Does the salt mist spray test appy to hot glavanized steel according to ISO 1461 ?
They are used for the purpose of determining the corrosion resistance of metals and metal coatings. One of the most used tests is that of salt spray according to ASTM B-117 or EN ISO 9227 developed for the automotive industry.
For galvanized steel, however, there is a substantial and systematic difference between the estimated laboratory life of the salt spray test and the actual performance when exposed to actual atmospheric conditions.
Zinc is well known for its very good resistance to corrosion. What is often less well known is that this property is the result of the formation of a “zinc patina” of the galvanized surface exposed to the atmosphere. This zinc patina is composed of zinc hydroxycarbonate which is formed by reaction of zinc with the CO2 of the atmosphere in a humid environment.
The zinc patina is a very dense and impenetrable layer which among others, has a very high chemical stability (poorly soluble). It is only when this patina has had the opportunity to form that the corrosion resistance of hot-dip galvanized steel reaches its maximum level.
In the salt spray test, the dry / wet surface cycles that give rise to the patina are non-existent.
The surface of the galvanizing coating is permanently wet and the saline solution, very aggressive, attacks the zinc continuously. This artificial laboratory environment therefore gives false indications as to the true lifetime of hot-dip galvanized steel.
The scientific confirmation that the salt spray test is unsuitable for galvanized steel, resulting from the book: “Prevention and fight against corrosion” to download in PDF version.
The salt spray test is inapplicable as an accelerated corrosion test for hot-dip galvanized steel, according to ISO 1461, as it is not cyclic and does not correspond to the reality of natural atmospheric conditions.
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