In certain applications such as the medical industry and the semiconductor industry, the use of highly pure, corrosion and contaminant free stainless steel is required. Stainless steel tubing and fittings for the transportation of process gases and surgical stainless steel are examples.
A key process step that ensures that components such as these remain contaminant and corrosion free is the cleaning and 'passivation' of the stainless steel surfaces. A process using citric acid (as opposed to mineral acids such as nitric acid) is generally considered superior.
Austenitic stainless steel is a form of carbon steel that is often used for these applications because of its high ductility, low yield stress and relatively high ultimate tensile strength. Under most conditions this material has good corrosion resistance, but in order to achieve suitable performance under all conditions it is necessary for the surface of the steel to be properly prepared.
Citric acid passivation is a process used by WS Associates to ensure that the components manufactured are suitable for use in both the medical and semiconductor fields.
Why use citric acid to clean and passivate stainless steel components?
Austenite steel is an allotrope of steel formed by hearing steel to a high temperature and then cooling rapidly. This process changes the crystal structure of the metal from body centered cubic to face centered cubic. Type 304 surgical stainless steel is austenitic steel containing 18-20% chromium and 8-10% nickel.
Once it is properly cleaned and passivated austenitic stainless steel is highly resistive to corrosion and contamination. Without cleaning however even this kind of stainless steel can begin the process of corrosion and once corrosion starts it only gets worse, being continuous and self-catalyzing.
Cleaning and passivating means that any impurities have been removed and the creation of an outer oxide layer that protects the metal from future corrosion has been created - this is the 'passivation' layer. Stainless steel is primarily iron alloyed with small quantities of chromium, nickel and sometimes molybdenum and manganese along with other materials. Corrosion resistance is engendered by the formation of an outer layer consisting of oxides of chromium, iron and nickel, which do not corrode. The best passivation layer is achieved by maximizing the oxides of chromium.
There are two key elements to the formation of a successful passivation layer. First all impurities, such as grease or other organics, that would prevent oxidation must be removed. Secondly, the oxidation must be allowed to complete so there are no free iron atoms on the surface.
In the past acids such as nitric acid have been used extensively to passivate stainless steel, however there are several disadvantages to nitric acid. Nitric acid is corrosive by nature, a health hazard and is hard to dispose of. In many cases free iron can be redeposited on the surface again and nitric acid is also known to produce pitting corrosion. There are many problems with long term use of mineral acid applications, especially under corrosive environments. If the nitric acid is heated then these problems are increased.
Despite these difficulties many companies continue to use nitric acid at least for part of the passivation process.
Citric acid cleaning and passivation process on the other hand is relatively safe and environmentally friendly since citric acid is present in many fruits and is used by anaerobic organisms as part of the process for creating energy. Citric acid is thus non toxic, non corrosive and biodegradable, and can often be disposed of relatively easily, usually in the sewer system without extensive pre-treatment.
Citric acid has proven to have many advantages including:
The passivated surfaces will pass all the required tests including salt spray and high humidity.
The citric acid works by creating complex molecules that bind up many metallic ions that would have the effect of reducing the corrosion resistance of the stainless steel. These complex molecules are typically water soluble in the aqueous solution in which the citric acid is dissolved and they will not redeposit metal ions as mineral acids are known to do. The citric acid also works to enrich the chromium concentration at the surface of the metal.
The citric acid passivation process is used very successfully in the pharmaceutical, medical and semiconductor industries, and produces superior results to the use of nitric acid alone, which has continued in use for historical reasons. Current studies of the surface chemistry of stainless steel shows that citric acid passivation eliminates the contaminants from the surface more completely and helps to produce a truly passive surface by giving the best chromium enrichment to the surface.
Citric acid cleaning and passivation are the state-of-the-art technology that is highly effective in stainless steel passivation. The use of nitric acid has several well know drawbacks and has only continued in use because of industry inertia.
At WS Associates manufacturing facilities citric acid passivation processes have been developed, producing a safe, effective and economical stainless steel passivation process for the semiconductor industry, medical field and other industries.