The attractive and hygienic surface appearance of stainless steel products cannot be regarded as completely maintenance free. All grades and finishes of stainless steel may in fact stain, discolor or attain an adhering layer of grime in normal service. To achieve maximum corrosion resistance the surface of the stainless steel must be kept clean.
Provided the grade, condition and surface finish were correctly selected for the particular service environment, fabrication and installation procedures were correct and that cleaning schedules are carried out regularly, good performance and long life will be achieved. Frequency and cost of cleaning of stainless steel is lower than for many other materials and this will often out-weigh higher acquisition costs. These principles apply whether the item concerned is a simple kitchen utensil or a large and complex architectural installation.
Why Maintenance is Necessary
Surface contamination and the formation of deposits are critical factors which may lead to drastically reduced life. These contaminants may be minute particles of iron or rust from other non-stainless steels used in nearby construction and not subsequently removed. Industrial, commercial and even domestic and naturally occurring atmospheric conditions can result in deposits which can be quite corrosive. An example is salt deposits from marine conditions.
Working environments can also create more aggressive conditions, such as the warm, high humidity atmosphere above indoor swimming pools. This particular environment has in a small number of instances been found to be highly aggressive, and specialist advice should be obtained. Aggressive operating environments can increase the speed of corrosion and therefore require more frequent maintenance.
Modern processes use many cleaners, sterilizers and bleaches for hygienic purposes. These proprietary solutions, if appropriate for use with stainless steel and when used in accordance with their makers’ instructions are safe, but if used incorrectly (e.g. warm or concentrated) can cause discolouration and corrosion on the surface of stainless steels.
Maintenance During Installation
Cleaning of new fabrications should present no special problems, although more attention may be required if the installation period has been prolonged. Where surface contamination is suspected, immediate attention to cleaning will promote a trouble-free service life. Food handling, pharmaceutical and aerospace applications may require extremely high levels of cleanliness. Strong acid solutions (e.g. hydrochloric acid or “spirits of salts”) are sometimes used to clean masonry and tiling during building construction but they should never be permitted to come into contact with metals, including stainless steel. If this should happen the acid solution must be removed immediately by copious water flushing, but even if promptly removed the appearance of the steel may be unacceptably changed.
On-Going Maintenance
Advice is often sought about the frequency of cleaning of products made of stainless steel, and the answer is simply “clean the metal when it is dirty in order to restore its original appearance”. The rule for many exterior building installations is to clean the stainless steel whenever the nearby glass needs cleaning. This may vary from once to to quarterly per annun for external applications or it may be once a day for an item in sanitary/food grade or aggressive situations.
In most situations the cleaning frequency is after each use. Suggested cleaning schedules are as in this table – these should be modified by experience. Note that natural rain is an effective cleaner – those items that are not washed by rain water may need more frequent maintenance
cleaning.
- Standard Grade 304 Grade 31
- Off shore 3 – 6 months 6 – 12 months
- Polluted Metropolitan or heavy factory Not suitable 6 – 12 months
- On water/coastal (not splashed) Not suitable 3 – 6 months
Good Housekeeping During Manufacture
Stainless steel can be contaminated by pick-up of carbon steel (“free iron”) and this is likely to lead to rapid localised corrosion. The perfect condition is to have workshops and machinery dedicated to only stainless steel work, but in a workshop also processing other steels avoid pick-up from:
- Tooling used with other metals
- Steel storage racks
- Handling Equipment
- Grinding wheels, wire brushes, finishing belts
- Contamination by grinding or welding sparks from nearby carbon steel fabrication