Steel constructions and corrosion protection
The climate in Scandinavia is rather aggressive to iron and steel structures. They are soaked with rain almost all year around, and the rain brings salt from the sea, and the salt reacts with the iron right away when wet and humid. And the heat from the sun is often insufficient for a thorough drying out on these northern latitudes. For what concerns constructions within offshore and the maritime sector, it is especially necessary to corrode the steel so that it can last as long as possible in the harsh saltwater environment.
Clemco Denmark is dealing with corrosion protection on a daily basis, and has many years of experience within surface treatment of steel, and has therefore been a part of the development in relation to corrosion protection. Frederik Nielsen, Director of Clemco Denmark, explains: "Today, we get more demands for corrosion protection than we did 10-20 years ago. Some of the reasons could be that the number of steel structures has increased during the last couple of decades, and today the individual steel constructions are made thinner but stronger, and that requires a different approach to the corrosion protection. There is also the aspect that, together with the industrialization, the environment has become more polluted, which has brought acid rain and corrosive elements to the surrounding water and atmosphere, which make the conditions for corrosion more favorable. And furthermore, the development of relatively new technologies within oil & gas and wind energy has led to more strict requirements for corrosion protection. "
From a technical and economical point of view, the rust formation on iron and steel is the type of corrosion with the highest importance. What happens is that the steel only waits for the slightest reason to return to the stage of ore, and that happens as soon as the steel comes into contact with just a tiny bit of oxygen and water. The content of electrolytes in the water such as ions, salts and oxygen give the steel the best conditions to begin the corrosive process. Therefore, corrosion protection and surface treatment in general is a must when dealing with steel and iron structures. However, the degree to which the structures should be corrosion protected is estimated from the 5 corrosion classes.
The five corrosion classes
To be able to make the correct corrosion protection on a steel construction we work with 5 corrosion classes each indicating how big the risk of corrosion is under various wind and weather conditions. Most steel used in construction, are protected against corrosion - and still we often see damaged steel constructions after just a few years. A significant part of the damage - and thus repair and maintenance - could be avoided if the corrosion preventive measures are taken into consideration already when the steel constructions are constructed so that you can avoid rust and corrosion traps.
It is also important that the constructions are set up in a way making it able to perform a good and safe surface treatment with shot blastinging followed by a possible metallization with zinc / aluminum, to end by spray painting with epoxy primer and polyurethane painting.
Corrosion Class C1 = Very low risk.
Heated buildings with clean air, such as offices, shops, schools, hotels, etc.
Corrosion Class C2 = Low risk.
Atmosphere with low pollution. For example in the country.
Corrosion Class C3 = Middle risk.
Urban and industrial areas, moderate sulphur dioxide pollution. Coastal areas with low salt content.
Corrosion Class C4 = High risk.
Industrial areas and coastal areas with moderate salt impact.
Corrosion Class C5 = Very high risk.
C5 I (polluted industrial environment) Industrial areas with high humidity and aggressive atmosphere
C5 M (marine environment in saltwater) Coast and offshore areas with high salt content
Types of corrosion protection
The reason why you make corrosion protection is to eliminate or to prevent one or more of the conditions for the corrosion to happen. Depending on how big the risk of corrosion is, it is important to find an appropriate way to protect the steel.
1. The most simple solution in order to prevent corrosion is to select a material that does not corrode in the current environment. However, as the steel is strong and durable - in particular with the proper corrosion protection, the steel is selected as the preferred way to make structures within the industry.
2. There is also the possibility to influence the environment, so it can not react with the steel. You can for example remove the oxygen or add some anti-corrosion agents.
3. You can also design the structure in such a way that corrosion is not able to occur - for example, by avoiding the presence of water, and thereby keep the metal dry.
4. You can also avoid corrosion by changing the corrosion potential of the so-called cathodic protection. It aims to make the metal negatively charged, and thus counteract the positive metal ions moving from metal to liquid.
5. It is also possible to apply the metal coating in order to avoid the electrolytes getting in contact with the metal. It ranges from painting, galvanizing and metallization with zinc/aluminum 85/15.
Clemco Denmark has many customers in the offshore and maritime sectors, meaning they are occupied within Corrosion Class 5, which means that for them there is a great corrosion risk due to the very harsh and wet saltwater climate.
There are several methods to coat with metal - either with hot galvanization or thermal spraying (metallization). But first of all the surfaces need to be checked. With a PosiTector salt tester you can measure the concentration of soluble salts on the surface before proceeding with further surface treatment of your steel structure.
Both in terms of hot galvanizing and metallization, these methods are supposed to be performed by specialist companies with years of experience. This is why pretreatment with sand blasting and dipping processes are not discussed further here. Metallization can be performed with or without robot in specially designed metallization booths. Most metals can be sprayed, but the aluminum and zinc are the best for the protection of steel structures. Stainless steel has been used successfully for machine parts and cylinders in the pulp and paper industry. The special advantages of metallization over other methods is that it is possible to apply a thicker layer of the protective zinc / aluminum coating. Further more the equipment is easy to move around, which means that the metallization process can be carried out on very large steel components, and is often the only alternative.
The costs in connection with thermal spraying is greater than the comparative paint with 3 layers after shot blasting, but for structures which are to be placed in or by the sea for more than 30 years, the reduced maintenance costs (thermal spray) definitely offset this and even more. This applies to the arc metallization which is considered as the classic method of applying zinc and aluminum. It is considered to be beneficial to paint on top of the metal coating, as the zinc/aluminum coating will increase the life of the paint coating because the metallized surface prevents the rust.
When metal spraying zinc / aluminum with arc gun you get a better adhesion than flame spraying (oxygen and gas). By thorough preparation with sand blasting and correct metallization afterwards you obtain a very high adhesion strength.
For what concerns aluminum and zinc, the porosity does not cause any problems, and the surface roughness provides a good basis for the subsequent painting with epoxy and polyurethane paints.
In order to get a good result with metallization with zinc / aluminum, it is necessary that the surface is completely clean, dry, and shot blasted before you start up the metallization process. Metallization is applied manually with a special metallization system, and it is possible to make applications in different thicknesses depending on the customer needs and requests. Metallized steel surfaces with a zinc / aluminum alloy containing 85% zinc and 15% aluminum, provides a first-class corrosion protection with very long life under most conditions if you add an extra layer of paint. This alloy also has the advantageous property that it is self-healing, making the material resistant to, inter alia, corrosion due to scratches in the coating. The zinc / aluminum provides excellent corrosion protection for most tasks in the upper corrosion classes.
Metallization with zinc / aluminum is the preferred form for corrosion and rust protection and metallization also represents the best basis for a painting or coating build-up subsequently.