We can categorize surface finishing of our fabricated stainless steel products as below;
Chemical Finishes
Chemical Acid Treatments are used on stainless steel to remove contaminants on the surface. Pickling and Passivation don’t remove grease or oil on the surface, detergent or alkaline can be used to achieve this. Our guide for chemical applications is ASTM A380 - Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.
5.1.1. Pickling
When stainless steel is welded or heated, the chromium level on the top layers significantly decreases which also makes stainless steel vulnerable to corrosion. In order to remove this damaged layer pickling is an option which is generally a solution of nitric and hydrofluoric acids.
5.1.2. Passivation
Passivation is usually a nitric acid solution or paste which is used to clean iron contaminants and helps formation of a passive oxide film on the surface of stainless steel material. To achieve the best result first we are removing lubricants, oils and greases from the surface by detergents or alkaline, apply pickling to remove oxide layers after this and perform passivation as the last step.
Mechanical Finishes
5.2.1 Grinding
Grinding is an abrasive application to remove excess metal from the stainless steel surface by usually using disc grinders. Grinding Wheels with different grit sizes can be used for desired surface quality. Grinding is usually applied as a progressive process starting from larger grits initially and using finer grinding wheels for subsequent applications. Changing the direction of the wheel by 90 degrees with each grit is vital for removing grinding lines.
5.2.2 Polishing
Polishing is actually grinding with finer grits. Wheels or belts are used for application. In order to achieve a finer result again finer grits shall be used gradually. While the purpose of grinding is mainly to remove the excessive metal, for polishing the aim is to provide smoothness. (and gloss -brightness)
5.2.3 Buffing
When cloth wheels with an addition of a compound including very fine abrasives is used, it is called buffing. The first step is cut buffing during which polishing lines are removed. Final step is called finish/color buffing which provides a bright luster finish.
5.2.4 Blasting
Bead or Shot Blasting is another option as mechanical finish. Depending on the mediums used such as; Sand, Glass bead, Silicon carbide, Stainless steel shot, Ground quartz surface qualities starting from dark to light and coarse to smooth surfaces can be achieved.
Electropolishing
Electropolishing is an electrochemical process used to remove microlayers from stainless steel and smoothing the surface to reach an aesthetic, mirror finish. Electropolishing is usually not a standalone process but rather a complementary process after chemical and/or mechanical finishes. Electropolishing also improves the corrosion resistance. While it is usually a tank process large pieces can also be electropolished by wand polishing where the piece itself becomes the anode. Electropolishing is largely used at food processing, medical and pharmaceutical applications as it increases smoothness, reflectivity, cleanliness and passivity (and gloss -brightness) of the surface.
ISO 15730:2000 - Metallic and other inorganic coatings -- Electropolishing as a means of smoothing and passivating stainless steel is the European standard we apply for electropolishing.
