Pharmaceutical Stainless Steel – Types, Composition & Difference

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Stainless steel

Stainless steel is named “stain-less, it is really means “stain or rust resistant” for this purpose It is mainly used, this property comes from its ability to naturally form an oxide-rich, passive layer on the surface, where it used. Oxide rich layer works as a protection sheet to resist the surface.

Due to it’s rust resistant property stainless steel is usually the preferred substrate for good manufacturing practice (GMP) applications, and it constitutes the majority of GMP product-contact surface areas.

Why Stainless Steel?

  • Corrosion-resistant in chlorine, alkaline and acid-bearing environments
  • Heat- and fire-resistant
  • Exhibits exceptional hygienic properties by facilitating cleaning
  • Cost-effective, with excellent strength-to-weight leverage
  • Retains strength and resists scaling at extremely high temperatures
  • Impact-resistant

Types

There are over 200 different types of stainless steel with a range of properties to suit various manufacturing needs and customer requirements.

Stainless steel used by its grade, and grade is its chemical composition as per need of usage, e.g. of some grades are as follows:

  • Martensitic grades (401, 410, 416, 420, 420F, 431, 440A, 440B, 440C, S44020),
  • Ferritic grades (405, 430, 430F, 434),
  • Austenitic ferritic grades (329, S31803, S31260),
  • Austenitic grades (201, 202, 301, 302, 303, 303Se, 304, 304L, 304N, 304LN, 305, 309S, 310S, 316, 316L,, 316LN, 317, 317L, 321, 347, 304Cu, S17400, S17700).

Pharmaceutical companies must have pharmaceutical-grade stainless steel manufacturing equipment that passes the FDA’s rigorous standards.

Austenitic stainless-steel series (e.g., 304, 304 L, 316 and 316L) has been popular in pharmaceutical applications because of its high stain and outstanding corrosion resistant properties and affordability.

“L” indication in SS grades

L indicates low content of carbon. SS 304L and 316L has low amount of carbon (0.03%) in the compression of SS304 and 316 grade (0.08%).

Grade 304 & 304L have only difference of low carbon content in its chemical composition, same as in SS  316 and 316L grade. Rest chemical composition are same in both of grade. Low carbon content grade SS is more corrosion resistant than others, low carbon content also makes more ease to weld the SS.

Difference between SS 304 (304L) & SS 316 (316L)

  • Grade 316, 316L is used in all ‘product contact applications’ and 304 used in ‘non contact applications’. However, there are some instances where the product is not suitable to stainless steel and qualified guidance is advised.
  • Both type of Stainless steel are non magnetic, easily welded and formed.
  • 316L grade Stainless Steel is often specified in Pharmaceutical installations in order to prevent excessive metallic contamination. 316L grade Stainless Steel is also significantly more resistant to heat in comparison to 304.
  • 304 contains no trace of molybdenum, 316 contains 2% molybdenum. The biggest difference between the grades is molybdenum is added to grade 316 to increase corrosion and pitting resistance. The addition of Molybdenum makes 316L Stainless Steel stronger and more durable, capable of withstanding higher levels of corrosion and the affects of chemicals such as chlorides (found in salt, seawater, sweat, etc.) and sulphuric acid.

As e.g. Type 304 stainless steel bucket could pit through in less than 8 hours. When molybdenum is added to the stainless steel the pitting corrosion resistance improves.

SS 304 & SS304L Chemical Composition

                               304                           304L

Carbon                0.08 max.                  0.03 max.

Manganese        2.0 max.                     2.0 max.

Phosphorus        0.045 max.                0.045 max.

Sulfur                   0.030 max.                0.03 max.

Silicon                  0.75 max.                   0.75 max.

Chromium           18.0 – 20.0                 18.0 – 20.0

Nickel                   8.0 – 12.00                 8.0 – 12.0

Nitrogen               0.10 max.                   0.10 max.

Iron                       Balance                      Balance

    

SS 316 & SS316L Chemical Composition        

                                    316                          316 L

Carbon                 0.08 max.                  0.03 max.

Manganese         2.0 max.                    2.0 max.

Phosphorus         0.045 max.                0.045 max.

Sulfur                    0.03 max.                  0.03 max.

Silicon                   0.75 max.                  0.75 max.

Chromium            16.0 – 18.0                 16.0 – 18.0

Nickel                    10.0 – 14.0                 10.0 – 14.0

Molybdenum        2.0 – 3.0                     2.0 – 3.0

Effective role of other chemicals in SS chemical composition

  • Ni improves the resistant to hot and humid environment condition
  • Manganese increase the tensile strength
  • Silicon improve the resistant of SS to the strong acid and sulfuric acid.

Before use of new SS equipment, pipes and etc., to remove extra chemical or chemical layer from surface a process has been done usually called “Passivation”.

As per regulatories bodies Passivation is a compulsory process should be performed before use of SS made tools, equipment, pipes and all.

Passivation Process

Passivation means to make SS surface nonreactive as in surface  chromium content of SS reacts with oxygen and develops a layer of chromium oxide, this is a form of passive layer, passivation is a process to remove this outer layer from the surface of SS. Usually it has been done by using nitric acid in mild concentration.

Technically we can say:

Passivation is the formation of a thin adherent film or layer on the surface of a metal or mineral that acts as a protective coating to protect the underlying surface from further chemical reaction, such as corrosion, electro-dissolution, or dissolution. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or many other fluids that contain oxygen.

Once the layer has formed we say that the metal has become “passivated” and the oxidation or “rusting” rate will slow down to less than 0.002 of an inch”.

Related Article:

Passivation of Stainless Steel in pharmaceuticals 

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