Corrosion is the deterioration of a material due to chemical reactions resulting from exposure to atmospheric agents, moisture, chemicals or other factors in the environment
Corrosion can occur uniformly on the surface, follow the edges of the item or penetrate the metal at one point and form a honeycomb.
The rate of corrosion depends on the aggressiveness of the environment in which the material is located
In our lab, we have developed and tested a zinc-free primer that can withstand 2000 hours of salt spray in accordance with ISO 9227. The results of the tests are summarised in Table II
The 'barrier protection' system aims to isolate the steel from its surroundings by coating it with a water- and airtight layer such as paint, enamel, plasticiser etc.
Powder coatings offer barrier protection. In order to provide optimum barrier protection, powder coating formulations must have a high binder content.
This type of technology consists of bringing steel into contact with a less noble metal. This is why zinc or zinc-based coatings (zinc plating or galvanising primer) are used to protect steel. In the presence of oxygen and water, zinc turns into zinc salts, which are insoluble in most natural environments. These salts create a screen on the surface of the zinc which reduces the rate of corrosion and, in the event of scratches or damage, provides a sealing effect which in turn helps to slow down corrosion in the damaged or scratched spot.
By using powder or liquid coatings, it is possible to delay the progressive corrosion of the material thanks to the presence of inhibitor additives which can passivate the steel substrate. The effectiveness of these additives depends greatly on the chemical nature of the paint.
This two-layer 'duplex' system uses the cathodic protection provided by zinc or special additives, together with the barrier effect protection provided by paints.
The advantages of this system can be greater than simple mono-protection as the additives or zinc only 'wear out' in the event of damage or certain coating defects, and various aesthetic finishes and shades can be achieved.
BEFORE PAINTING, IT IS NECESSARY TO PRE-TREAT THE SURFACE IN ORDER TO REMOVE DIRT WHICH COULD COMPROMISE THE ADHESION OF THE COATING TO THE SUBSTRATE AND NEGATIVELY AFFECT CORROSION RESISTANCE. THE TYPE OF TREATMENT DEPENDS ON THE SIZE AND/OR THICKNESS OF THE PARTS TO BE PAINTED, WHICH DOES NOT EXCLUDE THE POSSIBILITY OF USING EITHER CHEMICAL OR MECHANICAL TREATMENT.
A steel object exposed indoors and outdoors, the rate of corrosion varies depending on the aggressiveness of the surrounding environment, the treatment of the object, and on mechanical processes such as cutting, drilling, welding, bending, etc.
Corrosion can be accelerated if the materials are encrusted with dirt and salt despite having some form of protection; periodic cleaning, at a frequency dependent on the environment, slows down corrosion. If the steel protection system is scratched, the corrosion quickly spreads all around.
ISO 12944-2 specifies corrosive categories relating to types of atmosphere and environments, adopted to define appropriate protective systems (Table I).
Table I - Corrosive Categories (ISO 12944-2)
| CORROSIVE CATEGORIES | LEVEL | INTERNAL | EXTERNAL |
| C1 | VERY LOW | Heated buildings with a clean atmosphere; e.g. offices, shops, schools, hotels. | - |
| C2 | LOW | Environments with low levels of pollution, particularly rural areas. | Unheated buildings where condensation may occur; e.g. sports premises, warehouses. |
| C3 | MEDIUM | Urban and industrial environments, moderate sulphur dioxide pollution. Coastal areas with low salinity. | Production areas with high humidity and a corresponding level of pollution |
| C4 | HIGH | Industrial areas and coastal areas with moderate salinity. | Chemical plants, swimming pools, coastal boatyards. |
| C5 - I | VERY HIGH (INDUSTRIAL) | Industrial areas with high humidity and aggressive atmosphere. | Buildings or areas with almost permanent condensation and high pollution. |
| C5 - M | VERY HIGH (MARINE) | Coastal and offshore areas with high salinity. | Buildings or areas with almost permanent condensation and high pollution. |
Over time, painted items are subject to degradation, depending on the corrosive environment, weather conditions and the use of the items themselves.
Degradation of a paint film can manifest itself in different ways, for example by blistering, cracking, film peeling, rusting or chalking.
Specific regulations assess the degradation of a paint film:
- ISO 4628-3 degree of rusting
- ISO 5628-2 degree of bubble formation
- ISO 2409 degree of adherence of the coating
Table II - Different dust cycles developed according to different corrosivity classes according to ISO 12944-2
|
Salt spray test (ISO 9227) |
0-500 hours | 500-1000 hours | 1000-2000 hours | > 2000 hours |
| Mechanical preparation/ Pretreatment | Fe phosphatisation |
Fe or Zn phosphatisation + passivation |
Sa 2.5 sandblasting Zn phosphating + passivation | Sa 3 Sandblasting Phosphating Zn + passivation |
| Primer | Not required | MPZ170010001 EPZ108080001 EPF47001H001 |
MPZ170010001 EPZ108080001 EPF47001H001 |
MPZ170010001 EPZ108080001 EPF47001H001 |
| Topcoat Interior (80-100µm) |
EPO Series MP Series PES IND Series PES Series |
EPO Series MP Series |
EPO Series MP Series PES IND Series |
EPO Series MP Series PES IND Series |
| Topcoat Exterior (80-100µm) |
PES IND Series PES Series |
PES IND Series PES Series |
PES Series | PES Series |
Legend:
Sa 1 light sandblasting Sa 2 deep sandblasting
Sa 2,5 very deep sandblasting
Sa 3 super deep 'white metal' sandblasting