In practice, this is rarely performed for the determination of corrosivity for a paint specification. The scientific method for determining corrosion rate is determined by calculating the rate of metal loss on sample coupons (mild steel or galvanized steel) that are placed in the given environment.
The categories of corrosivity are taken from a separate standard, ISO 9223 2012 – “Corrosion of metals and alloys - Corrosivity of atmospheres - Classification, determination and estimation”. However, if the indoor asset is poorly ventilated or suffers from high humidity, then this increases the potential for condensation and, therefore, corrosion. If the asset is indoors, the potential for corrosion is usually lower because atmospheric pollutants are usually reduced. When specifying the protective coatings that should be used on assets, ISO 12944 considers the type of atmosphere in which the asset is located, and categorises these from rural (away from corrosive agents such as sulphur dioxide) through to marine (where airborne salts are present). In brief, atmospheric corrosivity calculations are made by summing the hours when the relative humidity is above 80% and the temperature is above 0☌.Ĭorrosivity is dependent on the corrosive agents present in the environment, especially gases such as sulphur dioxide, and salts such as chlorides and sulphates. The classification of environments considers temperature, relative humidity, and the time of wetness (the length of time that the metal surface is likely to be covered in a film of electrolyte that can cause atmospheric corrosion). Atmosphere (the gases – including aerosols and particles – that surround the asset to be protected against corrosion).Climate (the weather, which is established by reference to historical data).The environmental factors that determine an environment’s corrosivity are: The micro-environment, which is the environment at the interface between an element of a structure and the local environment.The local environment, which describes the atmospheric conditions around a particular component.When discussing the corrosive environment, two terms are used: Combined, these determine how corrosive the environment is, and therefore the types of protective paint systems that are needed to help prevent corrosion (covered in part 5 of the standard). There are many variables within corrosive environments. The corrosive environment describes the environment in which the asset to be protected is situated. This article introduces you to this part of ISO 12944 and the changes that were introduced in the latest revision in 2018. The second part of the standard discusses the corrosive environment. In a recent article, we introduced you to ISO 12944, the internationally recognised standard that provides the guidelines for the use of paint and coatings to protect assets from corrosion. Categorising corrosivity by type of environment