Salt spray test

  1. Pages with broken file links
  2. Metallurgical processes
  3. Measuring instruments
  4. Corrosion
  5. Coatings


The salt spray test is a standardized test method used to check corrosion resistance of coated samples. Coatings provide corrosion resistance to metallic parts made of steel, zamak or brass. Since coatings can provide a high corrosion resistance through the intended life of the part in use, it is necessary to check corrosion resistance by other means. Salt spray test is an accelerated corrosion test that produces a corrosive attack to the coated samples in order to predict its suitability in use as a protective finish. The appearance of corrosion products (oxides) is evaluated after a period of time. Test duration depends on the corrosion resistance of the coating; the more corrosion resistant the coating is, the longer the period in testing without showing signs of corrosion.

Salt spray testing is popular because it is cheap, quick, well standardized and reasonably repeatable. There is, however, only a weak correlation between the duration in salt spray test and the expected life of a coating, since corrosion is a very complicated process and can be influenced by many external factors. Nevertheless, salt spray test is widely used in the industrial sector for the evaluation of corrosion resistance of finished surfaces or parts.

Testing equipment

File:SS cabinet.jpg
A salt spray cabinet

The apparatus for testing consists of a closed testing chamber, where a salted solution (mainly, a solution of 5%sodium chloride) is atomized by means of a nozzle. This produces a corrosive environment of dense saline fog in the chamber so that parts exposed in it are subjected to severely corrosive conditions. Typical volumes of these chambers are of 15 cubic feet (420 L) because of the smallest volume accepted by International Standards on Salt Spray Tests - ASTM-B-117, ISO 9227 (400 litres) and now discontinued DIN 50021 (400 litres). It has been found very difficult to attain constancy of corrosivity in different exposure regions within the test chambers, for sizes below 400 litres. Chambers are available from sizes as small as 9.3 cu ft (260 L) up to 2,058 cubic feet (58,300 L). Most common machines range from 15 to 160 cubic feet (420–4,500 L).[1][2][3]

Tests performed with a standardized 5% solution of NaCl are known as NSS (neutral salt spray). Results are represented generally as testing hours in NSS without appearance of corrosion products (e.g. 720 h in NSS according to ISO 9227). Other solutions are acetic acid (ASS test) and acetic acid with copper chloride (CASS test), each one chosen for the evaluation of decorative coatings, such as electroplated copper-nickel-chromium, electroplated copper-nickel or anodized aluminium.

Some sources do not recommended to use ASS or CASS test cabinets interchangeably for NSS tests, as it is claimed that a thorough cleaning of the cabinet after ASS or CASS test is very difficult. ASTM does not address this issue, but ISO 9227 does not recommend it and if it is to be done, advocates a thorough cleaning.

Standardization

File:Whiterust.jpg
Electroplated and yellow chromated bolt with white corrosion
File:Redrust.jpg
Zinc flake coated bolt with red rust after testing

Chamber construction, testing procedure and testing parameters are standardized under national and international standards, such as ASTM B 117 and ISO 9227. These standards describe the necessary information to carry out this test; testing parameters such as temperature, air pressure of the sprayed solution, preparation of the spraying solution, concentration, pH, etc. Daily checking of testing parameters is necessary to show compliance with the standards, so records shall be maintained accordingly. ASTM B 117 and ISO 9227 are widely used as reference standards. Testing cabinets are manufactured according to the specified requirements here. However, these testing standards neither provide information of testing periods for the coatings to be evaluated, nor the appearance of corrosion products in form of salts. Requirements shall be agreed between customer and manufacturer. In the automotive industry requirements are specified under material specifications. Different coatings have different behaviour in salt spray test and consequently, test duration will differ from one type of coating to another. For example, a typical electroplated zinc and yellow passivated steel part lasts 96 hours in salt spray test without white rust. Electroplated zinc-nickel steel parts can last more than 720 hours in NSS test without red rust (or 48 hours in CASS test without red rust) Requirements are established in test duration (hours) and coatings shall comply with minimum testing periods.

Uses

Typical coatings that can be evaluated with this method are:

  • Phosphated surfaces (with subsequent paint/primer/lacquer/rust preventive)
  • Zinc and zinc-alloy plating (see also electroplating). See ISO 4042 for guidance
  • Electroplated chromium, nickel, copper, tin
  • Coatings not applied electrolytically, such as zinc flake coatings according to ISO 10683
  • Organic coatings

Hot-dip galvanized surfaces are not generally tested in a salt spray test (see ISO 1461 or ISO 10684). Hot-dip galvanizing produces zinc carbonates when exposed to a natural environment, thus protecting the coating metal and reducing the corrosion rate. The zinc carbonates are not produced when a hot-dip galvanized specimen is exposed to a salt spray fog, therefore this testing method does not give an accurate measurement of corrosion protection. ISO 9223 gives the guidelines for proper measurement of corrosion resistance for hot-dip galvanized specimens.

Painted surfaces with an underlying hot-dip galvanized coating can be tested according to this method. See ISO 12944-6.

Testing periods range from a few hours (e.g. 8 or 24 hours of phosphated steel) to more than a month (e.g. 720 hours of zinc-nickel coatings, 1000 hours of certain zinc flake coatings).

Examples of application of this test as an acceptance method for coated surfaces are the automotive, construction and aerospace industries.

References

  1. ↑ http://www.singletoncorp.com/scch.html
  2. ↑ http://www.autotechnology.net/Standard_Physical.asp
  3. ↑ http://www.mahipindustries.com/files/Why_Mahip_Salt_Spray_Chambers.pdf

Bibliography

  • Metal Finishing. Guidebook and directory issue. Published by Metal Finishing Magazine, 1996

Further reading

  • ASTM American Society for Testing of Materials. ASTM B 117-07a Standard Practice for Operating Salt Spray (Fog) Apparatus, 2007
  • Deutsches Institut für Normung e.V. DIN 50021 Sprühnebelprüfungen mit verschiedenen Natriumchloridlösungen. Beuth Verlag GmbH, 1988. This standard has been superseded by ISO 9227 and it is only mentioned for bibliographic purposes
  • ISO International Organization for Standardization. ISO 9227 Corrosion tests in artificial atmospheres—Salt spray tests, 2006
  • ISO International Organization for Standardization. ISO 4628-3 Paints and varnishes. Evaluation of degradation of coatings. Designation of quantity and size of defects, and of intensity of uniform changes in appearance. Part 3 Assessment of degree of rusting


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