Chrome-Manganese non-magnetic steel grades have been gaining market lately due to low cost. They are having company specific names as grades as they do not comply with any national or international standards of stainless steel. But in the market, they are referred to as SS 200 series material. These materials have Nickel as low as 0.20% and Chromium as low as 13% as against 4-6% Nickel and 17-19% Chromium in actual Stainless Steel of 202 grade. SS 304 grade is superior grade with 8-10.5% Nickel and 17.5-19.5% Chromium.
These "New 200 series" grades have been developed for certain niche applications like utensils for better drawability and formability. But due to cheaper prices and non-magnetic characteristics, they are now getting used in applications for which they are not at all recommended.
In a study done by M/s. International Stainless Steel Forum (Brussels, Belgium) called "New 200-series Steels An Opportunity or a threat to the image of Stainless Steel?", corrosion resistance of these grades against SS304 has been compared. The lowest grade Chrome-Manganese steel considered for the study was with 15% Chromium and 1% Nickel. So, the result with the material currently sold in Indian market will certainly be much worse than those. We reproduce here a graph for pitting corrosion resistance from the study:
The following image shows corrosion resistance of these Chrome-Manganese grade steels against SS304 in 24 days in a salt-spray test.
After a 575-hour salt-spray test on "304" chromenickel, 4% Ni and 1% Ni, the 4% Ni, 1% Ni samples showed signifi cant corrosion. These chrome-manganese grades are unsuitable for marine-environment applications.
Since these grades are non-magnetic and equally shiny in the beginning, they are not identifiable without sophisticated chemical testing. This, many times, leads to mislabelling and fraud in the long chain between the manufacturer and consumer and gets sold as SS304 material. With S304 Composite Tubes, you can be sure that the material used is of SS304 grade and if used appropriately, will last as long as pure SS304 material in the same application.
We sometimes feel that protection of SS304 is not required in indoor applications. But we forget that everyday usage articles also have many chemicals which lead to corrosion. Acetic acid is present in vinegar. When milk deteriorates, lactic acid is produced. Citric acid is commonly present in fruit juices. Soap has Fatty Acids. Chloride Salts are most commonly occurring substances in nature. All these add to corrosion. So, it is always better to use SS304. And best to use SCP-manufactured through finite engineering for similar protection at much lower cost.