Coating Systems for Steel Surfaces If you paint steel the industrial coating system you choose could be vital to the durability, safety and longevity of the finished product. Even VOC ratings (Volatile Organic Compounds) may come into play when painting substrates such as steel. This article serves as a quick guide for how to successfully paint steel and use metal paint across a variety of substrates which would fall into corrosivity environments as listed in ISO 12944 series.
If In Doubt Refer To Norway! All coating suggestions in the sections later are for carbon steel as the substrate. There are huge amounts of products and systems on the market and it’s difficult to list all possible coating combinations here from all manufacturers. One of the original and best manufacturers of industrial steel paint is Jotun Paints. This Norwegian company began in whaling, their fleet required so many litres of red oxide marine paint, for their steel ships they decided to move into paint manufacturing for themselves. The rest is history as this company made its mark by producing metal paints for the most harsh corrosive environments in the world. Today they are one of the worlds largest industrial paint providers.
Where To Start When Painting Steel If you are new to painting steel or paint infrequently your first port of call should be to an industrial paint expert. Most manufacturers have them internally and they can advise what paint to choose and if you should use a full system of coatings. They will ask what your end goal is for example, does the coating need to work in submersed environments or environments likely to come into contact with corrosive salts. If it does this could impact which system of coatings you may use compared to say a steel structure above the water line in the middle of a city. Most technical advice will be free from larger organisations as a courtesy that you may buy your industrial paint from them now or in the future. In general some sage advice to take from this is that when the advice is good so likely will be the product, do you want to risk your project or client for the sake of saving a little money short term.
Regardless of the advised system all metal paint recommendations will be proposed in accordance to the standard ISO 12944-2, Corrosion protection of steel structures by protective paint systems — Part 2: Classification of environments.
ISO 12944 For Steel Explained ISO 12944 is an internationally applied industry standard on corrosion protection of steel structures by protective paint systems. First published in 1998, it was developed as a guide for industry professionals on the selection of coating systems based on the different environments, expected durability periods and different surface preparation methods & grades. The standard does not extensively cover all possible types of structures, substrates, surface preparation methods and coating systems. Regardless, it can also be applied for those cases which are not covered by ISO 12944, through mutual agreement of all parties involved.
The standard consists of 9 parts ; • Part 1: General introduction • Part 2: Classification of environments • Part 3: Design considerations • Part 4: Types of surface and surface preparation • Part 5: Protective paint systems • Part 6: Laboratory performance test methods • Part 7: Execution and supervision of paint work • Part 8: Development of specifications for new work and maintenance • Part 9: Protective paint systems and laboratory performance test methods for offshore and related structures
The standard was substantially revised in 2018, with significant changes such as ; • Addition of new durability category - Very High • Addition of new corrosion category – CX • Incorporation of the revised ISO 20340:2009 to Part 9 • Changes to dry film thickness and minimum number of coats • Updated with new type of coatings
All the major changes can be found in detail on the website for ISO standards. Take a look at ISO.org for further information.
Metal Paints And Durability Durability refers to expected life of a protective paint system to the first major maintenance painting and it is a key technical consideration for establishing a maintenance programme. In all parts of ISO 12944 (2018), durability is expressed in terms of four ranges (i.e. low, medium, high & very high). Take note that the durability range stated is not the "guarantee" period. The "guarantee" period is a legal consideration in the administrative part of the commercial contract and in most cases, it is shorter than the durability range.
The general Durability range of a steel paint within ISO 12944 is regarded as the below, • Low Durability (L) – 7 Years • Medium Durability (M) – 7-15 Years • High Durability (H) – 15-25 Years • Very High Durability – Over 25 Years
Metal And Classification of Environment When choosing a paint for metal a major consideration is where will it be installed and what will it be affected by that is corrosive. The ISO standard analyses this to allow you to choose a suitable coating by giving the guidance below. In ISO 12944 Part 2, a classification system is established to present the corrosivity categories for the different environments to which steel structures are exposed. In Layman’s terms this refers back to how a piece of metal sits. Will this metal be in the air and in a city away from the majority of corrosive elements or will it sit submerged under the sea in a salty, wet highly corrosive atmosphere. The atmospheric corrosivity categories, based on mass loss (or thickness loss) by standard specimens, are defined and the various natural environments are described in this part of the standard. It also describes different categories of environment for structures immersed in water or buried in soil. The key changes in 2018 for Part 2 are illustrated below showing how corrosive categories begin at C1 very low chance of corrosion and work up to CX extreme corrosion.
• C1 – Very Low – Example heated building with clean atmosphere • C2 – Low – Example Rural area with low level pollution • C3 – Medium – Example Production room with high humidity and some air pollution • C4 – High – Example Chemical plants or high moisture areas • C5 Marine – Very High – Example Buildings with almost permanent condensation • CX – Extreme Example Aggressive industrial area with extreme humidity
Depending on the metal corrosive classification the recommended paint you may use can change dramatically. As an example if your metal substrate would fall into the C1 low level class, you may simply wish to use a direct-to-metal one coat system knowing that the metal is unlikely to be attacked at a cellular level by corrosive substances other than air.
However if your metal substrate fell into class CX or extreme, you may consider a complete metal paint system to include a primer, mid coat and top coat with at least one of the layers to include a product that is IMO Anti-fouling compliant for example Jotun Vinyguard.
This summarises just some of the considerations you should make when choosing a paint for steel. Any questions or comments please leave below for more information.
Additional information, All parts of ISO 12944 are copyright protected under the International Organization for Standardization (ISO).
If you require a copy of the standard, the latest revised version is available for purchase & the digital copy can be downloaded at the following link: https://www.iso.org/standards.html
Author Bio - Author - Andy Potts - A specialist in business, finance, Paint and spray shop consumables. View Industrial Paint Systems
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