In my last post we got to the point of corrosion protection by barrier methods and cathodic protection. There are of course other methods.
Protection by Steel Passivation
With paints or powder coatings, it is possible to delay corrosion under the corrosion protection coating layer by the use of inhibitors in the paint which provide a certain passivation of the steel substrate. These passivation inhibitors slow down the reaction of water-oxygen and can neutralize the acid ions caused by humidity.
The effectiveness of these inhibitors varies considerably depending on the coating chemistry and the formulation technique used.
Corrosion Protection by a Duplex System
When a zinc or zinc-rich covering is over-coated with a paint or powder coating, this is often called a duplex coating system.
This two layer approach combines a cathodic corrosion protection system provided by the zinc and a barrier corrosion protection system provided by the top paint or powder coating layer. The 2-layer corrosion protection system brings advantages:
1. a possibility of various finishes and shades
2. a greater corrosion protection when compared to singular cathodic protection such as zinc over steel
Prior to any coating application, a surface pre-treatment is a must.
Typically a pre-treatment has multiple aims:
- eliminate any surface dirt or contamination
- provide improved surface adhesion characteristics
- improve corrosion resistance of the coating system to be applied
The chemical process is generally made up of a core 5 stages, although many plants will have more processes, Northpoint Ltd for example has 10 stages on each of its two Fence coating lines.
- Degreasing – aims to eliminate oil, grease and dust and is used to create a clean surface
- Etching – eliminates rust & mill scale to obtain a surface free from oxides
- Chemical Conversion – builds a thin layer of well adhering insoluable metallic salts; iron oxids/iron phosphates, zinc phosphates, Mn, Ni phosphates, crystalline Zn-Ni phosphates, tri-cat phosphates, nanoparticles with zirconium
- Passivation – to eliminate active salts and to seal the surface
- Rinsing – after each operation to eliminate active residues
Anti-Corrosion Performance of Chemical Pre-Treatments
750 hour neutral salt spray on hybrid powder coating steel plate:
- Fe Phosphating (no passivation) – rust on scribe 7.7 mm
- Zi + nanoparticles (no post-treatment) – rust on scribe 4.3 mm
- Fe Phosphating + Cr- Passivation – rust on scribe 4.2 mm
- Zn Phosphating + Cr- Passivation – rust on scribe 1.7 mm
The Northpoint Ltd system is based on a sophisticated Zn Phos + Cr- Passivation pre-treatment and will meet or exceed 1000 hour neutral salt spray testing.