ANTI-CORROSION COATINGS TECHNICAL SUPPORT
STATEMENT OF EXTERIOR ANTI-CORROSION PROBABLE DURABILITY OF POLYESTER POWDER COATINGS
Anti-Corrosion Coatings applied to the Customers free issue galvanised steel products meet the general requirements of BS EN 13438 : 2005 & BS 1722 – 16 : 2009.
Powder coatings applied provide protection against corrosion determined in part by Environmental Aggressiveness.
For a steel item, whether it is exposed inside or outside, the corrosion speed varies according to the aggressiveness of the environment. The more aggressive the environment, the more corrosion takes hold. In 2018/2019, the ISO 12944 standard was revised and clause 9 was introduced – Protective Paint Systems and Laboratory Performance Test Methods for Offshore and Related Structures.
In addition, the corrosion categories, C1 – C5 (very low to very high) were also revised and a new category of CX was added relating specifically to offshore structures. In addition, a 25+ years durability classification was developed to correspond to a new immersion category, Im4.
ISO 12944 – 2 specifies atmospheric corrosion categories:
|ISO 12944-2 Tab. 1 – Categories of Corrosion|
|Category of corrosion|
|C1 – Very Low||Very little or no pollution.Heated buildings, offices, shops, schools, hotels etc.|
|C2 – Low||Low levels of pollution. Mostly rural areas.|
|C3 – Medium||Urban and industrial areas, moderate sulphur dioxide pollution. Coastal areas with low salinity [not under direct influence of salty air/wind]|
|C4 – High||Industrial areas and coastal areas with moderate salinity. Sea front areas|
|C5 – I Very High||Industrial areas with high levels of humidity (Middle East)|
|C5 – M Very High||Coastal and offshore areas with high salinity. Sea front protection barriers, oil rigs.|
|CX – Very High||Marine and offshore environments.|
The corrosion speed varies according to the design of the item as well as mechanical processes such as cutting, bonding, folding, jointing, welding…
- The presence of chinks such as air gaps constitute a confined environment favorable to accelerated local corrosion.
- Jointing of two materials creates galvanic couples starting up an accelerated corrosion – nuts or rivets often surrounded by rust marks.
- Welds, in particular when oxidised and irregular.
Some parts are subject to working stresses such as:
- Mechanical stress
- Rapid or cyclic temperature changes
- Permanent or repeated contact with fluids
These stresses increase the corrosion speed.
Corrosion can also be accelerated, when parts are encrusted with salts and dirt. Therefore periodic cleaning, with a frequency determined by the environment slows down the speed of corrosion. Quick and good repairs of the anti-corrosion coatings powder coating is vital to ensure the long-term protection provided by the powder coating.
Assuming good design and on-going appropriate care and maintenance Northpoint Ltd suggest the following as an indication of the powder coating life expectancy:
C1 – in excess of 15 years.
C2 – in excess of 15 years.
C3 – 5 – 15 years.
C4 – Non-standard processes required to offer any level of protection for these areas.
C5 – Non-standard processes required to offer any level of protection for these areas.
CX – All offshore systems must continue to go through 4,200 hours of cycling testing, which equates to 25 weeks.