Choosing the finish of stainless steel parts is just as important as choosing the right base material. The finish has a big influence on the performance of the material, particularly around the corrosion resistance and appearance. To maximise return on investment, prolong the life of the product and prevent it aging prematurely it is necessary to carefully consider the available finishing options.
Two finishing methods are Pickling (often called “Passivation”) and Electropolishing. Both use chemical processes, but passivation relies on the action of a very powerful acid working at room temperature, while the latter utilises the power of an electric current to polish the surface.
Understanding how each process works can help the decision of which method would better suit the goals of the stainless steel project. In this blog, Ensitech explains the differences between the two methods and the advantages one may have over the other.
Passivation
Passivation is a widely used metal finishing process for the prevention of surface corrosion. With stainless steel, the passivation process starts by using a combination of nitric and hydrofluoric acids to remove the surface of the metal along with any contamination from free iron. Once the underlying surface is exposed, the next step is applying nitric or citric acid to re-form a protective oxide layer. The temperature, duration, and concentrations of this final step are critical in obtaining a good result, and the details of the operation depend on the type of alloy being processed.
The new oxide layer “protects” the underlying metal by stopping it from chemically reacting with material in the environment that can initiate corrosion (such as chlorides). This oxide is called the “passivation” layer.
While this process improves the product’s corrosion resistance, it isn’t as effective as electropolishing. The process of removing the metal layer changes the appearance of the metal surface to a dull, grey look. To restore the usual bright look of stainless steel there is often the need of a third step to re-finish the metal using abrasives or other mechanical processes.
When to Choose Passivation
Passivation is used in the following situations:
- When the work is very large making it impractical to use other methods
- When there are not specific requirements regarding the appearance of the metal surface
- When safety and environmental protection are not high priorities
Benefits of Passivation
- Improved Corrosion Resistance
This finishing process improves stainless steel’s corrosion-resistant properties by generating a thick oxide layer that preserves the metal from chemical reactions that result in rust and tarnish.
- Minimal Maintenance
Passivation produces an even grey finish that does not require a lot of maintenance. Stainless steel’s smooth, shiny surface is generally more prone to scratches, fingerprints and water marks, which can be quite noticeable when appearance is important.
- Smooth, Clean Finish
The acids used in the passivation process removes oils, grease, and dirt from the metal’s surface, giving it a smooth, uniform appearance.
Downside of Passivation
- Time to Activate
In many situations it is not practical to use a heated bath to dip the work in. Instead, the pickling acid is applied as a paste, which sits on the surface until it works. This can take considerable time, especially in the colder temperatures of winter.
- Danger to User and Environment
The chemicals used to perform pickling are very strong acids. Applying these acids to metalwork in an industrial environment poses a danger to the user. Protective equipment needs to be worn to prevent skin contact or breathing of the vapours to prevent harm. The chemicals are also harmful to the environment, and disposal of the waste can be problematic.
- Hidden Costs
Due to the time to work, especially in cold temperatures, and the precautions that need to be taken to work safely with the pickling chemicals, there are hidden costs with Passivation that need to be taken into consideration in calculating the total costs involved with Pickling and Passivating. If the appearance of the finished result is important, the time involved in re-finishing the surface needs to also be considered.
Electropolishing
Electropolishing is a chemical process suitable for most metals. It removes the top of the microscopic undulations that make up the surface of the metal, producing a flatter surface. This improves the corrosion resistance of the metal by reducing the surface area exposed to the environment and removing features that initiate corrosion.
This process is popularly used for products that need to meet exacting requirements for surface roughness and corrosion resistance. Some common examples are medical implants, surgical instruments, food contact surfaces and aerospace components.
The electropolishing process occurs when the stainless steel parts are submerged in a temperature-controlled chemical bath. A D.C. electric current then passes through the bath which removes the microscopic peaks of the metal surface, smoothening out any flaws. The component comes out visibly smoother, shinier, and brighter.
Electropolishing can also eliminate heat tints or discolouration caused by oxide scales.
When to Choose Electropolishing
Electropolishing is best used for the following:
- When a superior level of smoothness, durability, and corrosion resistance is required
- Smaller parts or parts that have complex shapes and geometries
- When reverse electroplating, or the removal of a microscopic surface layer, is necessary
Benefits of Electropolishing
- Optimal Corrosion Resistance
One of the advantages of electropolishing over passivation is that it can provide the treated component up to 30 times greater corrosion resistance. Moreover, it delivers maximum tarnish resistance to various metals and alloys and withstands harsh environments.
- Enhanced Surface Appearance
Electropolishing provides stainless steel parts better visual appeal. It also improves the weldability of the product without sacrificing the hardness of the metal or causing the surface to peel or abrade.
- Improved Sanitation Control
Electropolishing makes maintenance easier by smoothening the metal’s surface, eliminating microscopic holes where contaminants can hide. This is especially advantageous for industries where a clean surface is a priority, such as in food, medical or pharmaceutical applications. Medical tools that have uneven surfaces might have spots where microbes can hide and spread, risking contamination.
Downsides of Electropolishing
- Alteration of the Surface Appearance
Despite creating a smooth surface that is clean and bright, many applications require a specific look or a specific surface roughness which electropolishing can spoil. By the removal of material Electropolishing will change the roughness of the surface, which is often quite visible.
- Requirement of Specialised Equipment
The high expense of running an Electropolishing facility means that work usually has to be transported to specialised supplier to perform the work. This can result in substantial risk of damage to the final work, together with the costs of freight and delay in delivery.
- Large Metal Work
In order to be electropolished the work need to fit into a pickling bath. Often this is impractical due to the size and/or shape of the finished product.
Choosing the Right Finish
Ensitech is a global leader in electrochemical weld cleaning technology and metal surface finishing. In 2004 the company discovered a passivation process with the advantages of electropolishing but without its disadvantages. This process (called “electrocleaning”) utilises A.C. current instead of D.C. current, which means the process does not remove material from the surface. Instead, the alternating current moves surface metal ions into a cleaning solution which dissolves any iron present. This results in the removal of contamination, and the formation of a passive layer which greatly enhances the anti-corrosion properties of stainless steel.
The machine used to perform electrocleaning is called a TIG Brush and being portable, the TIG Brush can be used at the customer premises. The requirement of a pickling bath is removed, and electrocleaning can be used quickly and safely in-situ. The process is especially effective in removing the heat-tint from stainless steel welds. From a cost perspective the process competes with Passivation and offers the superior finish of electropolishing. It is fast, does not alter the surface finish, does not require a separate passivation step and provides an excellent finish.
So, if you have the need to passivate stainless steel or stainless steel welds, make sure you investigate all the options available, including the advantages offered by the TIG Brush stainless steel electrocleaning solution. We offer weld cleaning machines and weld cleaning solutions to various industries, including aerospace, marine, and pharmaceutical.
To learn more about metal surface finishing, visit the TIG Brush blog and fill out our online form for more information. We’ll be more than happy to discuss the electrochemical weld cleaning process and demonstrate how the TIG Brush could change the way your company handles post-fabrication weld cleaning (and passivation).
