SALT SPRAY TEST

'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 of the corrosion resistance of the coating; the more corrosion resistant the coating is, the longer the period in testing without showing signs of corrosion.
There is no 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.

Contents

Testing equipment

Standardization

Uses

References

Testing equipment

The apparatus for testing consists of a closed testing chamber, where a salted solution (mainly, a solution of sodium chloride) is sprayed by means of a nozzle. This produces a corroding environment in the chamber and thus, parts in it are attacked under this severe corroding atmosphere. Typical volumes of these chambers are of 400 L, but they can be constructed larger.
Tests performed with a 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 acc. to DIN 50021). 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.
It is not recommended to use ASS or CASS test cabinets for NSS tests, since a thorough cleaning of the cabinet after ASS or CASS test is very difficult. NSS tests are widely used for the evaluation of many different types of coatings (see "Uses" below).

Standardization

Chamber construction, testing procedure and testing parameters are standardized under national and international standards, such as ASTM B117, DIN 50021, 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 B117, DIN 50021 and ISO 9227 are widely used as reference standards. Testing cabinets are manufactured according to the specified requirements. 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 zinc electroplated zinc and yellow passivation steel surface lasts 96 hours in salt spray test without red rust (RR). Zinc-nickel and transparent passivated steel surface can last more than 720 hours in 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:
• Hot-dip galvanized steel (see also galvanizing)
• Phosphated surfaces
• Zinc and zinc-alloy plating (see also electroplating)
• Electroplated chromium, nickel, copper, tin
• Coatings not applied electrolytically, such as zinc flake coatings acc. to ISO 10683
• Organic coatings
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

• ASTM American Society for Testing of Materials. ASTM B117 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