Salt spray testing of stainless steels

Introduction

Salt spray testing is an accepted method for assessing the suitability of stainless steel parts and fabrications that are likely to encounter chloride environments in service.

For any particular part, tested under laboratory conditions, a difference in performance between say, 430 (1.4016), 304 (1.4301) and 316 (1.4401) types would be expected, but the test outcome is sensitive to the shape of the parts (designed-in crevices), surface finish and the test conditions.
Using specific laboratory salt spray test data to assess if a particular steel grade is suitable for a specific or 'generic' environment is therefore not appropriate.

Salt spray test methods

Salt spray testing is covered by the standards such as
ASTM B 117 - Practice for Operating Salt Spray (Fog) Testing Apparatus
Tests durations may be up to 260 hours.
BS7479:1991 - Method for Salt Spray Corrosion Tests in Artificial Atmospheres
This supercedes BS5466:1977 and is equivalent to ISO 9227.
This standard covers methods in three types of atmospheres: -

    Neutral
    Acetic acid
    Copper accelerated acetic acid (CASS)
The test duration's range from 2 to 96 hours.
The pass / fail criteria is that thers should not be any 'visible' staining on parts tested.
The test acceptance criteria can therefore be subjective and need to be clearly defined for any set situation.

Comparison of 316, 304 and 430 type stainless steels in salt spray testing

Results from one testing laboratory, based on tests done to the ASTM B117 method suggest a 316 type part could be expected to pass a 96 hour test using a 3% salt spray.
Indications are that longer test times would not be expected to give satisfactory results.

In contrast, 304 type parts would not be expected to give satisfactory results in a 3% salt spray, but if the salt solution concentration is reduced to 0.3 %, then it is possible that a 304 type parts may be satisfactory for test times up to 120 hours.
This could also apply to 430 type ferritic or 431 type martensitic stainless steel parts.

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