Salt spray test
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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.
There is 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.
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[edit] 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 because for historical reasons that was the smallest volume accepted by ASTM-B-117, since the 90's there is no request about volume in ASTM, ISO recommends that the chamber shouldn't be smaller than 200 liters in order to receive an acceptable amount of test samples, chambers are available from sizes as small as 100 liters up to several cubic meters, most common machines range from 400 liters to 1000 liters.
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 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 for NSS tests, as it is claimed that a thorough cleaning of the cabinet after ASS or CASS test is very difficult. There is no official position about that, but ISO 9227: 2006 recommends not to do it and if it is to be used if a thorough cleaning not accomplished. NSS tests are widely used for the evaluation of many different types of coatings (see "Uses" below).
[edit] Standardization
Chamber construction, testing procedure and testing parameters are standardized under national and international standards, such as ASTM B117 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 B117 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 red rust (RR). Electroplated zinc-nickel steel parts 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.
[edit] Uses
Typical coatings that can be evaluated with this method are:
• Phosphated surfaces
• 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 acc. to ISO 10683
• Organic coatings
Hot-dip galvanized surfaces are not generally tested in salt spray test (see ISO 1461 or ISO 10684). Hot-dip galvanizing produces zinc carbonates when exposed to the environment, thus protecting the coating metal and reduce the corrosion rate. These by-products are not produced when a hot-dip galvanized specimen is exposed to a salt spray fog. Hence, hot-dip galvanized steel is not normally tested according to this test method and corrosion resistance is expressed as years of corrosion protection in different environments. See ISO 9223 for guidance.
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.
[edit] References
Salt spray testing
• 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
• 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
Referenced standards
• ISO 1461 Hot dip galvanized coatings on fabricated iron and steel articles -- Specifications and test methods, 1999
• ISO 4042 Fasteners -- Electroplated coatings, 1999
• ISO 10683 Fasteners -- Non-electrolytically applied zinc flake coatings, 2000
• ISO 10684 Fasteners -- Hot dip galvanized coatings, 2004
• ISO 12944 Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 6: Laboratory performance test methods , 1998.
Bibliography
- Metal Finishing. Guidebook and directory issue. Published by Metal Finishing Magazine, 1996
