09 Dec Not Just a Pretty Face
How correct surface finishing can maintain corrosion resistance
Stainless steel’s resistance to corrosion is only as good as its outer skin. In this article, John Swain of Anopol explains why the correct surface finishing of stainless steel plays a significant role in maintaining corrosion resistance when subjected to extreme conditions. The hygienic aspects of stainless steel finishes are also examined.
Many engineers regard stainless steel as virtually indestructible and an ideal material for specifying where weather and atmospheric conditions are challenging. Worthy example would be electrical enclosures and cable ladders installed on North Sea oil platforms. What these engineers possibly do not appreciate is that the material’s ability to withstand corrosive attack is dependent on surface finish. It is a known fact that a bright polished surface offers better corrosion resistance than that of a matt finish, usually produced by linishing or other abrasive means. The latter is often referred to as a brush finish.
Stainless steel is the universal name given to a number of different steels, used primarily for their anti-corrosive characteristics. Whilst grade 304, often referred to as 18/8 (18% chromium, 8% nickel), is the most prolific of these steels, low carbon grade 316L is generally specified for use in potentially highly corrosive environments. Stainless steel’s resistance to corrosion is due to the metal’s capacity to passivate in a sufficiently oxidative environment (such as air) by forming an invisible oxide film. However, in order to form this vital film or layer, the surface of the steel must be free of any contamination. Such contamination can result from welding, grinding, forming, and general handling or contact with other metals.
As mentioned above, a polished surface is preferred for anti-corrosion behaviour. However, mechanical polishing is carried out using a lubricant or smearing agent in the form of a compound, which can leave residues imbedded in the surface. These can impede the formation of the passive oxide layer. Viewed under a scanning electron microscope, the mechanically polished surface is a ploughed field in micro-terms. From a corrosion resistant aspect, such a surface is not ideal.
A finish, produced by electrochemical means, is very different. Known in industry as ‘electropolishing’, the finish obtained is as a result of a surface layer being removed electrochemically, usually to a depth of about 20 microns. Once referred to as ‘reverse plating’, the principles of electropolishing are basically similar to those for electroplating, but instead of depositing a metal on the matrix, a fine layer is removed. No thermal distortion occurs and the resulting surface is micro-smooth and contamination-free, allowing a superior passive oxide film to form. The corrosion resistant qualities of such a finish cannot be bettered.
An additional bonus is the appearance of the electropolished finish. Bright and highly reflective, the finish is aesthetically pleasing, which gives rise to the use of this finish for surgical, pharmaceutical and food preparation applications. The advantages are two-fold; the surface is easy to clean and micro-smoothness means there are no crevices to harbour harmful bacteria. A hygienic surface at its best.
by John Swain