Quality stainless steel dispatched from the mill contains an equal concentration (1:1) or less of iron (Fe) and chromium (Cr) atoms on its surface.
When developed, the chromium will interact with the oxygen in the air to create a chemically inert, passive layer. This passive layer helps the stainless steel to resist corrosion.
This naturally occurring layer, however, is just 1 – 3 nm (0.000001 – 0.000003 mm) in thickness and is not reliable across the surface. In addition, when exposed to water or other substances, it can oxidize the iron atoms and form rust that can shift throughout the metal and damage it.
Stainless steel is chemically passivated to eliminate the free iron and any surface contaminants, enabling an increase of chromium and a more consistent passive layer. The objective is to accomplish a higher ratio of chromium atoms to iron on the metal’s surface.
Passivation is not a method for eliminating discoloration or scale, nor does it alter the metal’s surface color. A surface plated, painted, or coated cannot be passivated after it is covered.
Chemistry is Key
Three chemicals are commonly used for passivating stainless steel; nitric acid, phosphoric acid, and citric acid. Each has its relative strengths and weaknesses, rendering them more appropriate for some applications than others.
The object or surface should always be cleaned before passivation to eliminate contaminants, including oils, grease, or any residue from the stainless steel's mechanical processing. Oils and grease can disturb the passivation process by developing thick films when exposed to acids.
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Phosphoric Acid
Phosphoric acid is a weak mineral acid used for a process known as electropolishing (EP). EP is used to even out the microscopic peaks and valleys remaining on the surface of the metal after it has been mechanically polished.
In contrast to the passivation process, electropolishing will remove metal from the surface. It can decrease or eliminate micro corrosion, shallow burrs, and other surface flaws that allow alien material to gather and put the passive layer at risk.
It can also eliminate discoloration in welded metal. As a result, electropolishing is the first step before a passivation treatment.
In some cases, electropolishing is adequate as the final treatment for external use or where commercial food preparation and handling takes place. Untreated stainless steel would contain a chromium-to-iron ratio (Cr:Fe) between 0.6:1 to 1:1, whereas a surface electropolished with phosphoric acid would have a ratio of 1.2:1 to 1.4:1.
Nitric Acid
Nitric acid is a highly corrosive mineral that has been used in various forms since the ninth century. It is currently used for diverse industries and applications.
When ASTMA-380 was initially published in July 1978, nitric acid was the approved chemical for passivating stainless steel. Its use in forming stainless steel can be traced to the mid-1800s when Christian Friedrich Schönbein, a German-Swiss chemist, discovered that dipping iron/chromium alloys in strong nitric acid would considerably decrease its chemical reactivity.
Nitric acid passivation usually achieves a Cr:Fe ratio of approximately 1.5:1, which boosts the corrosion resistance of the stainless steel compared to its untreated version. It has the benefit of being practical on the broadest range of stainless steel grades.
Nitric acid’s use and effectiveness in passivation are well understood, and it can be sufficiently regulated due to its long history of use. However, it is a hazardous material and hazardous waste.
When the ASTM A-380 standard was first developed, using citric acid under ambient conditions created the danger of possible organic growth, which would taint any product that was being contained or processed.
Citric acid was established as a cleaning solution for stainless steel but was not used in its passivation. Advances in the manufacturing of citric acid have now dispersed those concerns.
The most significant risk of applying nitric acid is its strength. As a robust oxidizer and strong acid compound, it requires dedicated training in handling hazardous materials.
It also necessitates dedicated equipment and personnel with personal protection equipment (PPE) to prevent burns, spills, and breathing in the deadly vapors released by the chemical.
The passivation process may occur at higher temperatures, raising the handling hazards and the formation of nitric oxide gas, which can cause headaches, choking, fatigue, and nausea among personnel exposed. Sufficient ventilation, therefore, has to be arranged and maintained.
Discarding solutions for nitric acid involve protocols such as neutralization in a secondary vessel. Neutralization during circulation cannot be done as the iron tends to precipitate back into the system, ruining the passivation process.
Nitric acid can also etch the surface of stainless steel, pulling heavy metals that would make the solution unsafe and necessitate off-site disposal.
Owing to its efficacy, nitric acid remains the mandatory default standard by several guidelines spanning numerous industries. The ASTM A-380 standard is also approved for use in the AMS QQ-P-35, AMS 2700, and ASTM A-967 standards.
Citric Acid
Contrary to nitric acid, citric acid is a weak organic acid found in citrus fruits. It has extensive applications in various industries, including as a preservative and a flavoring for food.
In 2013, the ASTM A-967 standard was assembled, which described the use of citric acid mixtures for passivation. This resulted in an update of the A-380 standard.
When the chemical is heated to a minimum of 60 °C (140 °F) and employed to process the metal for one hour, it can accomplish the same Cr:Fe ratio as nitric acid; 1.5:1.
When the metal is treated at 80 °C for 2–3 hours, citric acid combinations can realize ratios of 1.8:1 or even 2.0:1, offering higher corrosion protection than nitric acid and considerably more resistance to corrosion than untreated stainless steel.
Citric acid used at the usual 5–10% concentrations does not create the same toxicity and environmental hazards as nitric acid due to its comparatively lower acid strength and oxidation. This renders on-site treatment less disruptive, as harmful materials and ventilation protocols are not required.
Employees do not have to leave the premises while the equipment is processed. It also diminishes health hazards to the technicians carrying out the passivation service. The lower reactivity means a more substantial safety margin regarding process stability.
Another benefit is that citric acid molecules bind (chelate) the free iron and other metal atoms, rendering them incapable of chemically reacting and making it simpler to flush them out of the system in the passivation process.
Citric acid is readily available and economical. Citric acid necessitates blending with extra buffers, chelants, and surfactants to realize and enhance the quality of the passive film over nitric and other passivating substances.
Due to lower equipment degradation, reduced risk levels, and easier discarding, citric acid passivation is easily affordable for most clients.
Citric acid is not appropriate for passivating all varieties of stainless steel. Those with a ferritic structure, higher carbon content, or other alloy properties may not passivate well with citric acid.
Generally, however, citric acid passivation adheres to the ASTM A-380, AMS QQ-P-35, and ASTM A-967 standards and acts suitably on most stainless steel alloys. Based on the application, it needs additional approval to realize AMS 2700 stipulations.
The Bottom Line
As with any procedure, picking which acid to utilize for passivation is a case of selecting the appropriate instrument for the task. Just as the equipment, applications, and mandatory standards differ across industries, there is no single solution to meet all the requirements.
This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.
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