The Proper Handling and Cleaning of Stainless Steel
Stainless steel is corrosion-resistant but it is not rust-proof. You read it right.
The length of its service life depends profoundly on the proper handling, cleaning and storage of the machine or any installation that’s made from this material.
The clear, smooth surface of stainless steel may be affected by chemicals or a mechanical action. Some of these contaminants namely iron particles from non-alloyed metals, dust, grit, and iron-oxide arise from handling, welding, grinding, machining, paint, crayon marks, and tumbling during fabrication. Their presence may penetrate the passive film (chromium oxide) that may cause the stainless steel to rust.
Other surface contamination and the formation of deposits through cracks are critical factors which may lead to drastically reduced life of stainless steel.
Although the resistance of metals can be affected by a host of other factors such as concentration, temperature and presence of other chemicals, a huge part is played by the way they are cleaned to restore an acceptable surface quality for hygiene and corrosion-free maintenance.
- Welds and discoloration associated to welding should be cleaned within 1 to 2 days of being completed to eliminate rust. Electro-polishing, pickling, brushing, and shot blasting may be used
- Rusts and other corrosion-prone products, embedded or adhering free iron can be removed by 10% nitric acid or by pickling;
- Cover stainless steel appliances that are stored outdoor with a non-abrasive cloth to avoid exposure to moisture, salinity and chemicals
- At project design stage, choose the correct grade, condition, surface finish particular for the stainless steel service environment and avoid contact with dissimilar metals to reduce risk of galvanic corrosion
- After stamping, deep drawing and forming process, use clean tools to avoid residue-free iron from carbon steel
- Use appropriate oil.
- Use non-chlorinated solvents when degreasing
- Oil, grease, paints, foot prints, glue residues, and dirt can cause crevice-corrosion removed by organic solvents such as acetone, alcohol, methylated spirits, and degreasing agent that’s chlorine-free.
- Use only soap or mild detergent in warm water in routine cleaning
- Do not use bleach containing chlorine for this may cause pitting corrosion
- Do not use abrasive cleaners that will scratch the surface
- Do not use ordinary, everyday steel wool or steel brushes. These can leave particles that easily rust and contaminate the stainless steel. Microfiber is recommended
- Hard water can leave spotting and staining. It’s best not to assume that it’s caused by the cleaner
- Do not forget to rinse and dry the surface thoroughly
A few more helpful reminders…
Passivation is the removal of free iron and other contamination resulting from handling, fabrication, or exposure to contaminated atmosphere. This is performed by immersing the steel in an oxidant-like nitric acid.
- Grades with at least 16% chromium (except free machining grade such as 303): 20-45 volume % nitric acid, at 21oC to 32oC for 30 to 60 minutes.
- Thorough water rinsing must follow all passivating treatment
- Passivating solution, paste and gel are available commercially.
- Contact time should be in accordance with Supplier’s recommendation
Pickling is for the removal of high temperature scale (dark oxide film) from heat treatment or welding operations and iron contamination.
- All stainless steel (except free machining grades): 8-11 volume % of sulfuric at 65 to 80oC for 5 to 45 minutes
- Grades with at least 16% chromium (except free machining grades): 15-25 volumne of nitric acid + 1-8 volume % hyrdofluoric at 20-60oC for 5 to 30 minutes
- Thorough rinsing must follow pickling treatments;
- Free machining grades such as 303, 410 and 416: 10-15 volume % nitric acid + .5-1.5% hydrofluoric acid, at 20-260oC for 5 to 30 minutes;