The process of passivation is a mystery for most people and the outside observer. It would appear that the process of passivation is much like making a cocktail; pour the chemistry in, swish it around, and voila! However, a much better comparison would be baking a souffle where the right combination of ingredients, time and heat makes all the difference. Of course, with the souffle one gets a plate of rich, sweet, gooeyness, while passivation gives you a “clean plate” that helps ensure that the contents of the processed parts will be free of unwanted “ingredients.”
To recap, passivation is the removal of free iron from the surface of stainless steel, which is an alloy of iron (Fe), nickel (Ni) and chromium (Cr). This process leaves a higher concentration of chromium on the surface. This chromium combines with oxygen to form a non-chemically reactive, or passive, layer that not only protects the metal, but prevents it from reacting with – and contaminating – the materials that come into contact with it.
Iron and Chromium: A Balancing Act
The amount of chromium in stainless steels varies by the intended use of the material, but all stainless steels are made up of at least 10.5% chromium. The iron, of course, is needed to ensure the strength and durability of the structure. For its part, the nickel enhances the metal’s formability, weldability, ductility and also helps to increase its resistance to pitting corrosion. But, the chemical composition of the alloy is not uniform throughout its structure. To briefly return to the food analogy at the beginning of this article, stainless steel resembles a layer cake. The bulk of the metal is the iron-rich base metal, while at the surface is a thin transition layer where nickel is more prevalent. Above that is the equally thin passive layer where chromium is plentiful while the quantity of iron is greatly .
Left on its own, a cleaned stainless steel surface will naturally form its passive layer. The ratio of chromium to iron atoms in this layer is generally equal, or 1:1, but ranges from 0.5 to 1.2. However, it is neither very thick, nor is it durable. When treated with phosphoric acid – a weak mineral acid – in a process known as electropolishing, the ratio in the surface metal can be increased to 1.4:1. Electropolishing is usually done to create a very smooth cleanable surface and as a preparatory step for chemical passivation.
Passivation Takes Time and Temperature
Per ASTM A-380, passivation is done with the oxidizing nitric acid. This is the industry standard and it boosts the chromium to iron ratio up to 1.5:1, resulting in a much better quality of protective passive film. The corrosion resistance of a stainless steel surface passivated is higher than that of an untreated surface.
Citric acid is also accepted as a standard for use in passivation. Unlike nitric acid, citric acid is not considered a hazardous material. As a result, it is commercially available for companies to treat their equipment themselves. However, when applied at ambient temperature, the results are little better than those from a good cleaning and natural oxidation – about 1.2:1. In ASTM A-380, citric acid was only considered a cleaner precisely because it had been applied at room temperature. ASTM A-967 defined the standards for using heated citric acid in passivating stainless steel. Through a series of tests, Astro Pak found that when its citric acid based UltraPass® passivation chemistry is heated to 60°C (140°F), it becomes as effective as a nitric acid passivation with the chrome to iron numbers reaching 1.5:1. Moreover, bringing the temperature up to 80°C (176°F) could achieve even better results with ratios between 1.8:1 and 2:1. Whereas nitric acid passivation increased the corrosion resistance compared to untreated stainless steel, citric acid based UltraPass® passivation at 80°C provided double that level of protection, giving a corrosion resistance significantly higher than that of untreated stainless steel.
However, temperature was only half the equation. The results depended on how long the heated solution was applied. Using a piece of 316L stainless steel as a test sample, Astro Pak found that treating the material for 30 minutes with 60°C citric acid based UltraPass® passivation formula created a chromium to iron ratio to a level of 1.3:1. If the treatment was extended to an hour, the ratio increased to 1.5:1 – the same as with nitric acid. However, the best results came when the process was conducted for two hours at 80°C. This consistently produced the 1.8:1 to 2:1 ratios. Additionally, it was found that continuing the process for up to four hours did not result in a demonstrable increase in the ratios beyond those achieved at two hours. Astro Pak, which pioneered this breakthrough as part of their citric acid based UltraPass® system, typically treats the metal for a three-hour period to ensure consistent and uniform results that exceed the ASTM standards.
The Bottom Line
To borrow a line from basketball legend, Michael Jordan, “…if you put in the work, the results will come.” A proper passivation treatment relies more on just running some chemistry over the stainless steel. It is the combination of using the right chemistry at the right temperature and for the right amount of time that will produce the best results, leading to systems having a higher level of consistent performance for longer periods of time.
In summary, processing at temperature (80C) for 2 hrs achieves the optimal chrome/iron levels and a quality passive film with statistically validated results. Beyond that, Astro Pak’s UltraPass process falls under section 4 of ASTM A-967 as we use a citric acid solution with additional surfactants, chelants and buffers to achieve the highest chrome/iron or passive layer possible.