The 'Salt Spray' test results are often misleading or less realistic in comparison to those obtained by the 'Cyclic Corrosion' test. The 'Salt Spray' test has been in use for a very long period of time because of its ability to generate results very quickly, but it suffers from the inherent flaw in its test design, i.e. same/fixed experimental parameters mentioned above, which are not made to change over a time period. This leads to the lack of correlation between its own corrosion mechanisms and the actual corrosion mechanism that takes place in real life situations. On the other hand, with 'Cyclic Corrosion' testing, the parameters are periodically changed over a time period, for e.g. 24 hours, so that this method simulates the natural cyclic conditions in a better manner. Hence, the data/results obtained from CCT testing are more reliable in predicting the lifetime of a test component than the 'Salt Spray' test.
Cyclic corrosion testing is intended to produce failures representative of the type found in outdoor corrosive environments. CCT tests expose specimens to a series of different environments in a repetitive cycle. Simple exposures like Prohesion may consist of cycling between salt fog and dry conditions. More sophisticated automotive methods call for multistep cycles that may incorporate immersion, humidity, condensation, along with salt fog and dry-off.
Any or all of the following environments may be used for Cyclic Corrosion testing:-
Ambient Environment: As used in CCT procedures this term means laboratory ambient conditions. Ambient environments are usually used as a way to very slowly change the test sample's condition.
Chamber Environments: Non-ambient environments are usually chamber exposures. Cycling between different non-ambient environments can be performed by physically moving the test specimens from one chamber to another or, in automated chambers, by cycling from one condition to another.
Fog (Spray) Environment: Salt fog application can take place in a B117 type test chamber or be done manually in a laboratory's ambient environment. The fog nozzle should be such that the solution is atomized into a fog or mist. Commonly, in addition to NaCl, the electrolyte solution contains other chemicals to simulate acid rain or other industrial corrosives.
Humid Environment: CCT procedures often call for high humidity environments. Typically they specify 95 to 100% RH. These may be achieved by using ASTM D 2247. As an alternative, a B 117 chamber may sometimes be used to apply a pure water fog.
Dry-Off Environment: A dry-off environment may be achieved in an open laboratory or inside a chamber. The area should be maintained with enough air circulation to avoid stratification and to allow drying of the material. The definition of "dry-off" can be ambiguous. There is disagreement on whether a specimen should be considered dry when the surface is dry, or when the specimen has dried throughout. As corrosion products build up, the time necessary to achieve full dry-off may increase.
Corrosive Immersion Environment: This environment would normally consist of an aqueous solution with an electrolyte at a specified concentration, typically up to 5%. Typical pH is 4 to 8 and temperature is usually specified. The solution should be changed on a regular basis as it becomes contaminated with use.
Water Immersion Environment: Distilled or de-ionized water should be used. ASTM D 1193 provides useful guidance on water purity. The immersion container should be made of plastic or any other inert material. Acidity of the bath should be within a pH range of 6 to 8. Temperature should be 24°C-3°C. Conductivity should be < 50 mmho/cm at 25°C
Compliance to Various Standards:-
ASTM B117: As used in CCT procedures this term means laboratory ambient conditions. Ambient environments are usually used as a way to very slowly change the test sample's condition.
ASTM D2247: Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity
ASTM D2803: Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal
ASTM D3451: Standard Guide for Testing Coating Powders and Powder Coatings
ASTM D4585: Standard Practice for Testing Water Resistance of Coatings Using Controlled Condensation
ASTM D5894: Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal, (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet)
ASTM D6577: Standard Guide for Testing Industrial Protective Coatings
ASTM D6675: Standard Practice for Salt-Accelerated Outdoor Cosmetic Corrosion Testing of Organic Coatings on Automotive Sheet Steel
GM 4465P: Water Fog Humidity Test
ISO 7253: Paints and Varnishes- Determination of resistance to neutral salt spray (fog)
ISO 9227 : Corrosion Tests in Artificial Atmospheres - Salt Spray Tests