7/27/2019 The Corrosion Resistance of Stainless Steel is Dependent on the Presence of a Protective Oxide Layer

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The corrosion resistance of stainless steel is dependent on the presence of a protective oxide layer

(metal bonded to Oxygen) on its surface, which can break down in very strong acids. Stainless steel

resists corrosion by growing a hard shiny film of mostly chromium oxide to protect the base metal alloy

from corrosion. In the under water marine environment stainless steel needs to get some of the oxygen

which is dissolved in seawater to repair and grow this protective oxide surface film. If there is too little

oxygen available from the seawater. Pitting and Crevice Corrosion are usually the result. These are the

most common forms of Oxygen Depletion corrosion which affect stainless steel running gear in the

marine environment.

Crevice Corrosion can form on the surface of the stainless steel in salt water dependent uponthe environmental conditions, surface finish, contamination and the composition of the steel

itself. This pitting occurs under special conditions, involving sodium chloride (salt) in sea water

and greatly exacerbated by the elevated temperatures . Once started these small pits can

continue to grow and grow in a self sustaining cycle.

Hydrochloric acid is produced by the corrosion which also attacks stainless steel, making adeeper crevice, which produces more acid, which produces a deeper pit. Hence the honeycomb

effect of the corrosion damage. If left undisturbed, they can propagate to amazing depth over a

few months time.

Cryogenic hardening is acryogenicheat treatingprocess where the material is cooled to

approximately 185 C(301 F), usually usingliquid nitrogen. It can have a profound effect on the

mechanical properties ofaustenitesteels, It is designed to increase the amount ofmartensitein the

steel's crystal structure, increasing itsstrengthandhardness. Presently this treatment is being practiced

over tool steels, high-carbon, and high-chromium steels to obtain excellent wear resistance. Recent

research shows that there is precipitation of fine carbides in the matrix during this treatment which

imparts very high wear resistance to the steels.

The austenitic stainless steels such as 316L are tough at cryogenic temperatures and can be classed as

cryogenic steels. During the Cryogenic Tempering Process a material such as austenitic steel goes

through a phase change that transforms the crystal lattice structure from body-centered cubic to face-

centered cubic. The face-centered cubic structure has less space available for interstitial defects and

results in a stronger, more durable material.

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