Stainless Steel Post Weld Finishing-A Technical Review
By Metal Science Technologies
To a very large extent stainless steels are used because of the corrosion resistance of their surfaces. This excellent corrosion resistance can only be achieved if proper cleaning and finishing operations are carried out after any fabrication process which will have damaged the surface condition.
Stainless steels are iron alloys with a minimum of 10.5% chromium. The corrosion resistance of stainless steel arises from a chromium oxide film that forms a protective layer naturally on the surface of the steel. The oxide film is extremely thin,a few angstroms, but strongly adherent and chemically stable which is commonly referred to as ‘passive’. The film is formed when chromium reacts with oxygen. The film may be described in chemical terms as a Chromium oxy hydrate.
Maximum corrosion resistance only occurs when the film develops as an unbroken film across the entire structure. Any breaks in the film means that the anodic base metal is in contact with the corrosive environment. As such an electrochemical pathway opens which will result in corrosion of the base metal.
The high temperature of welding results in severe chromium depletion not only at the weld surface but through out the heat affected area. Therefore removing surface material such as oxide scale is only half the job as the weld area is still chromium depleted relative to the parent stainless steel.
Critically when the weld is depleted of chromium its corrosive and bacteria resistance are severely reduced. The weld is therefore vulnerable to pitting and crevice corrosion and what is often referred to as tea staining and of course it loses it reflectivity and cosmetic appeal.
Surface contamination with iron particles, grease and dirt also occurs during fabrication processes such as cutting and grinding as well as during general handling.
The best way to post-weld finish stainless steel is electro-polishing, which is an electrochemical process that increases density of the chromium throughout the weld surface and therefore the ability to create the oxide film. Electropolishing provides the most dense and durable
passive film that it is possible to achieve.
Electropolishing selectively dissolves the microscopic high points of the stainless and selectively removes iron so increases the chromium density and creates a microscopically smother surface on which contaminates can not bind or hide. It is usually accomplished by immersion of the item in a large, acid-containing bath. It is generally impractical to perform this on-site, meaning the work piece must be transported to and from an external location, at considerable and sometimes prohibitive expense. This process has become available in a portable machine that can in principle duplicate the bath immersion process and do it very quickly.
One such machine was a recent episode winner on the ABC TV show the New Inventors. The machine electro polishes using a food grade acid that combines a phosphoric acid with several sequestering agents to deliver a significant improvement in operator safety.
When Gary Mitchie, Welding Supervisor and Inspector and Fellow of the Welding Technology Institute of Australia heard about the machine he arranged a demonstration and was so impressed he said “This will change the stainless steel welding industry. Its application in heavy industry makes my job easier and delivers my clients a quality and OH&S benefit that will raise the industry benchmark. The Job Safety Environmental Analysis for the pickling paste is 10 pages and I will not allow pickling paste on any site I am contracted to work on.”
Passivation which could be simply called cleaning re-establishes an oxide film on the surface only and is carried is out by application of nitric acid, which is a highly corrosive and dangerous to exposed skin. The weld will remain chromium depleted relative to the parent stainless steel
Pickling using pickling paste which is typically a highly toxic chemical cocktail of nitric and hydrofluoric acids and prohibited for OH&S reasons on many commercial sites. Scanning Electron Microscopy has shown that pickling paste chemicals have an etching effect, which can increase the surface roughness profile, resulting in an obvious contamination problem. This can appear to the naked eye as a dull stain. The correct OH&S requirements for workers handling pickling paste are onerous in process, time and cost and while they reduce the health risk they do not remove it. There are a growing number of restrictions on storage and use of pickling paste. The pickling process should generally include a pre-treatment, an after-treatment and a waste treatment, to reach the required result and to meet environmental requirements.
Cleaning with tools such as mechanically grinding, buffing, wire brushing and/or polishing the weld is a surface treatment and leaves scale residues and abrasion and often some residue from cleaning so is typically a poor corrosive resistant finish. It can be used as a preparatory step. While it aids corrosion resistance by reducing surface roughness the smearing effect creates an amorphous, deformed surface layer. Iron-oxides, polishing compounds and other materials become embedded and entrapped in the distorted structure, creating a vulnerability point for surface corrosion.
Electro-polishing will return the characteristic properties of stainless steel to maximum effect being chromium rich, metallically clean and free from stress, a smoother surface with reduced microscopic cracks, minimum absolute surface area and chemically passive
There is an increasing use of mobile electrochemical post weld finishing machines that electropolish because technology has allowed them to be more powerful and faster than the weld pickling machines that have been available for some time. Furthermore the Electropolisher will give a chromium richer weld rather than a surface clean that weld pickling machines have historically delivered at relatively slow rate. Accordingly the safety benefits are now matched by the commercial efficiencies
Process Durability of Surface*
*This is in an indicative value only, score out of 10.
Contact: Richard Ray,
Metal Science Technologies 02 66809963 0r 0424030187