Monitoring Time and Temperature for Passivation

The passivation process is a mystery for most people. Although, it can be said that the passivation process has some things in common with making a cocktail; the chemistry is poured in, it is then mixed, and voila. However, a far better comparison would be baking a soufflé, where time and temperature make as much difference as getting the correct combination.

Indeed, with the soufflé, the end result is a plate  of sweet, rich, gooeyness, while passivation  provides  a "clean plate," which helps ensure that the contents of the processed parts will be rid of unwanted "ingredients."

Passivation is the removal of free iron from the surface of stainless steel - an alloy of iron (Fe), nickel (Ni), and chromium (Cr). This process results in a surface with a higher concentration of chromium.

This chromium can be combined with oxygen, forming a non-chemically reactive layer – also known as a passive layer -  that protects the metal and prevents it from contaminating or reacting with the materials it comes into contact with.

The chromium to iron ratio in the surface passive film is an excellent indicator of the surface's resistance to corrosion.

Monitoring time and temperature for passivation

Image Credit: Astro Pak Corporation

Iron and Chromium: A Balancing Act

The chromium content in stainless steels can depend on the intended materials,  but all stainless steels contain at least 10.5% chromium. The iron in stainless steel is used to provide the strength and durability of the structure, and the nickel enhances the metal's weldability, formability, and ductility as well as increasing its resistance to pitting corrosion.

The chemical composition of the alloy, however, is not uniform throughout its structure. The bulk of the metal is the iron-rich base metal, contrasting the surface, a thin transition layer where nickel is more prevalent. Above this is the passive layer which is equally thin but largely made up of chromium.

When untouched, a cleaned stainless steel surface will form its passive layer naturally. The ratio of chromium to iron atoms in this layer is equal overall, or 1:1, but ranges between 0.5 to 1.2. It is neither thick nor durable.

In a process known as electropolishing, in which the alloy is treated with phosphoric acid, the ratio in the surface metal can be increased to 1.4:1. Electropolishing is typically 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 performed with the oxidizing nitric acid. This is the industry standard, increasing the chromium-to-iron ratio to 1.5:1, resulting in a significantly improved quality of protective passive film. The corrosion resistance of a passivated stainless steel surface is greater than that of an untreated one.

Citric acid is not considered a hazardous material, unlike nitric acid. This means citric acid is commercially available for companies treating their equipment. When applied at ambient temperature, however, the results are less than those from a good cleaning and natural oxidation – a ratio of around 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 for the passivation of 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), its efficacy for passivation becomes equal to that of nitric acid passivation with the chrome to iron numbers reaching 1.5:1.

Astro Pak also found that increasing the temperature to 80 °C (176 °F) could achieve even better results with ratios ranging from1.8:1 to 2:1.

Citric acid-based UltraPass® passivation at 80 °C provided stainless steel with double the level of protection from corrosion than that of stainless steel passivated with nitric acid, resulting in a corrosion resistance substantially higher than untreated stainless steel.

However, the temperature was not the whole equation. The results depended on the length of time the heated solution was applied. With a piece of 316 L stainless steel as a test sample, Astro Pak found that treating the material with 60 °C citric acid based UltraPass® passivation formula for 30 minutes resulted in a chromium to iron ratio of 1.3:1.

When the scientists extended this to an hour, the ratio increased to 1.5:1 – the same as with nitric acid. The best results, however, came when the process was conducted at 80 °C for two hours. This consistently produced a ratio between 1.8:1 and 2:1.

Continuing the process for up to four hours also saw no demonstrable increase in the ratios compared with those achieved after two hours. At Astro Pak – the pioneer of this breakthrough as part of its citric acid-based UltraPass® system - the metal is typically treated for three hours to ensure uniform and consistent results that exceed the ASTM standards.

The Bottom Line

A proper passivation treatment relies on more than just running chemistry over the stainless steel. It is the result of using the right chemistry, at the correct temperature, for the correct length of time. Only then will the optimal results be achieved, allowing systems to run with a higher level of performance for longer periods.

Processing at temperature (80 °C) for 2 hours achieves the optimal chrome/iron levels and a quality passive film with statistically supported results. Astro Pak's UltraPass process also belongs to section 4 of ASTM A-967, as the company uses a citric acid solution with additional surfactants​, chelants, and buffers to produce the highest chrome/iron or passive layer possible.

Acknowledgments

Produced from materials originally published by Daryl Roll, Astro Pak Consultant.

About Astro Pak Corporation

Astro Pak is the leader in providing Passivation, Precision Cleaning and High-Purity Chemical Cleaning services for a wide variety of “cleanliness sensitive” critical systems and components. Many of our customers have external agencies that drive their cleaning requirements, such as the FDA, NASA and others. We service such industries as Pharmaceutical, Biotechnology, Aerospace, Laser, Semiconductor, Water Treatment and more, including Industrial markets. Our services and products leverage decades of experience to deliver the most effective chemistries and techniques, resulting in increased equipment longevity, reduced corrosion-related downtime and regulatory compliance in client facilities.


Sponsored Content Policy: AZoLifeScience.net publishes articles and related content that may be derived from sources where we have existing commercial relationships, provided such content adds value to the core editorial ethos of News-Medical.Net which is to educate and inform site visitors interested in medical research, science, medical devices and treatments.

Last updated: Nov 15, 2022 at 10:23 AM

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