What is Passivation?
It is an industrial chemical finishing practice to protect different metal alloys from corrosion. Passivation makes use of mild oxidants such as nitric and citric acid. These acids get rid of exogenous free iron, sulphides and other foreign particles from the surface and create an oxide layer or film in place of that which subsequently acts as a protective shield. It is critical that such treatment with acid should not considerably affect the metal itself. This shield significantly reduces the risk of future corrosion due to air.
This technique of passivation can be traced back to 19th century when a chemist, Christian Friedrich Schönbein, discovered that iron attains a chemically inactive or passive state when it is treated by concentrated nitric acid. Later in the 20th century, it was found that nitric acid can be replaced with citric acid which is far more environmentally friendly [1].
Why do we need Passivation?
Passivation not only protects the metal against rust but also protects the metal parts against other surface contaminations resulting in prolonged life and lower maintenance cost.
It should be noted that metals such as stainless steel already contain nickel and chromium in addition to iron in its chemical composition. Steel has this unique property that its composition changes as we move from the center to the surface. The outer surface contains higher level of chromium as compared to iron. The chromium on the surface readily reacts with the air outside and makes a protective film of chromium oxide over the surface. Passivation simply optimizes and enhances this formation of chromium oxide layer in a controlled fashion. The chromium oxide layer is 2 to 5 atom thick which is formed due to replacement of reactive iron atoms with stable chromium atoms. The uniformity and thickness can be quantitatively measured using multiple techniques such as Auger Electro Spectroscopy (AES), Electron Spectroscopy for Chemical Analysis (ESCA), and Energy Dispersive X-ray Analysis (EDX) [2].
Under ideal conditions, stainless steel might not even require passivation but it is common practice to subject steel to different processes such as cutting, grinding, welding which render the steel vulnerable to rust as chromium oxide is not properly formed. This causes ‘flash attack’ in which no uniform oxide layer is formed on the surface rather the metal surface becomes dark and etched. Red rust formed due to formation of iron oxide causes further corrosion in inner layers. Hence, the need for passivation arises in medical, dental, aerospace, power transmission/distribution, heavy equipment and petrol chemical industries [3].
What is the Process of Passivation?
Firstly, the metal is cleaned with the help of an alkaline solution (e.g. sodium hydroxide) to remove any contaminants, oil and grease. Secondly, it is rinsed with deionized and demineralized water preferably using a jet nozzle. At times, a hard bristle brush may also be used to remove foreign objects which would later result in bubble formation when oxide film forms. Afterwards, the stainless steel is immersed completely in acid for around 20 to 30 minutes with a temperature around 130 °F depending upon the chemical composition of the metal at hand. It is important to make sure that acid is contamination free and this can be achieved by regularly replacing acid solution with a fresh batch. It is also equally important to immerse metal of same composition and grade at a single time. The acid cleanses the surface of any free iron, leaving behind a surface which has higher proportion of chromium as compared to the composition of rest of the metal. Subsequently when this surface is exposed to air, a thicker surface of chromium oxide is formed rendering the metal non-reactive as well as making the metal resistant to rust. Currently, ultrasonic machines are used to produce oxide layer formation even while the metal is immersed in the acid bath. As a final step, the metal alloy may also be tested using salt spray, exposure to high humidity or copper sulphate [4].
At times, a compound of nitric acid with sodium dichromate is also used. This causes rapid formation of protective film over the surface. The main con of this technique is that sodium dichromate is extremely toxic. Ultimately it is the customer requirement and metal composition which decides which method is to be employed. It is noteworthy that passivation is not an electrolytic process and is not needed on metal surfaces which would be subsequently painted over, or powder coated [5].
What are the different methods of performing passivation?
Different methods are employed at the stage of acid immersion. The first and most common technique is immersion in acid tank. This method provides uniform and optimum protection. The size of tank may range from 1.25 gallons to more than 500 gallons depending upon the dimensions of the metal requiring passivation. Another method which is known as ‘Circulation’ involves circulating acid through the system. This method is appropriate for metal pipes which would be carrying corrosive material. Acid could also be sprayed on the metal, but this method needs special safety and disposal protocols.
What are the different industrial standards in place for Passivation?
There are a number of industry standards such as ASTM A967, AMS 2700, ASTMA380 and SAE-QQ-P-035 in place for conducting passivation of metals such as stainless steel, titanium etc. ASTM A967 specifically oversees passivation of stainless-steel parts. It is based on United States Defense Department standard SAE-QQ-P-035 and is one of the most common standards in use. AMS 2700 provides guidelines for passivation of steel used in aerospace. ASTM A380 regulates cleaning and descaling of stainless steel, titanium and related alloy equipment [6].
What Precision, Fabricating & Cleaning (PFC) can offer?
Coupled with years of experience, PFC can suggest best solution for your passivation needs. Our dedicated team of engineers and technicians are well aware of all the modern and state of the art techniques and tools to meet your particular specifications. We are equipped with completely automated system which can conveniently perform multiple steps of passivation such as washing, rinsing and drying. We can also provide passivation services at your location. In addition to this PFC also provides a wide range of other industrial services which can be seen here.
Sources
[1] https://www.besttechnologyinc.com/passivation-systems/what-is-passivation/
[2] https://advancedplatingtech.com/passivation-cleaning/passivation-of-stainless-steel/
[3] https://www.mmsonline.com/articles/how-to-passivate-stainless-steel-parts
[4] https://www.theruststore.com/What-is-stainless-steel-passivation-W85.aspx
[5] https://www.eng-tips.com/viewthread.cfm?qid=153287