Corrosion protection and steel constructions

The Danish climate is quite aggressive on iron and steel constructions. They are soaked by rain leading salts from the sea, and the solar heat is often insufficient for a thorough drying process. And for what concerns the constructions for off-shore use, as well as in the maritime sector, it is especially necessary to protect the steel from corroding so it is able to stay as long as possible in the harsh saltwater environment.

Clemco Denmark has many years of experience within coating of steel and corrosion protection, and for a long period of time we have followed the development closely in relation to corrosion protection. Frederik Nielsen, Director of Clemco Denmark, explains: "Today the demand on corrosion protection is greater than before. Some of the reasons why there is a change in the demand is that steel construction are more frequently used today. And the individual steel components are made thinner but stronger, which requires a different approach to the corrosion protection than before. There is also the fact that due to the industrialization the environment has become more polluted which is leading to acid rain and corrosive elements in the surrounding water and atmosphere, making conditions for corrosion more favorable. And the development of relatively new technologies within the industries of oil & gas and wind has led to stricter requirements within corrosion protection ".

The type of corrosion, which from a technical and economic point of view is the most important, is the formation of rust on iron and steel. What happens is that the metal is only waiting for the slightest opportunity to return to the ore stage and it happens as soon as the metal comes in contact with just a little oxygen and water. The water content of electrolytes such as ions, salts and oxygen gives the steel the best conditions for boosting the corrosive process. Therefore, it is impossible to avoid the steel to corrode if it hasn’t got some kind of surface coating. But the extent to which it is possible to protect the steel constructions, can be estimated from the 5 corrosion classes :

The 5 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 blasting, a possible metallization with zinc / aluminum and a following spray painted coating.

  • 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.