CWCT FACETS

12.02 Metal protection

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Categories: Finishes & Corrosion

Selection of material/composition
The most common and easiest way of preventing corrosion is through the judicious selection of materials, once the corrosion environment has been characterised.  However, it is not always feasible to use the material that offers the optimum corrosion resistance and sometimes another alloy and/or some other measure must be used.

Copper, aluminium, lead, and stainless steel have good resistance to corrosion and do not generally require protection provided the factors affecting corrosion discussed above are avoided. Although zinc corrodes when exposed to normal external conditions, the rate of corrosion is slow and a 200 micron thickness can be expected to last 20 to 40 years depending on the level of pollution. Where a longer life is required additional protection can be provided. Plain carbon steel normally requires protection against corrosion.
 


Organic coatings
Organic coatings can be applied on site or under factory conditions and create a physical barrier to the corrosive elements. They must maintain a high degree of adhesion to the metal substrate through careful surface pre-treatment and will have a finite life requiring periodic maintenance. Some coatings (e.g. polyester powder coatings) do not provide protection against corrosion of steel and steel substrates should therefore be pre-treated (e.g. galvanised) before application of such finishes. Coatings are discussed in greater detail in Section 12.03, metal finishes.
 


Metal coatings
The performance of a metal coating will depend on whether it is anodic or cathodic to the base metal. A cathodic coating (for example copper on steel) will only protect the base metal by forming a barrier. If there are defects in the coating, rapid local corrosion of the base metal may occur. An anodic coating (for example zinc on steel) will act as a barrier but if there are small defects it will continue to provide protection by corroding preferentially to the base metal as a result of galvanic action.

The most commonly used metal coating is zinc on steel which may be achieved by a number of processes including hot dip galvanising, zinc plating and sherardizing.  Aluminium coating of steel and chromium plating of steel and brass are also used.  Chromium coatng of steel is a cathodic coating.
 


Zinc coated steel
Zinc coating is the most common way of protecting steel against corrosion, although it is often pre-treated and then over-painted to enhance corrosion resistance and appearance.  Zinc protects the underlying steel in two ways:

  • The zinc acts as a physical barrier between a potentially corrosive environment and the steel substrate. Under most conditions zinc corrodes more slowly than steel; under normal atmospheric exposure zinc corrodes at a rate of between about 1/10 and 1/50 of that of steel.
  • The zinc gives galvanic or sacrificial protection at unprotected cut edges and at scratches.

For minor defects in the zinc coating it will continue to protect the underlying steel by galvanic action, giving very good performance in clean, neutral conditions, performing well in mildly alkaline conditions, but subject to chemical attack in acidic (industrial) environments.  Larger areas of damage (maximum 40mm2) should be repaired using either zinc-rich paint or low melting point zinc alloy repair rods or powders, which should be at least equal to the thickness of the original zinc coating.  Exposed steel and cut edges in galvanised sheet should be painted to prevent corrosion.

The degree of protection also depends on the area of zinc exposed adjacent to the defect. Where the zinc is painted the area of zinc exposed adjacent to a defect will be reduced and consequently the degree of protection will also be reduced.

There are many methods of zinc coating steel:

  • Continuous hot-dip galvanising
  • Hot-dip galvanised coating of articles
  • Thick hot-dip galvanised coating
  • Centrifugal galvanising (BS 729)
  • Zinc spray (BS EN 22063)
  • Zinc plating
  • Sherardizing
  • Coatings incorporating zinc dust

The most widely used methods are continuous hot-dip galvanising of coil, hot-dip galvanised coating of articles, zinc plating and sherardizing, which are described below.

The specifier must select the zinc coating appropriate for the type of component and the finish that offers the required economy, strength, formability and level of corrosion resistance depending on the exposure conditions.  Wear resistance and surface appearance may also be important properties of the zinc coating.
 


Continuous hot-dip zinc galvanising of coil
Hot-dip galvanising is one of the most common methods of zinc coating and provides the thickest protective zinc layer. A mixture of zinc and aluminium can also be used and gives better performance for the same thickness. When thin steel sheeting is manufactured, it is wound into large rolls known as ‘coil’. In the galvanising of coil, the steel, in one continuous process, is:

  • De-coiled;
  • Chemically cleaned by immersion in dilute hydrochloric acid (‘pickling’) to remove all corrosion products (i.e. traces of rust and mill scale);
  • Dipped into a bath of molten zinc maintained at a temperature of about 450oC;
  • Cooled, inspected and re-coiled.

Continuous hot dip coating of steel strip and sheet is covered by the following standards (BS EN 10214, BS EN 10215, BS EN 10142, BS EN 10143 and BS EN 10147). When specifying coated steel sheets, it is necessary to specify both the steel sheet and the coating.  The thickness of coating is the fundamental criterion of the quality of a hot dip coating.

The standard notation for describing coated products is as follows:

  • Number of standard,
  • Steel name,
  • Letter identifying coating; Z for zinc, ZF for zinc/iron, AZ for aluminium/zinc and ZA for zinc/aluminium,
  • The coating weight in kg/m2,
  • Surface finish, N for normal spangle, M for minimised spangle and R for regular iron zinc alloy due to diffusion of iron through the zinc coating.(only applicable to zinc coatings),
  • Surface quality, A for as coated, B for improved and C for best quality,
  • Surface treatment, C for chemical passivation, O for oiling, CO for passivation and oiling, and U for untreated.



Hot-dip galvanising of steel articles
In this process steel components (e.g. window frames) are dipped into a bath of molten zinc.  The zinc reacts with the surface of the steel to form a layer of zinc/iron alloy and a coating of zinc is deposited on the surface as the article is withdrawn from the bath. The appearance of the coating should be continuous, smooth and free from flux stains.

The specification for hot dipped galvanised coatings is defined by a single standard, namely BS EN ISO 1461. At an early stage it is necessary to define the type of post-treatment required, (e.g. chromating, phosphating or a heavier coating for additional protection) and the coating weight and thickness.

Attention should be paid to the conditions of storage to avoid wet storage staining, which can occur on freshly galvanised components when stored under damp and badly ventilated conditions.  The use of additional surface treatments, such as chromate treatments, reduces the formation of storage stains.

Some fabricated assemblies may suffer from warping and distortion at the temperatures used for hot dipping; at particular risk are assemblies that are dipped more than once to cover the entire surface area (e.g. asymmetrically shaped sections).  The risk of distortion is affected by a number of factors including differential expansion/contraction effects caused by the use of sections of unequal thickness, and the relief of internal stresses from prior welding or cold forming operations.  It is advisable to discuss design of the fabrication with the fabricator and the galvanizer.
 


Electroplated zinc
Here, coatings are produced by plating zinc onto a steel electrode in an electrolysis bath.  The procedure for zinc plating is covered in BS 1706.  This method of zinc protection is suitable for metals used in occasionally corrosive conditions.
When specifying zinc plating it is necessary to specify the surface treatment as well as the coating thickness.  A typical specification might be Fe/Zn 8c 1A, where:

  • ‘Fe’ refers to the base metal (iron or steel);
  • ‘Zn’ refers to the type of coating on the base metal (zinc);
  • ‘8’ refers to the minimum local thickness of 8mm of the zinc coating;
  • ‘c’ refers to the chromate conversion coating, if omitted this means a chromate conversion coating is not required (details of the type of conversion coating should be given separately);
  • ‘1’ refers to the class of finish required;
  • ‘A’ designates the type of chromate coating.

The electroplated articles should be free from clearly visible plating defects such as blisters, pits, roughness, cracks or unplated areas.  The coating should be bright, although heat treatment may cause slight dullness.  If heat treatment is required for the purpose of stress relief, the correct grade of steel needs to be specified according to its maximum tensile strength.

The types of conversion coating used on electroplated zinc are specified in BS 6338.  The colourless coatings, which may have a bluish tinge, have limited protective value and are used mainly for temporary protection during storage and handling.  The yellow coatings have good bare corrosion resistance and are good bases for paint and powder coatings, whereas the olive green coatings are used exclusively for corrosion protection.

All chromate conversion coatings harden with age and therefore should be handled carefully for the first 24 hours after treatment, with any tests deferred until the expiry of that period.
 


Sherardizing of steel
This method of zinc coating is by a process of diffusion in which articles are heated in close contact with zinc dust and an inert operating medium.  The process is normally carried out in a slowly rotating, closed container at a temperature of approximately 385oC and forms a uniform coating on all articles, including those of irregular shape.  This process is only suitable for relatively small pressings, forgings, nuts and bolts, and gives a rough surface finish affording good adhesion of most paints.
When specifying a sherardized coating it is necessary to specify the class of coating depending on the environment and the minimum thickness.  A typical specification might state that the zinc dust shall contain not less than 94 per cent by mass of metallic zinc, not more than 0.2 per cent by mass of lead and not more than 0.0005 per cent by mass of copper, with all particles passing through a 7 mm sieve.

Some articles may be too fragile to withstand the rotary movement required during sherardizing and it is advisable to consult the sherardizer at the design stage.  The life of sherardized coatings in any given environment is proportional to the thickness of the coating.  Where doubt exists concerning the appropriate class of coating, advice should be sought from the sherardizer.

The surface finish of sherardized metals is defined in BS 4921.  The type of finish should have a matt grey appearance and may show superficial scratches.  The coatings are relatively hard and scratches are not detrimental to the corrosion resistant qualities.
 


Cathodic protection
One of the most effective means of preventing corrosion is cathodic protection, which can in some situations completely stop corrosion.  Cathodic protection simply involves supplying electrons, from an external source, to the metal to be protected, making it a cathode.  The electrons may be supplied by a sacrificial anode or a low voltage power source. Cathodic protection is being increasingly used for remedial work on reinforced concrete structures and ha also been used to protect steel encased in masonry.  However, it is unlikely to be appropriate in most cladding situations.
 


Design
Since the longer the period of wetness the greater the corrosion, it is important that structures should be designed as far as possible to shed rather than trap water and allow complete drainage. Prevention of dirt build up by allowing rain-washing of exposed surfaces will also help to prevent corrosion. Design to prevent condensation and ventilation to aid drying of condensation or penetrating water will also reduce the risk of corrosion.

Local extreme conditions can have a severe, adverse effect on the durability of metals.  The strength of the prevailing wind and the regularity of its direction, as well as humidity and the duration of exposure affect the rate of corrosion.  For example, severe corrosion can occur when a metal is partly exposed but sheltered, such as under the eaves or canopy of a building.  This is particularly true in coastal and severely polluted environments where aggressive salts can lodge themselves in areas that are not washed by rainwater, but where moisture is present due to condensation. The effect of different environments on metals is described in Section 12.08.
 


Maintenance
The risk of corrosion can be reduced by regular maintenance. This should include:

  • repair or replacement of protective coatings,
  • cleaning metal surfaces (particularly in the case of aluminium and stainless steel),
  • maintenance of drainage systems,
  • maintenance of seals.

Cleaning is covered in detail in Section 12.09.