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 metallic coating used to protect steel construction products is hot dip galvanising. Very simply, the process involves coating the surface of the steel with a very thin coating of a corrosion-resistant metal, usually zinc or an aluminium/zinc alloy.

Many flat steel construction products such as cladding and roofing products have an organic coating 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

Surface cleanliness and surface preparation are essential for good protection by anticorrosive paints. Surface preparation and corrosion protection of steel through 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:

ISO 8501-1

Preparation of steel substrates before application of paints and related products -- Visual assessment of surface cleanliness -- Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.

ISO 8501-2

Preparation of steel substrates before application of paints and related products -- Visual assessment of surface cleanliness -- Part 2: Preparation grades of previously coated steel substrates after localized removal of previous coatings 

The cleanliness of the surface can be estimated according to standard:

ISO 8502 parts 1 ... 9 and parts 11 ... 12.

Preparation of steel substrates before application of paints and related products -- 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 the followingstandards:

ISO 8503-1 parts 1 ... 5.

Preparation of steel substrates before application of paints and related products -- Surface roughness characteristics of blast-cleaned steel substrates.

The pre-treatment methods for steel surfaces are given in standard:

ISO 8504 parts 1 ... 3.

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 standards:

ISO 11124 parts 1 ... 4.

Preparation of steel substrates before application of paints and related products -- Specifications for metallic blast-cleaning abrasives.

ISO 11126 parts 1, 3 ... 10

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 the following standards:

ISO 12944-1.

Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 1: General introduction. 

  • the standard 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 -- Corrosion protection of steel structures by protective paint systems -- Part 2: Classification of environments. 

  • the standard 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 -- Corrosion protection of steel structures by protective paint systems -- Part 3: Design considerations.

ISO 12944-4.

Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 4: Types of surface and surface preparation.

  • the standard makes reference to surface preparation standards

ISO 12944-5.

Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 5: Protective paint systems. 

  • the standard 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 -- Corrosion protection of steel structures by protective paint systems -- Part 6: Laboratory performance test methods.

ISO 12944-7.

Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 7: Execution and supervision of paint work. 

ISO 12944-8.

Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 8: Development of specifications for new work and maintenance.

  • The standard gives detailed instructions for the development of specifications.

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 coating

Zinc coating can be executed by:

  • hot-dip zinc coating
  • electroplating
  • spray galvanizing
  • zinc-rich paint

The corrosion resistance of zinc coating varies depending on zinc coating thickness. Hot dip zinc coating (hot dip galvanizing) is therefore the most common and most important form of zinc coating for steel structures. Table 3 gives typical properties of different zinc coatings.

Table 3.  Comparison of zinc coatings.

Coating thickness
Hot dip zinc coating Normally 50 to 100 µm (up to 250 µm). 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, being usually 80 to 150 µm (seldom exceeds 250 µm)
Zinc-rich paint One coat about 10 to 60 µm.
Adhesion of zinc to steel
Hot dip zinc coating Metallurgical bonding
Electroplating 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
Structure of the coat
Hot dip zinc coating Piece galvanizing:Iron-zinc coat plus pure zinc layer. Continuous galvanizing: very thin iron-zinc layer, pure zinc layer (99 %)
Electroplating Entirely pure zinc
Zinc spraying The coat is built up of drops of pure zinc. The coat is slightly oxidized and porous
Zinc-rich paint About 90 weight-% of the paint coat is zinc
Evenness and continuity
Hot dip zinc coating Good. Some excessive zinc at runnings
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.
Pretreatment
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)
Corrosion resistance
Hot dip zinc coating Good
Electroplating Limited (depending on coating thickness)
Zinc spraying Good
Zinc-rich paint Limited.
Standards
Hot dip zinc coating EN ISO 1461, EN ISO 14713
Electroplating ISO 2081
Zinc spraying  
Zinc-rich paint  
Notes
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 corrosion resistance of zinc is considerably lower 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.

Life expectancies for zinc coatings.

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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