Stainless steel passivation

The passivation of stainless steel occurs naturally, when the alloy is in an environment capable of bringing oxygen to the surface of the metal. When oxygen (coming from air, water, an oxidizing agent or other sources) comes into contact with the surface of stainless steel, it reacts with the chromium present in the alloy forming the oxides and compounds responsible for passivation.

When we talk about passivation of stainless steels we are referring to the specific properties for which a stainless steel is chosen for a given application. The passivation, or layer of passivity, of a stainless steel is what makes it resistant to corrosion.

Chromium alloys, such as stainless steels, are usually preferred for applications involving corrosive environments. They are selected steels, in particular, because they offer strong resistance to chemical attack without requiring large additional protection costs. The minimum chromium content, for which a steel can be considered stainless (and therefore give rise to passivation), is set by law at a minimum of 10.5% by weight (and a maximum of 1.2% of carbon).

The corrosion resistance of stainless steels is due to an invisible protective film formed mainly by chromium oxides and hydroxides of the Cr2O3 and Cr(OH)3 type. This film, to impart passivation to stainless steels, must have some very specific characteristics such as compactness, insolubility and adhesion to the metal substrate. The properties indicated are not the only ones, but they guarantee that the passivation film is effectively protective towards the stainless steel, giving good passivation and therefore protecting it from attack by external agents.

The contrast with the iron oxides that are created on carbon steels is very clear. What we currently call "rust" is nothing but a set of different porous oxides and hydroxides, which allow aggressive and oxidizing agents (even simple oxygen in the air) to flow to the metal substrate.

When we talk about passivation of stainless steel, we are referring to a perfectly adherent and compact oxide, a passivation layer that physically acts as a barrier against the passage of aggressive agents from the outside to the metal substrate. This film, which is formed and reformed rapidly by atmospheric oxygen, also leads to an increase in the surface electrochemical potential of the metal.

Sensitivity to contaminants (metallic residues deriving from production/manufacturing treatments, dirt, dust, etc.) compromises its homogeneity and resistance, causing its technological properties to decline. Therefore, for the natural passivation of stainless steel to occur satisfactorily, the environment in which the passivation process takes place must be free from pollutants, metal dust, chlorides, fluorides, sulphur, etc.

A protective oxide film is present on the passive surface of the stainless steel, before the part is fabricated. A clean, freshly machined, brushed or pickled piece of stainless steel automatically acquires this oxide film upon contact with the atmosphere (oxygen).

Under ideal conditions, the protective oxide film completely covers all surfaces of the workpiece and is considered extremely thin, approximately 2.5 nanometers. Some elements (such as nitrogen, titanium, nickel, molybdenum, and so on) influence in a more or less important way the formation, thickness, stability, adhesion and reconstruction speed of the passivating film and therefore the resistance to corrosion of steel.

Contaminants such as dirt or iron particles can be transferred from cutting tools to the surface of stainless steel parts during processing. If not removed, these foreign particles can reduce the effectiveness of the original screen protector. When this happens, corrosive attack can begin. Although the metal may appear shiny when freshly machined, invisible particles of impurities and iron can cause corrosion on the surface after exposure to the atmosphere.

Exposure to sulfides can also be a problem if not taken into consideration. These particles come from the addition of sulfur to stainless steels to improve their workability. Sulfides improve the alloy's ability to form chips, which cleanly separate from the cutting tool during machining. If the part is not passivated correctly, sulfides can facilitate the onset of surface corrosion of the product.

Method

The final choice regarding the type of passivation depends on the supply standards imposed by the manufacturer, for which the parts or components can be applied. Regardless of the direction in which the choice goes, there are two essential steps to guarantee the best possible resistance to corrosion: cleaning and passivating acid bath.

Phase no. 1: Cleaning

Cleaning must precede any other intervention. Grease, coolant or other particles present in the workshop must be immediately removed from the surface to obtain the best possible corrosion resistance. Processing chips or other dirt can be removed from the product with caution. To remove processing oils or coolants, a simple commercial degreaser or detergent may suffice. Foreign materials, such as thermal oxides, can be removed by grinding, or by methods such as acid immersion pickling.

Assuming that the simple immersion of the fatty part in an acid bath simultaneously gives rise to both cleaning and passivation is a thoughtlessness that costs a lot.

Contaminated grease may react with the acid, forming gas bubbles that settle on the metal surface and interfere with passivation. Even worse, contamination of the passivation solution, sometimes with high levels of chlorides, can cause so-called "flash attack" where, instead of the desired shiny oxide film, you end up with a heavily etched or dark, even deteriorated surface compared to the starting situation.

Phase no. 2: Passivating acid bath

After careful cleaning, the stainless steel product is ready for immersion in an acid passivation bath. Depending on the grade of stainless steel and the required criteria, a choice is made between three different methods: passivation in nitric acid, passivation in nitric acid with sodium dichromate, passivation in citric acid.

The more resistant chromium-nickel alloys (300 series) can be passivated in a 20% nitric acid bath. Less resistant stainless steels can instead be passivated by adding sodium dichromate to the nitric acid bath, to make the solution more oxidizing and capable of forming a passive film on the surface. As an alternative to nitric acid + sodium dichromate, the concentration of nitric acid is increased to 50%, in order to prevent a possible flash attack.

As for free-cutting stainless steels, which are not suitable for high-speed machining, the procedure is different. The sulfides of free-cutting steels (containing sulphur) are partially or totally removed during passivation in a nitric acid bath, creating microscopic discontinuities in the surface of the machined piece.

Even rinsing with water, which is normally effective, can leave residual acid trapped in these discontinuities after passivation. If not neutralized or removed, this residual acid can subsequently attack the surface of the steel. It is possible to use an alkaline-acid-alkaline process, which neutralizes the trapped acid and is carried out in less than two hours, through 8 different steps:

• degreasing
• immersion for 30' in 5% sodium hydroxide at 71/82 °C
• rinsing with water
• immersion for 30' in 20% nitric acid + 22 g/L of sodium dichromate at 49/60 °C
• rinsing with water
• immersion for 30' in 5% sodium hydroxide at 71/82 °C
• rinsing with water
• drying

As an alternative to nitric acid treatments, citric acid can be used alone, to remove light iron contamination from the surface, and also facilitates passivation of the clean steel surface. The use of citric acid is much less dangerous and offers environmental advantages in terms of fume emission (oxides – NOx) and disposal of the acid. To passivate stainless steel, citric acid treatments require 4-10% solutions.

Citric acid passivation procedures are more prone to flash attack than nitric acid treatment. Factors that cause this type of attack include bath temperature that is too high, excessive immersion time, and bath contamination. There are citric acid inhibitor products capable of significantly reducing flash attack.

Citric acid passivation is useful for a large number of stainless steel families and is increasingly used by manufacturers who want to avoid the use of mineral acids or solutions containing sodium dichromate.

Controls

Once passivation has been carried out, it is necessary to check whether the treatment has removed the contaminating particles and optimized the corrosion resistance. For this reason, there are tests to evaluate the surface of the treated pieces, usually in a humidistatic chamber at 35°C for 24 hours.

The cross section is generally the most critical surface, particularly for free cutting steels, because sulfides, distributed in the machining direction, intersect this surface. Critical surfaces should be positioned upwards, angled at 15-20° to allow moisture to flow downwards.

It is important that the test method corresponds to the grade of the steel being tested. The test must be neither too rigid nor too tolerant. Correctly passivated material will be substantially free of rust, although it may show some coloration/reflection.

A quicker method is also available: the solution provided by ASTM A380 (Recommended Standard Practices for Cleaning and Descaling Stainless Steel Parts, Equipment and Plant) applies. The test consists of dabbing the piece with a solution of copper sulphate and sulfuric acid, maintaining the humidity for six minutes and observing whether there is any copper plating. Alternatively, the piece can be immersed in the solution for six minutes. Copper plating occurs if iron melts.

This last test cannot be applied to components intended for the food industry. Furthermore, it should not be used for low chromium martensitic or ferritic stainless steels (400 series), as false positive results are likely. Historically, salt spray testing has also been used to test passivated samples, but this test is too stringent for some steel grades and is typically not necessary to confirm that passivation is effective.