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Stainless steel corrosion resistance properties

Stainless steel grades 316, 304L, 321, 347 are discussed.

Chemical composition

  316 304L 321 347
Carbon .08 % .03 % .08 % .08 %
Chromium 16 to 18 % 18 to 20 % 17 to 19 % 17 to 19 %
Molybdenum 2 to 3%
Iron Rem Rem Rem Rem
Titanium 5 x carbon
Columbium +tantalum (min) 10 x carbon
Nickel 10 to 14 % 8 to 12 % 9 to 12 % 9 to 13 %
Highest application temp oC 649 427 649 649
oF 1200 800 1200 1200

There are above 70 standard kinds of stainless steel. Normally, entire are iron based with 12 to 30% chromium, 0-22% nickel and various magnitudes of other constitutes.

There are three classes of stainless steel alloys: 1. Martensitic 2. Ferritic 3. Austenitic. The martenisitic alloys comprise of 12 to 20% chromium with the limited magnitude of carbon and other elements. The grade 410 is a distinctive member of this class. These alloys can be hardened by heat processing that increase the tensile strengths from 80,000 to 200,000 lbs psi.

Ferritic stainless comprises of 15 to 30% chromium with nominal carbon content. The strength of ferritic stainless steel can be improved by cold processing but not by heat processing. These are quite easily machinable with high speed apparatus. Welding is not an issue. The corrosion resistance is rated sufficient however the ferritic alloys are not better against the reducing acids like hydrochloric acid. The grade 430 is commonly utilized in handling nitric acid.

Austenitic stainless steels are highly corrosion resistant among the three classes. These steels comprise of 16 to 26% chromium and 6 to 22% nickel. These alloys can be work hardened but the heat processing will not cause hardening. The austenitic stainless steels are hardand ductile. Welding of these stainless alloys can produce carbide precipitation and corrosion attack in these regions. To prevent precipitation, special stainless steels stabilized with titanium or columbium have been produced. Another approach to the issue is using low carbon steels like stainless steel 304L and stainless steel 316L.

The stainless steel 304 is the basic alloy of this group. The grade 304 and 304L are low carbon grades of 302. The grades 316, 316L and 317 containing 2.5 to 3.5% molybdenum are the highly corrosion resistant grades.

A confusion lies in determining the low carbon content in stainless steels such as alloy 304 is unlike to 304L as 304L consists of lower concentration of carbon. The lower carbon content is essential when welding is to be done on the metal in fabrication or installation. If no immediate welding is done, the additional cost of receiving low carbon alloy is not beneficial. The weld socket valve ends need after welding and low carbon containing steel is essential when the material is to be utilized in the corrosive conditions.

Galling is an irritating issue with the stainless steels. Various grades of stainless steels with the remarkable variation in hardness, sufficient lubrication and different forms of plating are the options to decrease the stainless steel galling. The use of a specific method depends significantly on the final use of the product.

Uses of stainless steel grades

  • 316: Paper pulp industry-sodium and calcium brines,hypochlorite solutions, phosphoric acid, sulfite liquors and sulfurous acid.
  • Chemical plants- Nitric acid application,phosphoric acid, bleaches, photographic chemicals, printer’s ink, artificial fibers.
  • Food processing- cleanliness
  • Stainless steel 304L
  • Food processing- cleanliness and sterilizing.
  • Chemical units- highly oxidizing acids, normal rusting, dye stuff.

304L is needed instead stainless steel 304 when the application includes welding of object in its production or application.Otherwise this expensive low carbon grade is unnecessary.

321 and 347:

  • Chemical plants- applications where fitting orvalve will be used for welding in installation.
  • Nuclear reactors- Equipment and process lines where welding is needed.