residue leftover from the mechanical processing of the stainless steel. Grease and oils can disrupt the passivation process by forming impenetrable films when in contact with the acids.
Phosphoric acid, a weak mineral acid, is used for a process called electropolishing. Electropolishing, or EP, is used to smooth out the microscopic peaks and valleys left in the metal’s surface after being mechanically polished. Unlike the passivation process, electropolishing will remove metal from the surface. It can reduce or remove shallow burrs, micro corrosion, and other surface imperfections that allow foreign material to collect and threaten the passive layer.
It can also remove the discoloration in welded metal. For that reason, electropolishing is the first step before a passivation treatment.
In some cases, for external use or where commercial food handling and preparation occurs, electropolishing is sufficient as the final treatment. Where untreated stainless steel will have a chromium-to-iron ratio (Cr:Fe) of between 0.6:1 to 1:1, a surface electropolished with phosphoric acid will have 1.2:1 to 1.4:1.
Nitric acid is a highly corrosive mineral acid used in a wide variety of industries and applications and has been in use in some form or another since the 9th century. When ASTM A-380 was first published in July of 1978, nitric acid was the prescribed chemical accepted in passivating stainless steel. Its use in developing stainless steel dates back to the mid-1800s when German-Swiss chemist Christian Friedrich Schönbein discovered that dipping chromium/ iron alloys in concentrated nitric acid would significantly reduce its chemical reactivity.
Nitric acid passivation typically achieves a Cr:Fe ratio of about 1.5:1, which increases the corrosion resistance of the stainless steel compared to its untreated state. It has the advantage of being usable on the widest range of grades of stainless steel. Due to its long history of use, nitric acid’s application and efficacy in passivation were well understood and could be precisely controlled but is a hazardous material and hazardous waste.
At the time when the ASTM A-380 standard was created, using citric acid at ambient conditions ran the risk of potential organic growth which would contaminate whatever product was being processed or contained. It was accepted as a cleaning solution for stainless steel but not for use in its passivation. Since then, however, developments in citric acid production have removed those concerns.
Not surprisingly, the most significant hazard of using nitric acid is its strength. As a strong oxidizer and potent acid compound, it requires specialized training in handling hazardous materials. In addition, it requires specialized equipment and personnel with personal protection equipment (PPE) to avoid burns due to spills or from breathing the toxic vapors the chemical emits. The passivation process may occur at elevated temperatures, which also increases the handling risks and the development of nitric oxide gas, which can cause choking, headache, nausea and fatigue among those exposed. As a result, proper ventilation must be set up and maintained when it is being used.
Likewise, disposal of nitric acid based solutions require special protocols including neutralization in a secondary vessel. Neutralization during circulation is not an option because the iron would precipitate back out into the system, thereby undoing the passivation process. Additionally, if incorrectly prepared/diluted, nitric can etch the surface of stainless steel pulling heavy metals that would render the solution hazardous and require off-site disposal.
Because of its effectiveness, it remains the default standard required by many guidelines across a wide number of industries. In addition to the ASTM A-380 standard is also accepted for use in the AMS 2700, AMS QQ-P-35 and ASTM A-967 standards.
By contrast to nitric acid, citric acid is a relatively weak organic acid most notably found in citrus fruits. It too has wide use in various applications across a large number of different industries, including as a flavoring and preservative for food. In 2013, the ASTM A-967 standard was created, which detailed the application of citric acid blends for passivation. This led to an update of the A-380 standard. When the chemistry is heated to a minimum of 60°C (140°F) and used to process the metal for an hour, it can achieve the identical Cr:Fe ratio as nitric acid; 1.5:1. When the metal is processed at 80°C for 2-3 hours, then citric acid blends can achieve ratios of 1.8:1 or even 2.0:1, the latter providing much higher corrosion protection achievable with nitric, or significantly more resistance to corrosion as untreated stainless steel.
Because of its relatively lower oxidation and acid strength, using citric acid at the typical 5-10% concentrations does not impose the same environmental and toxicity risks as nitric acid. That makes on-site treatment less disruptive as hazardous materials and ventilation protocols are not needed. Workers do not have to be evacuated while the equipment is being processed. Likewise, that greatly reduces the health risks to the technicians performing the passivation service. Additionally, the lower reactivity means a more significant safety margin overall in terms of process stability.
Another advantage is that the citric acid molecules bind (chelate) the free iron and other metal atoms and render them incapable of chemically reacting, making it easier for them to be flushed from the system as part of the passivation process. Citric acid itself is readily available and inexpensive. Citric acid requires blending with additional chelants, buffers and surfactants to reach and improve the quality of the passive film over nitric and other passivating agents. Combined with reduced hazard levels, reduced degradation of equipment, and easier disposal, the cost of citric acid passivation can be lower for most clients.
Citric acid is not suitable for passivating all types of stainless steel. Those with higher carbon content, ferritic structure, or other alloy properties may not passivate well with citric acid. Overall, however, citric acid passivation meets the AMS QQ-P-35, ASTM A-380 and ASTM A-967 standards and performs appropriately on most stainless steel alloys. Depending on the application, it requires additional approval to meet AMS 2700 requirements.
The Bottom Line
As with any process, selecting which acid to use for passivation is a case of choosing the proper tool for the job. Just as the applications, equipment, and required standards vary across industries, there is no one-size-fits-all solution.