ASTM G85 Annex 5 Prohesion Testing
Prohesion, also known as ASTM G85 Annex A5 - dilute electrolyte cyclic fog/dry test is a method of cyclic accelerated corrosion testing. Prohesion, a contraction of "protection is adhesion" was developed by F.D. Timmins in conjunction with Mebon Paints, a UK manufacturer of surface coatings, during the 1970's when British Rail wanted to develop a method of evaluating corrosion performance of maintenance coatings on steel.
The cyclic nature of ASTM G85 Annex A5 Prohesion is more like long-term natural exposure than ASTM B117 salt spray, or other static corrosion tests where exposure conditions are constant. Prohesion-tested samples exhibit blistering and peeling which correlate with exterior exposure.
ASTM G85 Annex A5 Prohesion involves mounting the test panels in a temperature-controlled chamber and exposing the samples to a fine mist of dilute corrosive salts in demineralized water. Prohesion differs from standard salt spray exposure primarily due to the Prohesion salt solution being significantly more dilute and the panels are cyclically wetted by the electrolyte and then allowed to dry.
ASTM G85 Annex A5 Prohesion is a derivation of the standard salt spray tests and is widely viewed as providing a corrosive environment that better emulates outdoor exposure. Prohesion testing is particularly useful in evaluation of the corrosion performance of industrial maintenance coatings.
The ASTM G85 Annex A5 Prohesion test employs an electrolyte of 0.4% ammonium sulfate and 0.05% sodium chloride featuring electrolyte spray at ambient temperature, drying at elevated temperature, and rapid cycling between spray and dry conditions. Specifically, the Prohesion cycle consists of one hour of exposure to a continuous indirect fog of dilute acidified salt water solution as described above, which condenses on the specimens at a rate of 1.0 to 2.0 ml/80cm²/hr, at ambient temperature ranging between 21oC and 27oC, followed by one hour of drying at a chamber temperature of 35oC. The introduction of purge air provides a fresh supply of oxygen which in conjunction with the corrosive agents in the slightly acidic electrolyte creates the conditions for accelerated corrosion.
As with most chamber-based laboratory corrosion resistance testing, ASTM G85 Annex A5 Prohesion should be utilized for comparative testing of different coatings or coating formulations rather than for direct correlation with “real world” conditions, because real world conditions vary so much with geographic location. It is recommended that samples with known performance be used as controls when conducting ASTM G85 Annex A5 Prohesion testing on coated samples with unknown performance.
ASTM G85 Annex A5 Prohesion is often combined with UV Weathering in ASTM D5894 Cyclic Salt Fog/UV Exposure of Painted Metal, also described in ISO 11997-2. Samples are exposed for one week in accordance with ASTM G151 and G154, and Cycle 2 of ASTM D4587, followed by one week in ASTM G85 Annex A5. ASTM D5894 combines the corrosivity of Prohesion with UV weathering to even better emulate outdoor exposure conditions in an accelerated laboratory manner.
ASTM G85 Annex A5 Prohesion is also specified in AAMA 2605 for evaluation of coatings on aluminum fenestration products.
There are no pass/fail criteria defined in the ASTM G85 Annex A5 Prohesion standard, which only teaches how to perform the Prohesion test. Acceptance criteria can be found in a governing standard, specification, or drawing.
The other reference documents in ASTM G85 Annex A5 are listed here:
ASTM D609 “Practice for Preparation if Cold-Rolled Steel Panels for Testing Paint, Varnish, Conversion Coatings, and Related Coating Products”
ASTM D1141 “Practice for the Preparation of Substitute Ocean Water”
ASTM D1193 “Specification for Reagent Water”
ASTM D1654 “Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments”
ASTM E70 “Test Method for pH of Aqueous Solutions With the Glass Electrode”
You can download the most recent version of ASTM G85 Annex A5 Prohesion Testing from ASTM's website at the link below: