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 an increased chromium and 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 that will be 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 mechanical processing of the stainless steel. Grease and oils can disrupt the passivation process by forming impenetrable films when in contact with the acids.
A weak mineral acid, phosphoric acid is not used so much for passivating, but rather 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 it has been 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 can allow foreign material to collect and pose a threat to the passive layer.
It can also remove the discoloration in welded metal. For that reason, electropolishing is performed prior to a passivation treatment for an item before being installed or commissioned into service. It is also conducted after an item or structure has been repaired or modified.
In some cases, such as for external use or for 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 a ratio of 1.2:1 to 1.4:1.
Nitric acid is a highly corrosive mineral acid that is 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 to provide the standard recommendations and precautions for cleaning, descaling, and passivating of new stainless steel parts, assemblies, equipment, and installed systems,” nitric acid was the prescribed chemical accepted for use 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 greatly 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, the application and efficacy of nitric acid in passivation was 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 for use 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 biggest hazard of using nitric acid is its strength. As a strong oxidizer and strong acid compound, it requires specialized training in handling hazardous materials. In addition, it requires specialized equipment and personnel working with it must be equipped with personal protection equipment (PPE) to avoid burns due to spills or from breathing the toxic vapors the chemical emits. The passivation process may take place at elevated temperatures, which also increases the handling risks as well as 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.
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, it is also accepted for use in the AMS 2700, AMS QQ-P-35 and ASTM A-967 standards.
By contrast to the strong mineral nitric acid, citric acid is a relatively weak organic acid most notably found in citrus fruits. It too has a wide use in a variety of applications across a large number of different industries, including as a flavoring and preservative for food. As was mentioned earlier citric acid was accepted for use as a cleaner. 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 is heated to a minimum of 60°C (140°F) and used to process the metal over the course of 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 with nitric acid. That makes on-site treatment less disruptive as hazardous materials and ventilation protocols do not need to be enacted and 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 that there is a larger safety margin overall in terms of process stability.
There is another advantage in 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 the reduced hazard levels, the reduced degradation of the equipment used to conduct the passivation and the easier disposal, the cost of citric acid passivation can be lower for most clients.
It should be noted that citric 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, but should be passivated with nitric acid instead. Overall, however, citric acid passivation meets the AMS QQ-P-35, ASTM A-380 and ASTM A-967 standards and performs properly on most stainless steel alloys. Depending on application, it requires an additional approval to be used 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 not a one-size-fits-all solution.