Corrosion protection of structural steel

Words by Basheer Bava, Wednesday 31 August 2016

BSBG Senior Structural Engineer, Mohamed Basheer Bava, writes an in-depth guide to the corrosion protection of structural steel.

Introduction

When it comes to choosing materials, steel is the most popular among designers because of its exceptional physical and mechanical characteristics. Steel can be used to rapidly build economical, durable and safe structures. However, due to its inherent nature, it tends to react with atmospheric agents such as oxygen and water to form a more stable compound called ferrous oxide - which in general terms is called rust, a process referred to as corrosion.

The primary quality of steel is its ability to retain its physical and mechanical strength, or in the case of a steel structure, its load bearing capacity. This is generally a long-lasting feature and is only compromised when corrosion reduces its cross section to such a degree that safety is adversely affected. The service life of a structure depends on the rate of reaction between the steel and its environment. These reactions depend upon the nature and concentration of the corrosive agents present.

Corrosion protection

Corrosion protection of structures must therefore be considered to intervene in this process in order to prevent the reaction, or to greatly reduce the rate of corrosion. Cost effective corrosion protection of structural steelwork should present little difficulty for common applications and environments if the factors that affect durability are recognised from the outset.

The purpose of this guide is to explain, in terms of concepts, the basic requirements for protecting structural steel with paint and metallic coatings, the systems commonly used, and their significance in relation to the protective properties required.

Figure 1. Schematic representation of corrosion process

What is corrosion?

The ability of steel to revert to its natural or original state is called corrosion.
The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties.
For iron and steel to corrode it is necessary to have the simultaneous presence of water and oxygen. In the absence of either, corrosion does not occur.
All corrosion occurs at the anode; no corrosion occurs at the cathode(Figure 1).

Rate of corrosion

The time of wetness - This is the proportion of total time during which the surface is wet, due to rainfall, condensation, etc.
Atmospheric pollution - The type and amount of atmospheric pollution and contaminants, e.g. sulphates, chlorides, dust, etc.

Atmospheric corrosivity categories

The rate at which the corrosion process progresses depends on a number of factors relating to the ‘micro-climate’ immediately surrounding the structure; principally the timing of the presence of moisture and the atmospheric pollution level. Because of variations in atmospheric environments, corrosion rate data cannot be generalised. However, environments can be broadly classified, and corresponding measured steel corrosion rates provide a useful indication of likely corrosion rates. More information can be found in BS EN ISO 12944-2.

As per BS EN ISO 12944-2, the atmospheric environments are classified into six atmospheric-corrosivity categories as shown below:

Figure 2. Atmospheric-corrosivity categories as per table – BS EN ISO 12944-2

Corrosion prevention and control

Figure 3. Detailing to minimise corrosion

In external or wet environments, design can have an important bearing on the corrosion of steel structures. In dry heated interiors, no special precautions are necessary. The prevention of corrosion should therefore be taken into account during the design stage of a project.

The main points to be considered are:

Entrapment of moisture and dirt

Avoid the creation of cavities and crevices, etc.
Welded joints are preferable to bolted joints.
Lap joints should be avoided or sealed where possible.
HSFG faying surfaces should be edge-sealed after connection.
Provide drainage holes for water, where necessary.
Seal box sections, except when they are to be hot-dip galvanised.
Provide for free circulation of air around the structure.

Contact with other materials

Avoid, where practical, bimetallic connections or insulate the contact surfaces if necessary.
Provide adequate depth of cover and correct quality of concrete.
Separate steel and timber by the use of coatings or sheet plastics.

Coating application

Design should ensure that the selected protective coatings can be applied efficiently.
Hot-dip galvanising should not be used for sealed components. Such items should be provided with vent holes and drain holes.
Adequate access should be provided for paint spraying and thermal (metal) spraying, etc.

General factors

Large flat surfaces are easier to protect than more complicated shapes. Complex shapes and structures should have adequate access for initial painting.
Access should be provided for subsequent maintenance.
Lifting lugs or brackets should be provided where possible to reduce damage during handling and erection.
BS EN 12944-3 provides details of designing for the prevention of corrosion.

Corrosion Protection period

The protection period of coatings is defined in BS EN 12944-1 as the expected service life up until the moment where the system needs attention for the first time.
The protection period is a vital factor when selecting and defining corrosion protection systems. This technical concept helps the owner of the structure to specify his maintenance schedule.
The protection period is not a guarantee period. Generally, the guarantee period, a legal concept, is shorter than the protection period.
The protection period of a zinc coating depends mainly, for a given corrosive load, on the thickness of the applied coat. Figure 4 shows the relationship between the protection period/the thickness of the zinc coat and the corrosive category.
Special microclimatic features and higher loads particular to the structure, such as an accumulation of dust with a prolonged exposure to moisture can considerably reduce the protection period.

Figure 4. Protection period of zinc coatings as a function of the thickness of the coat and the corrosive category

Surface preparation

Why it is required?

Hot rolled structural steelwork leaves the last rolling pass at a temperature of about 1000°C. As it cools, the surface reacts with oxygen in the atmosphere to produce mill scale. This is a complex oxide which appears as a blue-grey tenacious scale completely covering the surface. Mill scale is unstable and with time water in the atmosphere penetrates fissures in the scale and rusting of the steel occurs. The corrosion process progressively detaches the mill scale and produces a variable surface that is generally unsuitable for overcoating.

The amount of rusting is dependent upon the length of time that the steel has been exposed to a damp or wet environment. The four categories of ‘rust grades’ for steelwork are described in BS EN ISO 8501-1:

A – Steel surface largely covered with adhering mill scale, but little if any rust.

B – Steel surface which has begun to rust and from which mill scale has begun to flake.

C – Steel surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting under normal vision.

D – Steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.

The majority of new steelwork usually conforms to A and B conditions and occasionally C condition. The surface preparation of steel is therefore principally concerned with the removal of mill scale, rust and other contaminants to provide a satisfactory substrate for coating.

Preliminary treatment

Residues of oil, grease, marking inks, cutting oils etc. after fabrication operations will seriously affect the adhesion of applied coatings and must be removed.
Failure to remove these contaminants before blast cleaning results in them being distributed over the steel surface and contaminating the abrasive.
Suitable organic solvents, emulsion degreasing agents or equivalents should be applied to remove contaminants in preparation for subsequent treatments.
Further guidance can be obtained from BS 7773 ‘Code of Practice for Cleaning and Preparation of Metal Surfaces’.

Methods of preparation

Various methods and grades of cleanliness are presented in ISO 8501-1 are listed below:

Water, solvent and chemical cleaning

Water cleaning
Steam cleaning
Emulsion cleaning
Alkaline cleaning
Organic-solvent cleaning
Cleaning by means of chemical conversion
Stripping
Acid pickling

Mechanical cleaning

Hand & power tool cleaning
Abrasive blast cleaning

Flame cleaning

By far the most common methods used for the surface preparation are acid pickling, hand and power tool cleaning, and abrasive blast cleaning which are discussed in detail below:

Acid pickling

This process involves immersing the steel in a bath of suitable inhibited acids that dissolve or remove millscale and rust, but do not appreciably attack the exposed steel surface. This cleaning method can be 100% effective. Acid pickling is normally used for structural steel intended for hot dip galvanising.

Hand and power tool cleaning

Surface cleaning by hand tools such as scrapers and wire brushes is relatively ineffective in removing millscale or adherent rust. Power tools offer a slight improvement over manual methods and these methods can be approximately 30% to 50% effective but are not usually utilised for new steelwork fabrications. Where it is not possible to clean by abrasive blasting, hand and power tool methods may be the only acceptable alternative methods.

Modern power tooling has been developed not only to achieve a good standard of surface cleanliness and profile but also to provide near total containment of all dust and debris generated. New equipment is now available to use percussive reciprocating needles, rotary abrasive coated flaps and right-angle grinders, all within a vacuum shroud to enable on-site surface preparation to be environmentally acceptable.

Abrasive blast cleaning

By far the most significant and important method used for the thorough cleaning of mill-scaled and rusted surfaces is abrasive blast cleaning. This method involves mechanical cleaning by the continuous impact of abrasive particles at high velocities on to the steel surface, either in a jet stream of compressed air or by centrifugal impellers. The abrasives are recycled with separator screens to remove fine particles. This process can be 100% effective in the removal of mill scale and rust.

The standard grades of cleanliness for abrasive blast cleaning are:

SA 1 – Light blast cleaning

SA 2 – Thorough blast cleaning

SA 2^1⁄2– Very thorough blast cleaning

SA 3 – Blast cleaning to visually clean steel

The particle size of the abrasive is also an important factor affecting the rate and efficiency of cleaning. In general terms, fine grades are efficient in cleaning relatively new steelwork, whereas coarse grades may be required for heavily corroded surfaces.

Standard corrosion protection systems for buildings

The standard corrosion protection system can be broadly classified into two categories:

Paint Coating system
Metal Coating system

Hot Dip Galvanising
Sprayed metal coating
Zinc Electroplating
Sherardizing

Paint Coating system

Painting is the principle method of protecting structural steelwork from corrosion. Paints are applied to steel surfaces using many methods, but in all cases this produces a ‘wet film’. After solvent evaporation, a ‘dry film’ layer is formed. In general the corrosion protection afforded by a paint film is directly proportional to its dry film thickness.

Paints are usually applied one coat on top of another and each coat has a specific function/purpose (Ref Figure 5 & 6).

Figure 5. Schematic diagram of a paint system

Figure 6. Multi-coat paint system

These are described as follows:

Primers

The primer is applied directly onto the cleaned steel surface. Its purpose is to wet the surface and to provide good adhesion for subsequently applied coats. For primers applied directly to steel surfaces, these are also usually required to provide corrosion inhibition.

Intermediate coats

The intermediate coats are applied to ‘build’ the total film thickness of the system. Generally, the thicker the coating the longer the life. This may involve the application of several coats

Undercoats/Intermediate coats are specially designed to enhance the overall protection and, when highly pigmented with laminar pigments, such as micaceous iron oxide (MIO), reduces or delays moisture penetration in humid atmosphere.

Finish coats

The finish coats provide the required appearance and surface resistance of the system. Depending on the conditions of exposure, they must also provide the first line of defence against weather and sunlight, open exposure, condensation, highly-polluted atmospheres in chemical plants, impact and abrasion at floor or road level, and bacteria and fungi (in food factories).

Composition of paints

Paints are made by mixing and blending three main components

(a) The pigments: Pigments are finely ground inorganic or organic powders which provide colour, opacity, film cohesion and sometimes corrosion inhibition.

(b) The binder: Binders are usually resins or oils but can be inorganic compounds such as soluble silicates. The binder is the film forming component in the paint.

(c) The solvent: Solvents are used to dissolve the binder and to facilitate application of the paint. Solvents are usually organic liquids or water.

Main generic types of paint and their properties

(a) Air drying paints For example alkyds.

These materials dry and form a film by an oxidative process, which involves absorption of oxygen from the atmosphere. They are therefore limited to relatively thin films. Once the film has formed it has limited solvent resistance and usually poor chemical resistance.

(b) One pack chemical resistant paints For example acrylated rubbers, vinyls.

For these materials, film formation requires only solvent evaporation and no oxidative process is involved. They can be applied as moderately thick films, though retention of solvent in the film can be a problem at the upper end of this range. The formed film remains relatively soft and has poor solvent resistance, but good chemical resistance.

Bituminous paints also dry by solvent evaporation. They are essentially solutions of either asphaltic bitumen or coal-tar pitch in organic solvents.

These materials are supplied as two separate components, usually referred to as the base and the curing agent. When these two components are mixed, immediately before use, a chemical reaction occurs. These materials therefore have a limited 'pot life' before which the mixed coating must be applied. The polymerisation reaction continues after the paint has been applied and after the solvent has evaporated to produce a densely cross-linked film, which can be very hard and has good solvent and chemical resistance. Liquid resins of low viscosity can be used in the formulation thereby avoiding the need for a solvent. Such coating are referred to as 'solvent less' or 'solvent free' and can be applied as very thick films.

A summary of the main generic types of paint and their properties is shown, see Figure 7.

Figure 7. Main Generic Types of Paint and their Properties

Prefabrication primers

Prefabrication primers are also referred to as blast primers, shop primers, temporary primers, holding primers, etc.

These primers are used on structured steelwork, immediately after blast cleaning, to maintain the reactive blast-cleaned surface in a rust-free condition until final painting can be undertaken. They are mainly applied to steel plates and sections before fabrication, which may involve welding or gas cutting. These materials dry and form a film by an oxidative process, which involves absorption of oxygen from the atmosphere. They are therefore limited to relatively thin films. Once the film has formed it has limited solvent resistance and usually poor chemical resistance.

Many proprietary prefabrication primers are available, but they can be classified under the following main generic types:

(a) Etch Primers:

Etch primers are based on polyvinyl butyral resin reinforced with a phenolic resin to increase water resistance. These primers can be supplied in a single pack or two-pack form, the latter providing better durability.

(b) Epoxy Primers:

Epoxy primers are two-pack materials utilising epoxy resins and usually have either polyamide or polyamine curing agents. They are pigmented with a variety of inhibitive and non-inhibitive pigments. Zinc phosphate epoxy primers are the most frequently encountered and give the best durability within the group.

(c) Zinc Epoxy Primers:

These primers can be either zinc-rich or reduced zinc types. Zinc-rich primers produce films which contain about 82 to 85% by weight of metallic zinc powder while corresponding figures for the reduced zinc type are as low as 55% by weight. When exposed in either marine or highly industrial environments, zinc epoxy primers are prone to the formation of insoluble white zinc corrosion products which must be removed from the surface before subsequent overcoating. This cleaning process is usually known as ‘secondary’ surface preparation. All zinc epoxy primers produce zinc oxide fume during welding and gas cutting and this can cause a health hazard.

(d) Zinc Silicate Primers:

Zinc silicate primers produce a level of protection which is comparable with that provided by the zinc-rich epoxy types and they suffer from the same drawbacks, e.g. the formation of zinc salts and production of zinc oxide fume during welding. There are currently different categories of zinc silicate primers based upon the binder (organic or inorganic) and the zinc content. Low-zinc primers in this group have been developed to improve their weldability and to minimise weld porosity. However, their durability is also reduced. The organic silicate primers are the most suitable as prefabrication primers.

The application of paints

The method of application and the conditions under which paints are applied have a significant effect on the quality and durability of the coating. Standard methods used to apply paints to structural steelwork include application by brush, roller, conventional air spray and airless spray.

Table 1 .Generic paint systems selection for Steel members for different corrosivity categories (Table A.1 BS EN 12544-5)

Durability ranges:

Low (L): 2 years to 5 years

Medium (M): 5 years to 15 years

High (H): more than 15 years

List of corrosion protection paint suppliers available in UAE

Dulux Protective Coatings

For more info, visit www.duluxprotectivecoatings.com

Oasis AMERCOAT Industrial coatings

For more info, visit www.oasisamercoat.com

JOTUN Protective Coatings

For more info, visit www.jotun.com

Leigh’s Protective Coatings

For more info, visit www.aglpuae.com

National Paints

For more info, visit www.national-paints.com

HEMPADUR range by HEMPEL paints

For more info, visit www.hempel.com

Hot-dip galvanisation

Hot-dip galvanising is a process that involves immersing the steel component to be coated in a bath of molten zinc (at about 450°C) after pickling and fluxing, and then withdrawing it. The immersed surfaces are uniformly coated with zinc alloy and zinc layers that form a metallurgical bond with the substrate. The resulting coating is durable, tough, abrasion resistant, and provides cathodic (sacrificial) protection to any small damaged areas where the steel substrate is exposed.

Figure 8 schematically represents the principle of the galvanisation process.

Figure 8. Principle of the galvanisation process

Since hot-dip galvanising is a dipping process, there is obviously some limitation on the size of components that can be galvanised. However, ‘double-dipping’ can often be used when the length or width of the workpiece exceeds the size of the bath.

Some aspects of the design of structural steel components need to take the galvanising process into account, particularly with regards to the ease of filling, venting and draining and the likelihood of distortion. To enable a satisfactory coating, suitable holes must be provided in hollow to allow access for the molten zinc, the venting of hot gases, and the subsequent draining of zinc. Further guidance on the design of articles to be hot-dip galvanised can be found in BS EN ISO 14713-1.

For many applications, hot-dip galvanising is used without further protection. However, to provide extra durability, or where there is a decorative requirement, paint coatings are applied. The combination of metal and paint coatings is usually referred to as a ‘duplex’ coating. When applying paints to galvanised coatings, special surface preparation treatments must be used to ensure good adhesion. These include light blast cleaning to roughen the surface and provide a mechanical key, and the application of special etch primers or ‘T’ wash, which is an acidified solution designed to react with the surface and provide a visual indication of effectiveness.

Hot-dip galvanisation of structural steel components exposed to atmospheric conditions constitutes a highly effective durable corrosion protection method. In many applications, the corrosion protection lasts as long as the structure itself. Zinc coatings require no or almost no maintenance. Hot-dip galvanising, in terms of lifetime of the structures, including maintenance and repair costs, is by far the most cost-efficient corrosion protection system available for steel structures.

Notes:

Minimum coating thickness on samples that are not centrifuged.
Larger coating thicknesses can only be achieved when the steel has a specified silicon content.
Minimum local coating thickness according to EN ISO 1461.
Mean coating thickness according to EN ISO 1461.
In category C5 it may be necessary to use duplex (galvanising + paint) to reach longer life times.

Sprayed metal coating

Thermally sprayed coatings of zinc, aluminium, and zinc-aluminium alloys provide long-term corrosion protection to steel structures exposed to aggressive environments.

The metal, in powder or wire form, is fed through a special spray gun containing a heat source, which can be either an oxygas flame or an electric arc. Molten globules of the metal are blown by a compressed air jet onto the previously grit blast cleaned steel surface. No alloying occurs and the coating consists of overlapping platelets of metal and is porous. The adhesion of sprayed metal coatings to steel surfaces is considered to be essentially mechanical in nature. It is therefore necessary to apply the coating to a clean roughened surface and blast cleaning with coarse grit abrasive is normally specified.

Figure 9. Cross-section of thermally sprayed aluminium coating

Due to the thermal spray process, the coating typically has around 10% porosity. The pores are subsequently sealed by applying a thin organic coating, which penetrates the surface. Sealers may be either un-pigmented, with colouring agents or aluminium flake. Typically specified coating thicknesses vary between 100-200 µm (microns) for aluminium, and 100-150 µm for zinc.

Thermally sprayed metal coatings can be applied in the shop or at site. No drying time is required, they do not sag or run, and can be applied to the required thickness in a single operation. There is no limitation on the size of the workpiece that can be coated, as there is with hot-dip galvanising, and since the steel surface remains cool, there are no distortion problems. Thermal spraying is considerably more expensive than hot-dip galvanising.

For some applications (e.g. bridges), thermal spray coatings are over-coated with paint coatings (after the application of a sealer coating) to form a ‘duplex’ coating system. The combination of metal and paint in a duplex protective treatment has greater durability in comparison with that of the individual components.

The protection of structural steelwork against atmospheric corrosion by thermal sprayed aluminium or zinc coatings is covered in BS EN ISO 2063.

Zinc electroplating

Electroplating is the process of coating one metal over another by using electricity, mainly done to provide protection from corrosion. Using the electroplating process enables us to change the chemical and physical properties of a metal.

Electroplating is done by the process of electrodeposition, and involves the formation of an electrolytic cell consisting of the cathode (the object to be plated) and the anode (the metal used for plating), immersed in an electrolytic solution. The object to be plated and the metal are dipped into the aqueous solution containing the metal ions. When direct current is applied to the aqueous solution, the metal at the anode begins to dissolve, and the free metal ions reach the cathode to form a thin layer of coating on the object. The object to be electroplated is also called a substrate. Zinc is mostly used to provide electroplating to steel or iron material.

It is a very cost-effective process, and is mostly used to provide a protective coating to metallic substances such as nuts, bolts, fasteners etc.

Figure-10.-Zinc-Electroplating-process-.png
Figure 10. Zinc electroplating process

Figure 11. Zinc electroplated bolts

Advantages and disadvantages of zinc electroplating

Advantages:

Cost-effectiveness and ease of application
Provides a decorative finish to metals and can be applied in a variety of colors
It can also be used as an undercoat for paints
It prevents the formation of white rust for a long period of time
Excellent ductile and adhesive properties

Disadvantages:

Lack of durability in sea water
Inability to form a uniform thickness due to the shape of the metal being electroplated
It easily forms a coating over the external parts of the metal, but is not easily attached in the internal areas of the object

Reference code: BS EN 12329:2000 – Corrosion protection of metals. Electrodeposited coatings of zinc with supplementary treatment on iron or steel.

Sherardizing

Sherardizing is a thermal diffusion process and involves placing the pre-cleaned components to be coated into a container, along with a distribution media and a calculated mass of zinc powder.

The container is sealed and placed into a furnace which is raised to a temperature of between 330˚C and 425˚C in order to vaporise the zinc, allowing it to form an alloy with the substrate. At the same time, the container is rotated for a pre-determined length of time, usually between 2-4 hours.

The resultant Sherardized coating is smooth, matt grey in appearance and uniform in thickness, the normal range being between 15 to 80 µm, although thicker coatings up to 100 µm are possible.

After the components have been Sherardized, they are normally cleaned and zinc passivated, making them ready for the application of additional coatings if required.

Figure-12.-Sherardizing-process-.png
Figure 12. Sherardizing process

Figure 13. Sherardized bolts

Sherardizing is a versatile process and component size and shape can range from small washers to larger, more geometrically complex and intricate components. As the components to be Sherardized have to be packed into a container, their size is limited by its dimensions of 2000mm x 500mm x 400mm.

This process is used mainly for small parts and fasteners, particularly for threaded work where only small change of dimension is acceptable. After suitable surface preparation, the items are tumbled in hot zinc dust. The thickness of the coating varies with the processing conditions.

Reference code: BS EN 14713-3:2009 – Zinc coatings. Guidelines and recommendations for the protection against corrosion of iron and steel in structures.

Basheer Bava

Associate Director

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