Salt spray testing is a standard and popular corrosion testing method used to check the corrosion resistance of materials and surface coatings. Typically, the material to be tested is metallic (although stone, ceramic and polymers can also be tested) and is coated with a surface coating designed to provide some level of corrosion protection to the base metal. The salt spray test is an accelerated corrosion test that has a corrosive effect on coated samples in order to (mostly comparatively) assess the suitability of the coating for use as a protective layer. Corrosion appearance products are evaluated after a predetermined time (rust or other oxides). The test time depends on the corrosion resistance of the coating; generally, the more corrosion resistant the coating, the longer the test time before corrosion/rust occurs. Salt spray testing is one of the most extensive and longest established corrosion tests. ASTM B117 was the first internationally recognized salt spray standard, originally published in 1939. Other important relevant standards are ISO9227, JIS Z 2371 and ASTM G85.
The test apparatus consists of a closed test cabinet/chamber in which a saline (5% NaCl) solution is atomized through nozzles (using pressurized air). This creates a corrosive environment of concentrated salt water mist (also known as fog or spray) within the chamber, so test samples exposed to this environment are severely corroded. Chamber volumes vary by supplier. If a minimum volume is required by a particular salt spray test standard, it shall be clearly stated and shall be adhered to. There is a general consensus historically that larger chambers provide a more uniform test environment.
Typical coatings that can be evaluated by this method are:
Phosphating (pre-treatment) of the surface (and subsequent paint/primer/varnish/rust inhibitor)
● Zinc and zinc alloy plating (see also electroplating). See ISO 4042 for guidance
●Electroplated chrome, nickel, copper, tin
Non-electrolytically applied coatings, such as zinc flake coatings according to ISO 10683 Organic coatings, such as rust inhibitors
●Paint coating
Hot-dip galvanized surfaces are not normally subjected to salt spray testing (see ISO 1461 or ISO 10684). When exposed to the elements, hot-dip galvanizing produces zinc carbonate, which protects the coated metal and reduces the rate of corrosion. When hot-dip galvanized samples are exposed to salt spray, no zinc carbonate is produced, so this test method cannot accurately measure corrosion protection performance. ISO 9223 provides guidance for the correct measurement of the corrosion resistance of hot-dip galvanized samples.
Painted surfaces with an underlying hot-dip galvanized coating can be tested according to this method. See ISO 12944-6.
Test times range from a few hours (e.g. 8 or 24 hours on phosphating steel) to over a month (e.g. 720 hours on zinc-nickel coatings, 1000 hours on certain zinc flake coatings).
