Passivation (Times Four!)

Q. My company currently uses citric acid to passivate various stainless steels and titanium. I know of the related ASTM procedures for this, but I would love to be able to get my hands on a book that can teach me a great deal more on the passivation process. Is there such a book out there? S.M.

Q. We rinse parts in a closed-loop bath that is not controlled for pH or any other content parameters. We see Type 304 stainless steel parts periodically rust along sharp edges—bolt threads and sheared edges—and discolor on rounded formed edges. I am thinking we don’t need to do an additional bath so much as control the existing process—water ought not make Type 304 rust unless the water is acting as a caustic, correct? Under what conditions do you need to take the acid bath step? V.M.

Q. I am setting up passivation and chem film lines in a shop and am wondering what are some good resources for getting started? D.F.

Q. Are there biocompatibility issues with passivating stainless steel? D.F.

A. It is unusual that I would receive all four of these questions on different aspects of the same topic at one time. Because of that, I am going to answer all of these at one time by providing a brief overview of passivation which should answer the questions above along the way.

While passivation is an important and widely used process in today’s manufacturing, it is often misunderstood. The vast majority of users of this process are deploying it on stainless steel parts. The use of passivation has likely also risen along with the increased use of stainless steel to replace other metals, primarily painted mild steel. The subject of passivation is covered in several articles, books, etc., but I am not aware of a book written just on the subject of passivation. A good reference that details passivation methods and evaluation criteria is ASTM A 967.
Background

Stainless steels are used where a corrosion-resistant metal is needed for a specific application. Due to the significantly higher cost of stainless steel versus mild steel, there is rarely an occasion when rusting and corrosion of the stainless steel would be considered acceptable. Therefore, it is important to have a very general understanding of stainless steels and their corrosion protection mechanism.

The three broad, metallurgical categories of stainless steels are ferritic, austenitic and martensitic. Ferritic stainless steels typically employ chromium as the major alloying element, producing a body-centered cubic (BCC) crystal structure. These materials are designated as 400-series stainless steels, with some of the most common examples being Type 409 and Type 430.

Martensitic stainless steels also share the 400-series designation and differ in alloying from ferritic materials through the addition of carbon, which produces a body centered tetragonal or cubic structure. The addition of carbon promotes formation of martensite when quenched from an elevated temperature. This allows martensitic materials to have higher strength and toughness (after tempering) than other stainless steels. Common examples of this are Types 420 and 440(-A, -B and -C).

The third major category of stainless steels is the austenitic type that uses the 300-series designation. This type of stainless relies on chromium and nickel additions to achieve a face centered cubic (FCC) crystal structure. Austenitic grades cannot be heat treated to improve strength, but can be cold worked. The most common varieties are Types 304 and 316.

While there are several different classes of stainless steel, they all share one common thread in that they must have a minimum of 11 wt% chromium to be considered a stainless steel. It is this alloying addition, along with the ability of that element to form a stable chromium oxide layer on the surface of the material, that imparts much of the corrosion resistance of stainless steels. Higher levels of chromium, with addition of several other alloying elements, determine the general category of stainless steel along with its associated corrosion resistance.
Reasons for Passivating

The general handling and welding of stainless steels can reduce corrosion resistance through the presence of “free” iron. A part that goes through a metalworking process may be exposed to tool steels, iron chips and contaminated coolants. A welding operation will produce some free iron in the weld as it solidifies. If it is near the surface, it will act just like mild steel regarding its corrosion resistance. It is these operations, and the resulting embedded free iron, that will reduce the overall corrosion resistance of the part.

Additionally, the “building” of the chromium oxide layer is thermodynamically favorable at room temperature (i.e., will occur spontaneously) while the reaction rate (kinetics) can be sped up in several ways. One way is the passivation process we are discussing. Through use of an oxidizing acid such as nitric or citric, the oxide layer will grow on the surface more quickly and uniformly. Many of the other mineral acids are not candidates since they are reducing acids.
So, in summary, there are two primary reasons to passivate a part. 1) Removal of free iron from the surface that has been embedded due to welding or metalworking and 2) Establishmernt of a uniform and consistent chromium oxide layer on the surface such that the stainless steel will remain stainless. Based on these reasons, all stainless steel can benefit from the effects of passivation to increase or restore their original corrosion resistance. That said, it is possible that these steps are not needed if the corrosion resistance of the part is adequate for the intended design or the manufacturing steps do not significantly alter the stainless properties.
Passivation Solutions

The oldest and most common method of passivating stainless steel is with nitric acid. This is an oxidizing acid that will both dissolve iron and build the oxide layer on the stainless surface. When used on 300-series stainless steel, it is often in the range of 20-40% acid by volume and from room temperature up to about 140°F. Time is typically 20–30 min. When passivating a lower-chromium ferritic stainless steel (<17% Cr) or a martensitic stainless, sodium dichromate is used to minimize etching or attack of the base material and temperature is usually limited to 120°F on the top end.

While this procedure has been the mainstay of passivating solutions for many years, citric acid has seen wider use in the past 10 years and is included as an option in the ASTM A 967 specification. This may involve the use of citric acid alone or a purchased, proprietary mixture. Treatment is usually done at elevated temperature (140–160°F) with a solution containing 4–10% citric acid in water. The time is generally in the range of 10–20 minutes. The citric acid method would be the preferred one when setting up a new line since it will be safer and reduce hazardous waste and associated treatment.

Evaluation
There are several different available test methods called out in the ASTM specification to evaluate the effectiveness of passivation, including water immersion, high humidity, salt spray, copper sulfate and the potassium ferricyanide test. All of these will primarily check for the presence of free iron on the surface through reaction with it, but can also be generally effective to evaluate the relative stability of the oxide layer that has been developed.

Line Design and Set Up
Setting up a new line is not an insignificant process, and I would recommend utilizing the companies on the Products Finishing website (www.pfonline.com) that specialize in finishing equipment since they will have the most experience. You will first need to know the type of passivation you are interested in (nitric or citric) for them to recommend the required materials of construction, line length, ventilation, etc.
Biocompatibility

Although I am no expert in biocompatibility, I have no reason to believe that a passivated and well rinsed part would have any biocompatibility issues. The surface of stainless steels is passive and largely inert. Additionally, highly alloyed stainless, nickel and titanium materials are frequently used for implant devices. It is highly probable that, in the manufacture of these devices, the final product will see a passivating process. ?