Corrosive tests

Salt spray samples
Salt spray samples

Corrosive tests, or corrosion resistance tests, are procedures designed to induce the corrosion of painted and unpainted products to verify their resistance in the short term. These tests represent empirically but reasonably the effects that natural corrosion would have over many years. To make these tests possible, painted specimens are placed inside climatic chambers which, thanks to vapors or technologies, induce corrosion which, day after day, manages to penetrate the areas below the painting substrate, finally attacking the metal surface. Following specific ISO or ASTM standards, it is possible to evaluate them to determine the corrosion resistance of the pretreatment of the paint in question at the end of the test.

Traditional corrosion resistance tests are still widely used and represent fundamental tests for the validation of products and paints. There are different types of climatic chambers, the most famous is certainly the neutral salt spray chamber, where a mist of water and 5% sodium chloride at 35 ° C can recreate an ideal environment for corrosive development. This method is suitable for ferrous materials, less for aluminum where instead it is necessary to add acidity to remove aluminum oxide which would otherwise hinder corrosion. This variant is called acetic saline mist, for the addition of acetic acid up to a pH of 4.5 - 5.00. There are many other variables, such as cupro acetic salt spray and others that simulate smog and acid rain.

For some sectors, such as automotive or painting, cyclical climatic chambers are used, which alternate wet / salty mists with dry periods and vice versa or even subjected to lights that simulate the solar spectrum.

The salt spray still represents the most used tool for this purpose, however there are other faster methods even if more empirical if not correctly carried out. We are talking about immersion baths such as the so-called BAC FORD or the MACHU BATH. In the first case the pieces are simply immersed in demi water, which can be at room temperature or thermostated, while in the second they are immersed in a highly corrosive solution based on acetic acid, hydrogen peroxide and salt. In general we can say that the BAC FORD is more suitable for ferrous metals while the MACHU for aluminum and its alloys. Based on the requirements or regulations followed, the pieces are checked gradually and engraved to check the corrosive trend. The tests end with the detachment of the paint or after a pre-established level of corrosion.