Architectural Finish puts Galvanising in a New Light

Little has changed in almost 160 years in the metallurgy of hot dip galvanising to protect steel structures against corrosion. On the aesthetic front however, there has been a seachange in the way in which galvanised products are treated.

As architects increasingly put bare galvanised steel on show in their structures, the demand for architectural finishes increases. This brings into play a whole new set of considerations which can affect the visual qualities of the finished coated steel.

In the galvanising process, fabricated steelwork or steel components are dipped into a bath of molten zinc, forming an alloy coating of zinc and steel on the surface of the workpiece.

This coating fuses to the steel rather than forming a separate coating. The zinc-rich galvanised surface forms a hard, outer barrier to the environment. Even when there is surface damage, the zinc provides protection for the steel by cathodic action.

Galvanising has a sound track record in most atmospheric environments, with service life ranging from 25 to 50 years commonplace throughout Australia. In some instances, service life of up to 110 years has been achieved. There are also numerous documented case histories of hot dipped galvanised transmission towers in windswept coastal regions around the world lasting up to 75 years before requiring maintenance.

After-fabrication galvanising is now covered by the new Australian Standard AS/NZS 4680:1999, but this standard addresses product description and process method only, not covering an increasingly significant element of galvanising - aesthetic appearance.

Galvanised steel can be overcoated in an almost limitless range of colours, providing there is suitable preparation and a compatible paint system. Until recently, that treatment and use of such steel "as is" were the only available options.

Self Finish

Designers are now increasingly calling for an "architectural finish" on galvanised components, creating in effect a new product needing specific consideration from the outset.

Architects value the metallic finish and natural texture of uncoated galvanised surfaces and often select this for architectural work on the basis of its impact as a self finish. It is not always possible to judge that a "normal" coating will be acceptable.

Variations in steel composition can result in notable differences in appearance, even though these may be well within standard specifications required for protective properties. The use of some imported steels can also make the creation of the desired effect less likely, due to vagaries in the way they accept the galvanising process. With the knowledge that architectural appearance is required, the galvanizer can take extra care to achieve uniform appearance and normal coating weight.

Design specifiers require information which must be presented for steel procurement, so that the additional detailing required is considered at bidding and observed before dispatch. Without this advice the necessary dressing will occur only as required by the normal standard. It is also highly desirable that the galvanising be inspected before it leaves the plant, since it becomes the responsibility of other parties from that point. Risks of a galvanised surface being damaged are very low where hardness greater than steel exists at the alloy level.

Site storage can expose galvanised steel to oil, grease and dirt which may disfigure the surface and require extensive washing and degreasing to remove it. Disciplined freight, site storage and erection procedures must be observed by the builder or fabricator, to ensure that the cleanliness of the coating is protected and no undue abuse inflicted on the surface. The normal assumption that galvanising is immune from damage must be set aside and the enhanced "metallic" appearance characteristic preserved.

Specification Details

To specify the additional after-fabrication galvanising characteristics of wear resistance and metallic appearance, the following specification details should be additional to calling up the after-fabrication galvanising standard.

Scope

This specification refers to the standard of galvanising finish required when aesthetic appearance is needed.It is additional to the standard finish referred to in AS/NZS4680:1999 for the purposes of decorative use or post treatment where there is to be a decorative finish such as paint or powder coating.

Galvanising for use as a Self Finish

The surface finish of the galvanised coating should be free from runs, dags, spikes, uneven surfaces and roughness. The appearance and surface condition of the coating should take account the need for visible uniformity and should be able to cope with handling and physical contact in public areas. These aspects should be defined and agreed with the galvaniser prior to the commencement of the galvanising process.

Steel Composition

Silicon and phosphorous levels can affect coating appearance. Certain levels of silicon content will result in excessively thick galvanised coatings. These very thick coatings result from the increased reactivity of the steel with molten zinc, and rapid growth of zinc-iron alloy layers on the steel surface. Excessive growth in coating thickness takes place on steels with silicon contents in the range of 0.04 to 0.14 per cent. Growth rates are less for steels containing between 0.15 and 0.22 per cent silicon, and increase with increasing silicon levels above 0.22 per cent. Most Australian made steels fall within acceptable limits for these constituents.

The presence of phosphorous above a threshold level of approximately 0.05 per cent produces a marked increase in reactivity of steel with molten zinc, and rapid coating growth. When present in combination with silicon, phosphorous can have a disproportionate effect, producing excessively thick galvanised coatings. Allowances for thicker and rougher galvanised coatings should be made if steels containing excessive levels of silicon and phosphorous are galvanised.

Design for Dipping

The design of the articles to be galvanised should be discussed with the galvanizer during the detailing stage, as the provisions of adequate filling, venting and drainage requirements can have a significant effect on the ability to achieve a smooth finish.

As a general rule, the drainage should be a minimum of 40 per cent of the cross sectional area of the article. In order to achieve the best finish, the article should be designed to fit inside the galvanising bath in a single dip. The welding quality should be specified to be free from porosity and weld slag. Grinding marks should be minimised as they may appear in the finished galvanised coating.

Inspection

The finished galvanised work should be inspected by the interested parties at the galvanizer's yard prior to dispatch, alternatively a sample of the type of finish could be agreed to before galvanising.

Post Treatments

The type of top-coat, if any, should be stipulated to the galvanizer in advance. This will determine the type of repair method used for the galvanised coating.

Galvanised steel is a proven and reliable product in terms of durability, but its use as an architectural finish presents challenges as well as opportunities. If these guidelines are observed, the finish can be assured, and the combination of appearance, performance and cost can make this product increasingly attractive to the specifier, contractor and client.

Visit the Galvanisers Association of Australia site for more information on galvanising.

By Nick Karakasch