Does Stainless steel corrode?
Steels like 316L are called “stainless” because in oxidizing atmospheres (air and water), they form a protective layer on the surface through a phenomenon called “self passivation”. Oxygen from the atmosphere combines with the chromium in the stainless steel to form this passive chromium oxide film that protects from further corrosion.
Stainless steel is not a single alloy, but rather the name applies to a group of iron-based alloys containing a minimum 10.5% chromium. Other elements like nickel and molybdenum are added and the chromium content increased to improve the corrosion resistance and heat resisting properties, enhance mechanical properties, and/or improve fabricating characteristics.
304 is the basic chromium-nickel austenitic stainless steel and is suitable for a wide range of applications. It is the most readily available in a variety of product forms and is extensively used in commercial and industrial kitchen equipment, cutlery is another well known application. This grade is easy to form and fabricate with excellent resistance to corrosion.
316 offers more corrosion-resistance through the addition of molybdenum. This grade is desirable where the possibility of severe corrosion exists, such as heavy industrial atmospheres and marine environments.
316L 316L differs from 316 due to its reduced carbon content, so the chrome present in the alloy has less possibility of combining with the carbon and forming chromium carbides. The free chrome, in other words chrome not combined with carbon, creates the passivating oxide layer, which provides protection against corrosion.
Cleanliness and stainless steel are closely related and, in many applications, each is dependent upon the other in the handling of food, chemicals, pharmaceuticals and in the use of stainless steel as a construction material. Stainless steel provides the degree of corrosion resistance that is necessary to prevent product contamination or surface rusting. However, stainless steel performs best when clean — cleanliness is essential for maximum resistance to corrosion.
Some form of routine cleaning is necessary to preserve the appearance and integrity of the surface. Stainless steels are easily cleaned by many different methods. They prolong their life with frequent cleaning, and, unlike some other materials, it is impossible to “wear out” stainless steel by excessive cleaning.
Air Energy recently were contacted by a large abattoir that had replaced most of their existing copper pipework with Europress stainless steel, partly because of corrosion issues with the copper. However rather alarmingly, after a while their SS pipework appeared to be rusting right through and leaking.
The plant engineers thought it was leaking from inside the pipe right through the pipe wall.
316 and especially 316L Stainless steel have a high resistance to corrosion, but are suspect to chlorine attack eg high salinity water. Tests on the water supply showed that it was well within acceptable limits.
Tests and spectroscopic analysis on the composition of the stainless steel proved it complied and was midrange on all specification limits.
Cutting out a section and examination showed the SS was failing with a deep pit corrosion typical of chlorine attack, but it seemed to be only on the bottom half of the pipework.
Trouble shooting investigation by Air Energy ascertained that the worst affected were pipes carrying 84º hot water. At the end of each shift the maintenance team washed down all surfaces with an industrial foaming detergent sanitizer. Once all surfaces were wetted in the area they then washed down with water.
But what was happening was the hot water evaporated the cleaner before it was washed down. This concentrated the chemical especially on the bottom half of the pipes. The cleaner that was being used had its active ingredient approx. 2000ppm of chlorine!
Europress pressfittings in AISI 316L stainless steel can be used in environments with moderate salinity. They cannot be used to transport saltwater (and seawater) Cupronickel pressfittings must be used for this application.
316 Stainless steel is corrosion resistant but not corrosion proof.