Stainless steel needs to be maintained through suitable surface finishing and cleaning methods. Nigel Willcock, 3M’s Abrasive Systems Division, discusses the properties of stainless steel and how to optimise its benefits
Stainless steel is a highly attractive material for use in environments where it will be exposed to chemicals, liquids or foods. All metals oxidise when exposed to air, causing materials such as regular iron and steel to rust. However, the composition of stainless steel ensures that an extremely thin passive chromium oxide layer is created on the surface which then protects the bulk metal from further oxidation. This passive layer is ‘self-healing’ in that if the surface is damaged or removed the passive layer re-forms, protecting the bulk material.
The addition of nickel to stainless steel offers valuable fabrication advantages, such as better formability and weldability, as well as improved corrosion resistance. There are several main groups of chromium-nickel stainless steels and those most commonly used within the pharmaceutical industry are known as ‘austenitic’. An 8–12% nickel content makes them ductile, meaning they can be easily fabricated while remaining tough. Additionally, their 18% chromium level gives them an excellent defence against general corrosion.
Storage tanks that are used for acidic liquids are often made of stainless steel because of its corrosion resistance
Having decided to use a form of stainless steel, there are a number of reasons why a company might need to modify the surface of the metal. These needs fall into two broad categories – cleaning and finishing.
The alloy is formed into coils, sheets, plates, bars, wire and tubing for various products including industrial equipment for chemical processing, tank and vessels, surgical instruments and major appliances. Storage tanks and tankers that are used to transport acidic liquids are often made of stainless steel because of its corrosion resistance. This also influences its use in commercial kitchens and food processing plants as it can be steam cleaned and sterilised without needing paint or other surface finishes.
There are five identifiable applications that fall into the surface finishing and cleaning requirements of stainless steel:
Light stock removal: When thin sheets of stainless steel are finely welded together via the tungsten inert gas (TIG) method there will be excess weld material left on the surface that needs removing. Often, an abrasive will be used for this process; however, this can sometimes pose a risk of substantially affecting the shape of the component and there is also potential for a mistake to be made depending on operator skill.
3M has developed a range of Scotch-Brite Products that are suitable to use as an alternative, providing surface refining at the same time. Scotch-Brite Light Grinding and Blending Discs provide both time and control benefits by removing the weld with less aggression leaving the more rounded edge expected from a Scotch-Brite Product in one step. The durable non-woven abrasive product utilises Cubitron Technology from 3M for consistency and measurable results.
Surface refining, finishing and blending: These steps are more prevalent in the manufacturing process of items such as pharmaceutical tanks and vessels or professional kitchen equipment. If there is a significant amount of excess weld material left then a more aggressive approach will be required.
The grinding process often removes the passivation of stainless steel as it creates a jagged surface prone to corrosion
If the weld needs to be removed using a coated abrasive step then Scotch-Brite Surface Conditioning Discs by 3M are best placed for this type of refining application and finish the surface in a process that is time efficient. The technology is impregnated with non-woven nylon web with mineral throughout its structure providing a uniform finish that also offers aesthetic properties. By confining the area to a minimum and reducing the risk of gouging or undercutting there is less re-work required compared with traditional methods.
Furthermore, the grinding process often removes the passivation of stainless steel as it creates a jagged surface prone to corrosion. When harsher abrasives are used in conjunction with Scotch-Brite Metal Finishing Products, it creates a smooth surface that potentially reinstalls the passivation layer.
Cleaning and surface preparation: When stainless steel is welded together the heat creates oxidisation known as ‘heat discoloration’ and this is potentially corrosive if not removed. However, the grinding removal process weakens the weld and base material so in some situations this is replaced with a cleaning method instead.
Traditional methods of cleaning and surface preparation include ‘pickling’ with acid or wire brushes, both of which pose their own set of risks
Traditional methods include ‘pickling’ with acid, or wire brushes both of which pose their own set of risks. Nitric and hydrochloric acid release harmful fumes and can burn human skin. Similarly wire brushes can be dangerous if they come into contact with skin as they are highly aggressive, and eye and body protection is required due to the risk of wires becoming loose. Wire brush selection also needs consideration as a low grade stainless steel or a carbon steel brush could contaminate the steel.
The Scotch-Brite Radial Bristle Brush by 3M is one alternative to these methods. It is clean, dry and more efficient without damaging the underlying surface. The moulded construction is impregnated with the Cubitron Technology by 3M which gives a constant cleaning action throughout its life. Compared with wire brushes, the bristle disc construction eliminates the risk of flying wires and minimal pressure is required, making it extremely easy and comfortable to use.
Deburring: If the stainless steel used to manufacture tanks and vessels, professional kitchens or other pharmaceutical equipment is being cut to shape from a larger section then this can cause a burr on the underside of the material. The sharpness predominantly poses a material handling risk as well as the end-product safety for the user. Traditional methods utilising files, scraping tools or harsh abrasives suit the purpose; however, they are imprecise from a surface finishing point of view. These devices will only remove the corner, on a microscopic scale, leaving a 45° chamfer.
Scotch-Brite Deburring Wheel and Scotch-Brite Radial Bristle Brushes are ideally placed for an absolute deburr with a rounded edge. They work more efficiently across all metal applications from removing light burrs on precision parts to heavier deburring of rough edges depending on the product chosen.
In the pharmaceutical industry stainless steel is the most commonly used material due to its tolerance of constant cleaning
Surface cleaning: In the pharmaceutical industry stainless steel is the most commonly used material due to its tolerance of constant cleaning. Generic wire wool cleaning pads have traditionally been used for cleaning; however, they can cause a contamination issue due to the wire wool affecting the passivation of the surface. Scotch-Brite Hand Pads by 3M have an abrasive grain in the non-woven nylon web to remove excess chemical or liquid more gently without leaving any contamination.
There are many regulations to which companies involved in the manufacture, transportation and preparation of chemicals have to adhere. These include ensuring that any material coming into contact with the chemicals does not transfer any of its constituents in quantities that could endanger human health or bring about an unacceptable change in the composition of the chemicals or a deterioration in its characteristics.
Hazard And Critical Control Point (HACCP) analysis requires that the operators of commercial process, including chemical processing and packaging, should actively look for potential threats to health, safety and hygiene. A ‘threat’ is defined as anything from infection resulting from poor design or lack of hygiene, through the degradation of materials by erosion or corrosion, to component failure by fracture or fatigue.
HACCP systems must now consider the possibility of materials contaminating the chemicals with which they are in contact, including stainless steel. These regulations, together with the aesthetic appearance, ease of cleaning, durability and formability of stainless steel, make it an increasingly popular material for the pharmaceutical industry.
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