Steel is said to be stainless when it shows resistance to aggressive agents. Stainless steel will generally have a Chromium content of not less than 10,5 % by mass and a carbon content not exceeding 1,2 % by mass. All surface contaminants resulting from mill processing (e. g. hot forming, heat treatment or welding) have to be removed to provide a metallurgically clean surface, as this has a definite influence on its corrosion resistance. The 13 % Chromium grades need to have the cleanest surface, achieved by polishing or grinding. Surface contaminations as grease, discolouration etc. can have almost as much effect on corrosion resistance as choosing the wrong grade. 

Steel is said to be heat resistant when it displays the ability to withstand hot grases and products of combustion at temperatures above 550°C and at the same time retaining good mechanical properties during cyclical working.The extent of its heat resistance very much depends on working conditions (corrosion) and cannot be defined by any one single testing method. The highest temperature in atmospheric conditions could be up to 1150°C, depending on the alloying of the material, but with the addition of other gases e. g. sulphur bearing, water vapour, ash etc, this could be dramatically lower. Resistance to molten metals and salts is very limited, and in these instances we would recommend consultation. 

Highly corrosion resistant steels are those alloys that have the ability to resist attack from aggressive elements, and will have high Nickel and Chromium contents with the addition of Molybdenum and Copper. The grade of material and its specific finish has to be matched to the application to provide optimum performance. Corrosion resistance can be affected by the following:

-          the surface is not metallic clean
-          mill scale and discolouration
-          grease, paint or other surface impurities
-          an unsuitable grade has been chosen 

Stainless steel: 1.4021, 1.4034, 1.4057, 1.4104, 1.4112, 1.4122, 1.4301, 1.4305, 1.4306, 1.4313, 1.4436, 1.4439, 1.4541, 1.4542, 1.4571, 1.4539, 1.4460, 1.4462 etc.
For all industrial activity. 

Heat resisting steel: 1.4713, 1.4742, 1.4762, 1.4828, 1.4828, 1.4841, 1.4876, 1.4878 etc.
For all industrial activity. 

Highly corrosion resistant steels: 1.4529, 1.4539, 1.4462, 1.4439 etc.
1.4529 and 1.4539: their resistance to chlorides and a good behaviour in sulphuric acid media allow is usage in problematical cases, e. g. in chemical process technology, in the food industry, in flue gas desulphurising systems, in seawater and sewerage treatment.
1.4462: it is used e. g. for movable parts in pumps and for pipelines, for pressure vessels up to 280°C and in general as substitution for normal stainless steels, using reducend material thicknesses.
1.4439: suitable applications for this steel are: chemical-, cellulose-, paper- and photographic industry.