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What Makes Stainless Steel “ Stainless ”? Stainless steel differs from carbon steel by the amount of chromium present . Unprotected carbon steel rusts readily when exposed to air and moisture . This iron oxide film ( the rust ) is active and accelerates corrosion by forming more iron oxide . The corrosion will continue until all the carbon steel is converted into iron oxide .
Stainless steel is a group of alloys that contain a minimum of 10.5 % chromium . Because stainless steel contains at least 10.5 % chromium , the oxidation of the iron is changed to produce a complex oxide that resists further oxidation and forms a passive layer on the surface . The passive layer is only a few microns in thickness and will reform if it is removed by scratching or machining . If stainless steel is properly selected and maintained , it should not suffer any corrosion . For highly corrosive areas , 316 grade or clear coat over 304L may be recommended . Hager ’ s standard grade stainless steel is 304L .
What Makes Stainless Steel Corrode ? Stainless steel is corrosion resistant not corrosion proof and will corrode under certain conditions . Corrosion in stainless steel is not the same type of corrosion as found in carbon steel . The most likely form of stainless steel corrosion is “ pitting ”. Pitting occurs when the environment overpowers the ability of stainless steel to reform the passive layer . Normally dark brown pits occur on the surface of the stainless steel .
The three key types of corrosion that normally effect door hardware products are :
1 . Atmospheric Corrosion Atmospheric corrosion occurs when stainless steel is exposed to airborne liquids , solids or gases . Some sources of atmospheric corrosion are sea spray , rain in coastal zones , salt ( possibly from de-icing ), and dirt . This form of corrosion is typically worse outdoors , especially near marine environments .
2 . Chemical Corrosion Chemical corrosion takes place when stainless steel comes in direct contact with a material that has a high chloride concentration or a strong corrosive solution . Some factors which affect the severity of chemical corrosion include : chemical / chloride concentration level , duration of contact , frequency of washing , and the operating environment .
3 . Contact Corrosion A small piece of carbon steel , scale , copper , or foreign material lodged in stainless steel may be sufficient to destroy passivity at the point of contact and cause pitting . Carbon steel brushes and steel wool should never be used to clean stainless steel .
Maintenance of Stainless Steel Finishes Cleanliness is essential for maximum resistance to corrosion . Surface contamination by dirt or other materials hinders the ability of stainless steel to reform the passive layer and traps corrosive agents , reducing corrosion protection .
In many cases , standard household cleaners can be used to clean and maintain stainless steel hardware items . Soap , ammonia , and detergent and water are used to clean normal dirt . Cleaning should always be followed by rinsing with clean water . When water contains mineral solids , which leave water spots , it is advisable to wipe the surface completely with dry towels .
In selecting cleaning practices , consider the possibility of scratching and the potential for post cleaning corosion caused by incompletely removed cleaners . Select only cleaners that are not abrasive , not acidic , and low in chloride . Many antibacterial cleaners used to clean and sanitize door hardware contain high levels of chlorides , which will cause corrosion if not properly removed from the surface by rinsing .
Routine cleaning is essential , especially in environments that have elevated levels of atmospheric corrosion . Proper care for your stainless steel door hardware will result in many years of durable use .
Acknowledgements : Stainless Steel Industry of North America Euro Inox - European Stainless Steel Development Association
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