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For architects and building professionals.
4. Corrosion protection
The most common surface protection methods for steel are:
- anti corrosive paint coating
- hot dip galvanizing
- electroplating
- spray galvanizing
- chromium plating
- aluminium spraying
- rubberising
- coil coating of sheet steel
The most common method of protecting steel construction products is hot dip galvanising. Very simply, the process involves coating the surface of the steel with a corrosion-resistant metal, usually zinc or an aluminium/zinc alloy. Zinc and zinc-based coatings protect steel in two ways. Like paint, they provide barrier protection. Secondly, they provide galvanic protection, i.e., zinc will sacrifice itself to protect steel.
In addition to the metallic coating, many flat steel construction products such as cladding and roofing products have an organic topcoat for increased durability and enhanced appearance. A range of different coatings is available depending upon the product and the application. Coating thicknesses vary from 25 to 200 µm.
Anticorrosive painting
Paints are barrier coatings that, when applied and used properly, give sufficient corrosion protection to steel for many common applications. They are, however, not impervious to moisture, and rust can occur under even a perfectly applied paint if exposure time to moisture is long enough. Nevertheless, surface cleanliness and surface preparation are essential for good protection by anticorrosive paints. Surface preparation and corrosion protection of steel by protective paint systems are addressed in many standards.
Pretreatment:
The surface to be painted must be completely clean before painting. The standards for inspection of steel surface cleanliness are: covered in ISO 8501-1 and ISO 8501-2
The cleanliness of the surface can be estimated according to standard:
ISO 8502 parts 1 ... 9 and parts 11 ... 12.
which covers the preparation of steel substrates before application of paints and related products and the tests for the assessment of surface cleanliness.The roughness of the steel surface influences the adhesion of the paint and the corrosion protection. Surface roughness can be estimated according to:
ISO 8503-1 parts 1 ... 5.
which describes the preparation of steel substrates before application of paints and related products and the surface roughness characteristics of blast-cleaned steel substrates.The pre-treatment methods for steel surfaces are given in standard:
ISO 8504 parts 1 ... 3.
which covers the preparation of steel substrates before application of paints and related products -- Surface preparation methods.Information of the blast-cleaning abrasives used in surface preparation is given in the standard:
ISO 11124 parts 1 ... 4.
covering the preparation of steel substrates before application of paints and related products -- Specifications for metallic blast-cleaning abrasives; andISO 11126 parts 1, 3 ... 10
which is for the preparation of steel substrates before application of paints and related products -- Specifications for non-metallic blast-cleaning abrasives.Protective paint systems
The protective paint systems are addressed in:
ISO 12944-1.
Paints and varnishes which classifies protective paint systems by durability. The durability class does not imply any guarantee period but the expected serviceable life before repainting for maintenance.
ISO 12944-2.
Paints and varnishes which specifies the corrosivity categories according to the type of atmosphere and stress caused by immersion (tables 1 and 2)
ISO 12944-3.
Paints and varnishes -- Design considerations.
ISO 12944-4.
Paints and varnishes -- Types of surface and surface preparation.
ISO 12944-5.
Paints and varnishes -- Protective paint systems. It specifies the most common types of anti-corrosive paint and gives instructions for the selection of these for different environmental classes.
ISO 12944-6.
Paints and varnishes -- Part 6: Laboratory performance test methods.
ISO 12944-7.
Paints and varnishes -- Part 7: Execution and supervision of paint work.
ISO 12944-8.
Paints and varnishes -- Part 8: Development of specifications for new work and maintenance.
Shop primers
Shop primer is applied immediately to the blast-cleaned steel surface for temporary protection against corrosion during fabrication, transport, installation and storage. The shop primer is then painted over with the finishing paint system, which usually includes a new primer coat. Usually shop primer is not part of the paint system. Therefore it may have to be removed. Products supplied with a shop primer coat can be welded.
Guidance on shop primers is given in standards EN ISO 12944-5, appendix B and EN 10238 Automatically blast cleaned and automatically primed structural steel products.
Zinc coatings
Zinc coating can be applied by:
- hot-dip galvanising
- electroplating
- spray galvanizing
- zinc-rich paint
The atmospheric corrosion resistance of a zinc coating is a linear function of its thickness. For example, a 20 �m thick coating will last twice a long a 10 �m coating in a given environment. Hot-dip zinc coating (hot dip galvanizing) is the most common method of zinc coating for steel structures. Table 3 gives typical properties of different zinc coatings.
Table 3. Comparison of zinc coatings.
| Hot dip zinc coating | Normally 50 to 100 µm (up to 250 µm) thick. Continuously coated steel sheet 10 to 30 µm. |
|---|---|
| Electroplating | Usually 5 to 15 µm. Thick coats cannot be produced economically. |
| Zinc spraying | Coat thickness varies, typically 80 to 150 µm (seldom exceeds 250 µm) |
| Zinc-rich paint | One coat about 10 to 60 µm. |
| Hot dip zinc coating | Metallurgical bonding |
|---|---|
| Electroplating | Interatomic bonding/mechanical adhesion |
| Zinc spraying | Mechanical adhesion. Good if shot blasting has been carried out correctly |
| Zinc-rich paint | Depends on binder and carefulness of shot blasting |
| Hot dip zinc coating | Piece galvanizing: Zinc-iron layers coat plus pure outerzinc layer. Continuous galvanizing: very thin iron-alluminium-zinc layer, pure zinc layer (99 %) |
|---|---|
| Electroplating | Entirely pure zinc |
| Zinc spraying | The coating is built up from droplets of pure zinc. It is slightly oxidized and porous |
| Zinc-rich paint | The best products have about 90 weight-% zinc in the paint |
| Hot dip zinc coating | Good. Some excessive zinc runnings from the batch process |
|---|---|
| Electroplating | Even, depending on the efficiency of bath |
| Zinc spraying | Depends on operators skills. The coating is porous, but the pores are quickly filled with zinc salts and after that the coating is compact. |
| Zinc-rich paint | Good. Pores, if any, are filled with reaction products. |
| Hot-dip zinc coating | Piece galvanizing: degreasing and acid pickling.Continuous galvanizing: cleaning in annealing furnace. |
|---|---|
| Electroplating | Degreasing and acid pickling |
| Zinc spraying | Shotblasting (minimum Sa3) |
| Zinc-rich paint | Shotblasting (Sa2 to Sa3) |
| Hot dip zinc coating | Good |
|---|---|
| Electroplating | Limited (depending on coating thickness) |
| Zinc spraying | Good |
| Zinc-rich paint | Limited. |
| Hot dip zinc coating | EN ISO 1461, EN ISO 14713, ISO 3575 (coated sheet) |
|---|---|
| Electroplating | ISO 2081 |
| Zinc spraying | |
| Zinc-rich paint |
| Hot dip zinc coating | The maximum size of the object to be dipped depends on the zinc bath. Reversing dipping can be used to handle long objects. The objects should be appropriately designed to allow successful hot dip zinc coating. |
|---|---|
| Electroplating | Zinc pot dimensions set the limits. Usually for small components of simple shape. Suitable for sheet and wire. No heat is developed in the process. |
| Zinc spraying | Size and form unlimited. Economical for objects that weigh a lot in proportion to surface area. Uneconomical for network structures. Less accessible spots limit its use. Best method for producing thick coatings. |
| Zinc-rich paint | Suitable for the same applications as painting in general. Narrow places present problems. |
The atmospehric corrosion rate of zinc is approximately ten times slower than that of steel. The corrosion rate of zinc is:
- rural atmosphere: < 1 µm/year
- urban atmosphere: ≈ 2 µm/year
- industrial atmosphere: 2 ... 10 µm/year
- marine atmosphere: ≈ 2 µm/year
The life expectancies for zinc coatings under different conditions are presented in figure 1.
Figure 1. Life expectancies for zinc coatings.
Stainless steels
Stainless steelsare the most corrosion resistant steels used in construction. Stainless steel contains a minimum of 11% chromium that produces a thin protective oxide film on the surface that protects the material from corrosion. If damaged, this protective layer simply re-forms. Stainless steel is rarely used for structural steel but is used in some specific structural products such as lintels and masonry support systems. The most common use of stainless steel is for building roofing and cladding and internal applications such as escalators, doors, railings, etc.
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