ARCHITECTURAL GALVANIZING

Last Updated 1 week ago by Kenya Engineer

Little has changed in almost 187years in the hot dip galvanizing industry to protect steel structures against corrosion, however on the aesthetic front there has been a sea change in the way in which galvanized products are being presented. Designers are increasingly calling for an “architectural finish” creating in effect a new product which needs specific consideration from the outset. The bare metallic finish and natural texture of uncoated galvanized surfaces is often selected for architectural work based on its impact as a self-finish. As a result, this brings into play a whole new set of considerations which can affect the visual qualities of the finished product.

Galvanizing has a track record in not all, but most atmospheric environments, where service life up to 25 years plus is commonplace. In some instances, particularly in dry rural environments, 110 years has been achieved. There are also documented case histories of transmission towers in windswept coastal regions around the world lasting up to 75 years before requiring maintenance.

The prime purpose of galvanizing is to provide corrosion protection for steel, throughout the world it is commonly referred to as either hot dip or static galvanizing. Both descriptions are covered by the internationally recognized standards ISO 1461, Australian Standard AS/NZS 4680 and USA ASTM A123, however it is important to understand these standards only address product description and process method and do not cover an increasingly significant element of aesthetic appearance of the work piece, fusing to the steel rather than forming a separate metal surface, commonly referred to as adhesion at the atomic level.

It is not always possible to judge that the alloy metal created 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 steels can also make the creation of the desired effect less likely, due to vagaries in the way in which they accept the galvanizing process. With knowledge that architectural appearance is required, the steel fabricator together with the galvanizer can take extra precautions to achieve uniform appearance and normal alloy weight.

Historically choosing a galvanized product was relatively simple. For the first 100years or so, it was the only product available; today many zinc-based products are on the market via differing processes which can and have confused the consumer. It is vitally important to understand what is on offer; otherwise, materials with lower performance qualities may be unwittingly chosen particularly where long term protection is the expectation.

The principle galvanizing types: –

• Hot dip galvanizing of fabricated steel articles is covered by ISO1461/ASTM A123 and AS/NZS 4680 standards. These standards were designed for optimum corrosion protection and abrasion resistance, which is widely accepted as the International Galvanizing benchmark for corrosion protection. Alloyed thicknesses range from 35-55microns for steel less than 3mm to a minimum of 85microns for steel over 6mm in thickness, thicknesses are generally 30-40% higher which is an important feature as thickness dictates the performance one can expect. In some cases, thicknesses can be 100% higher, particularly for heavy steel sections, or where the chemistry of the steel is high in silicon/phosphorous.
• Technology developments in the steel industry over the past half century, has introduced inline or continuous galvanizing method for steel components to accommodate modern building practises. Hollow sections are also available which offer a range of design options, basically to provide cost savings for steel and weight reductions. The processes were developed to suit a wide range of construction materials to provide ductility, alloy thickness control, and a measure of protection. The marketing philosophy was to reduce or replace the use of timber and conventional shop painted steel sections. There is no alloy created in these processes, the result is a pure zinc layer over the steel which is significantly thinner than traditional hot dip galvanizing. Whilst these processes have some similar features, they do not provide anywhere near the same level of corrosion resistance, nor do they have the same level of abrasion resistance. On average due to the speed of the operation, (180m/min) zinc thickness is controlled and is typically 25-35microns.

The protection provided is suitable for internal, limited exterior exposure or mild rural environments at best. It is extremely important that designers and consumers are aware of the radical differences between the two processes. Inline/continuous galvanizing should not be confused with traditional hot dip galvanizing where long term protection has been achieved. Nevertheless, when used correctly and in the appropriate circumstances these are outstanding materials.

Serious and costly failures have occurred due to misaligned specifications and use. Galvanizing’s reputation rests on its appropriate use where the flow on effect also impacts on specifying authorities and upon those in the construction industry who unwittingly base decisions on economic factors. In many cases procurement decisions are made with good intentions; however, there can be a severe lack of product knowledge where the consequences far outweigh the original construction value. One example which comes to mind is where the original cost of           balustrading for a multi storey apartment building was $600K, the remedial cost for replacement due to premature corrosion failure was more than $20M. Unfortunately, there appears to be a culture within the construction industry worldwide to look for the lowest common denominator, with the         overall view of cost minimization.

Appearance

Firstly, it is not all doom and gloom for inline/continuous products; they provide an attractive, very smooth uniform and shiny finish like paint. Hot dip galvanizing on the other hand, cannot be compared in the same light; for the most part it is relatively smooth, finely textured, and somewhat granular, characterised by an attractive flower or spangled appearance.

Spangled or matt finishes are related to the alloy layers created; a good covering of surface zinc ensures a bright finish. Brightness can be increased with a small addition of aluminium to the galvanizing bath. The other influencing factor is cooling of the steel after galvanizing; slow cooling of zinc crystalizes into large grains and spangled appearance; conversely rapid cooling produces a fine grain where the spangle is less obvious. Where spangle is the desired result, small amounts of tin are added to the galvanizing bath which aids in the creation of the spangled effect. Dull grey colouring is frequently found on heavy sections that spend longer time in the bath and cool slower or where the steel chemistry is high in silicon/phosphorous.

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 which is aimed primarily toward corrosion protection. It is also highly desirable that the galvanizing be inspected before it leaves the plant, since it becomes the responsibility of other parties from that point on where it may be subject to abuse if not handled correctly.

Site storage can expose galvanized steel to damage, oil, grease, dirt, and other foreign matter 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 fabricator and builder, to ensure that the cleanliness of the steel items are protected, and no undue abuse inflicted on the surface, in the same manner as conventional painted surfaces are treated.

Architectural Specification Comment

To specify the additional galvanizing characteristics for architectural purposes, the following details should be additional to calling up the after-fabrication galvanizing standard.

Scope: This specification refers to the standard of galvanizing finish required when aesthetic appearance is needed. It is additional to the standard finish referred to in ISO 1461, ASTM123 and AS/NZS4680 for the purposes of decorative use, or post treatment where there is to be a decorative finish such as paint or powder coatings applied.

Galvanizing for use as a self-finish: The surface finish of the galvanizing should be free from runs, dags, spikes, uneven surfaces and roughness. The appearance and surface condition should take account of 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 fabricator and galvanizer prior to the commencement of the galvanizing process. Some of the problems associated with the process that can occur are bare spots, blisters, lumps runs, roughness, dross protrusions, ash and flux inclusions. All of these can influence the eventual appearance, nevertheless the industry is aware of these potential problems and can take the appropriate measures when there is an architectural requirement. The surface condition of the steel also needs consideration, as a rule hot rolled sections will not provide the same smoothness and shiny surface as with cold rolled sections, which are free if mill scale prior to galvanizing. Rusted steel will also highlight surface variations.

Steel composition: Silicon and phosphorous levels can affect surface appearance, particularly colour and roughness. Certain levels of silicon content will result in excessively thick galvanizing. These very thick alloys result from the increased reactivity of the steel with molten zinc, and rapid growth of the alloy layers on the steel surface. Excessive growth in thickness takes place on steels with silicon contents in the range of 0.04% to 0.14%. Growth rates are less for steels containing between 0.15% and 0.22% silicon and increase with increasing silicon levels above 0.22%. Most steels fall within acceptable limits for these constituents. A large proportion of steel used in construction throughout the world is produced by the electric arc furnace process, where there exists a propensity for higher silicon content than steel made by the continuous casting method.

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

Design for dipping: The design of the articles to be galvanized 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. To achieve the best finish, the article should be designed to fit inside the galvanizing bath in a single dip. Welding quality should also be specified to be free from porosity and weld slag, welding rods are also known to contain high levels of silicon and phosphorous, therefore it is important that this facet of the work be given consideration. Grinding marks should be minimised as they may appear in the finished galvanized surface.

Inspection: The completed work should be inspected by the interested parties at the galvanizer’s yard prior to dispatch, or alternatively a control sample of the type of finish required could be agreed to before galvanizing commences.

Galvanized 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 the appropriate industry guidelines are known and observed, the desired finish can be achieved. The combination of appearance, performance and cost can make this product increasingly attractive to the specifier, contractor and client alike. The most effective way of keeping potential problems at bay is through consultation between those in the supply chain, designer, steel fabricator, and the galvanizer prior to and after treatment