What Does Prohesion Mean?
Prohesion is a method of cyclic accelerated corrosion testing. This test relates more closely to long-term natural exposure than do conventional salt spray or humidity/sulfur dioxide tests. This test is particularly relevant to paints on steel.
The results of prohesion testing are more representative of outdoor corrosion than salt spray test results. Prohesion shows blistering and delamination which correlate with exterior exposure. These tests are designed to evaluate how well surface coatings protect substrates from corrosion.
Prohesion, which is a shortened form of "protection is adhesion," is also known as ASTM G85 annex A5 - dilute electrolyte cyclic fog/dry test.
Corrosionpedia Explains Prohesion
Prohesion is a less severe version of the standard salt spray tests and is generally regarded as giving a better correlation with outdoor exposure results. Prohesion testing has been found especially useful for industrial maintenance coatings.
The method involves mounting the test panels in a chamber into which is introduced an aqueous solution of salts in the form of a fine aerosol. The method differs from the standard salt spray tests in that the salt solution used in the prohesion test is much more dilute and the panels are not exposed to it continuously. In addition, the spray atomizing air is not saturated with water.
The test method was developed by British Rail and Mebon Paints. There is no pass/fail criteria defined in the standards—this is a matter of agreement between the parties concerned.
The prohesion test usually uses an electrolyte of 0.4% ammonium sulphate and 0.05% sodium chloride and one hour exposure at 662°F (350°C). It also uses:
- Electrolyte spray at ambient temperature
- Drying off at elevated temperature
- Rapid cycling between spray and dry conditions
The test should be regarded as producing comparative rather than absolute data and ideally samples with known outdoor exposure performance should be used as controls.
In prohesion during a dry cycle, the high air flow combined with the elevated temperature changes the levels of humidity considerably in the chamber. Not only do the humidity levels change due to this drying effect, but the introduction of air may enhance the oxidation process and consequently increase the rate of corrosion.