Below is a series of brief answers to some Frequently Asked Questions. Scroll through and click on the question to reveal the answer.
If you have a question that is not listed here, would like more information or would like to discuss a specific scenario, please contact us.
What is the durability/performance of hot-dip galvanized coatings in different atmospheric environments?
The durability or service life of all galvanized coatings is generally directly proportional to the coating thickness and these are defined in each of manufacturing Standard for the different hot dip galvanized coatings available, with the Standard for ferrous articles hot dip galvanized after fabrication being AS/NZS 4680. Two key Australian Standards, AS 4312 and AS/NZS 2312.2 provide considerable information on the corrosion rate of steel and zinc respectively under various conditions of atmospheric service. The chart below graphically displays estimated zinc corrosion rates versus galvanizing coating thickness, allowing for a simple calculation of life to first maintenance.
To use the chart, let’s say you have a hot dip galvanized structure using steel thicker than 6 mm and located in a C4 environment. According to AS/NZS 4680, a piece of steel greater than 6 mm thick must have a minimum average of 85 µm zinc. If you follow the 85 µm marker on the chart up to the green C4 line and across to the “coating life to first maintenance” axis, you’ll see you have, in the worst case, approximately 20 years until first maintenance (or 5% rust of the substrate steel). In other words, 95% of your coating is still intact, so the structural integrity is not threatened. In the best case, the durability of the coating will be up to 40 years (following the line up to next zone).
How do I repair a hot dip galvanized coating (e.g. after welding)?
Appropriate coating repair methods in accordance with AS/NZS 4860 – Hot-dip galvanized (zinc) coatings on fabricated ferrous articles are outlined in Clause 8 Repair After Galvanizing. The most commonly recommended option is the application of an organic zinc rich epoxy paint complying with AS/NZS 3750.9. The recommended method is as follows:
Degrease and remove all surface contaminants.
Remove all welding scale, slag, rough edges and corrosion products.
Clean to either:
(i) For C1 – C3, low to medium corrosivity categories, AS 1627.2 St 3* OR
(ii) For C4 – C5, high to very high corrosivity categories, AS 1627.4 Sa 2 1/2 with a surface profile**
For Industrial finish in C1 – C3 (moderate) atmospheric corrosivity zones
Apply 2 coats of a one- or two-pack epoxy zinc rich primer to 100 – 150μm DFT.
For Industrial finish in C4 – C5 (coastal, marine) atmospheric corrosivity zones
Option 1: Apply 1 coat of an inorganic zinc silicate (IZS) to AS 3750.15 to 100μm DFT; for longer service life of repair and when a colour topcoat is not required.***
Option 2: Apply 2 coats of two-pack epoxy zinc to AS 3750.9 to 150μm minimum DFT followed by two-pack high build epoxy to 150μm DFT, when colour topcoat is required.
For Decorative finish – all environments
Apply appropriate repair from the options above, stippling edges of the painted area to achieve optimum appearance of the repair.
Where the unrepaired galvanizing has a ‘shiny silver’ appearance and if a very close colour match is essential, apply 1 coat of a ‘silver’ paint with an aluminium pigment**** over the zinc rich primer for appearance only. Otherwise a more uniform metallic colour match will be achieved over time.
It is important to observe normal good painting practice with respect to weather and application conditions. Apply all paint strictly in accordance with paint manufacturers’ recommendations.
Repaired areas of hot dip galvanized steel are normally considered to be most ‘at risk’ of early corrosion. Repaired areas should therefore receive an earlier maintenance inspection than the remainder of the structure.
* Suitable power tools include power wire brush, needle gun, disc sander, angle grinder and/or chipping hammer.
**The Bristle Blaster can achieve a Sa 2 1/2 level of cleaniness along with a profile for improved coating adhesion
***Inorganic zinc silicate paints introduce a risk of mud cracking when the coating is thicker than 120μm DFT.
****An aluminium (oxide) platelet or leafing aluminium pigment, which orientate parallel to the substrate and reflect light, giving the coating a shiny appearance.
For more information, download our Advisory Note on Field Repair of Welded Areas and Other Damage from our Technical Publications page.
How do I clean hot dip galvanized surfaces?
There are a number of ways of treating different types of stains or marks. It is advised that with any of the cleaning treatment of the galvanizing should be conservative at first and then if the situation demands, the treatment can become steadily more aggressive. It is also important that wherever some form of mechanical abrasion or “scrubbing” is required, a hard plastic bristle brush is recommended to be used. Steel bristle brushes are not to be used since they will cause discolouration.
It is important to note that mechanical methods of cleaning zinc surfaces can cause aesthetic issues. The “cleaned” areas are likely to contrast with adjacent untreated surfaces and may take a significant period of time to weather to a uniform colour. If aesthetics is a large concern, it is advisable to first test the cleaning method in an inconspicuous area in case the aesthetic effect is unappealing.
For general cleaning of bulk contaminants such as dirt and the like, ordinary laundry soaps can be satisfactorily used. For more stubborn or larger areas, the use of a low pressure wash (such as a gurney gun), with just pure water or in conjunction with proprietary cleaning materials such as car wash or truck wash, can be effective. The car and truck cleaners are made to minimise corrosion on the metallic parts of vehicles so are generally suitable for use on galvanized steel although it is important that the steel be washed down with freshwater after cleaning.
Many mild stains (such as those from water ponding and water runs or, in public areas, those from beverages such as beer, wine etc) can be removed with the use of common household ammonia cleansers, again being sure to thoroughly rinse the galvanized article with freshwater afterwards.
Often, water draining from other adjacent steelwork that is rusting can flow on to galvanized steel and cause conspicuous brown staining. This can be treated with the use of commercial oxalic acid or a proprietary solution that has been developed for descaling pots and pans. Thorough rinsing with water is again important to remove any corrosive residues of the cleaner.
Sometimes during building or renovations, cement and mortar can be dropped onto the galvanized steel and this can be very difficult to remove once it has hardened. Firstly remove the large parts of the deposit as close to the surface as practicable, then oxalic acid can be used to remove the remaining remnants from the galvanized steel, followed with a thorough rinsing. Other acids are more effective on the mortar or cement, but these can be very aggressive on zinc and are not recommended.
Paints, such as graffiti, can be removed using thinners. If some form of scraping is required, use plastic or wooden scrapers (not steel/metallic items). If the paint is wet or fresh, then normal thinners can be used. Once the paint has hardened, then a non-alkaline stripper can be used. Again, rinsing is important to remove residues that may cause discolouration later and/or encourage corrosion
How do I care for and maintain hot dip galvanized steel?
The following information offers some guidance on the general care and maintenance of hot dip galvanized steel products.
Avoid long periods of exposure of your galvanized steel product to environments where the pH is below 6 and above 12. Outside the range of pH 6-12 the galvanized coating can suffer greater corrosion than normal.
Avoid direct contact of your galvanized steel product with dissimilar metals, such as brass and copper, particularly in corrosive environments. Where dissimilar metals are to be used together ensure that there is an insulator between the dissimilar metal and the galvanized product.
Do not constantly abrade clean your galvanized product and, where possible, avoid abrasive washing of your galvanized product altogether. One of the ways in which galvanizing protects steel from corrosion is by the development of a thin barrier film of insoluble zinc corrosion products (known as a patina) on the outer surface of the galvanized steel through exposure to the atmosphere. Abrasive cleaning will wash away this protective patina and the galvanized article will have to build up this barrier protection again, consuming more of the zinc. Constant abrasive cleaning will consume the zinc more quickly and therefore may reduce the life of your galvanized steel product.
Galvanizing may be cleaned using a water-based emulsifier, alkaline-based cleaners with a pH of 12 or lower or organic solvents. Then rinse the area with fresh water and simply wipe clean with a soft cloth. Please consult the your galvanizer or the GAA if you have any concerns in regards to cleaning your product.
For galvanizing product situated in a highly corrosive environment eg. coastal, heavy industrial, etc. it is recommended the product be rinsed with potable water on a regular basis, particularly under sheltered conditions (i.e. not exposed to rain and sun).
Avoid long term storage of any galvanized product in damp and poorly ventilated conditions. Ensure the storage location is dry and there is effective ventilation.
If there is physical damage to the galvanized coating of the product (e.g. coating is chipped or fabrication after galvanizing has taken place), it is recommended that the damaged area be repaired in accordance with AS/NZS 4680. For more information, download our Advisory Note on Field Repair of Welded Areas and Other Damage from our Technical Publications page.
How does the cost of paint compare with (the cost) hot dip galvanizing?
The cost aspect of the coating is usually split into two sections, the initial cost and the maintenance cost. The life cycle cost of a coating combines the total initial cost and all of the maintenance costs together to give the overall coating cost for the duration the steel is in service
The maintenance cost of a coating will depend the durability of the coating system and how long you want what you are protecting from corrosion to last. The durability of a coating system can be estimated using Australian Standards after estimating the corrosivity of the environment, giving the user an idea of when maintenance costs will begin.
The GAA has developed a free Life-Cycle Cost Calculator (LCCC) that allows users to estimate the initial and total life costs of 30+ corrosion protection systems against hot dip galvanized steel. Based on the information you provide, the LCCC will generate a customised report detailing all estimated costs associated with maintaining the structure over the desired service life, including taking into account the time value of money.
Most galvanizers in Australia quench articles after galvanizing in a solution which contains a substance to passivate the zinc surface. This helps to protect the fresh zinc’s surface from early corrosion and typically washes off naturally within a month. AS/NZS 4680 requires the galvanizer to remove any wet storage stain from articles before leaving the galvanizing plant, after which the customer is responsible for storing the galvanized steel correctly and remedying any storage stain themselves.
Careful storage of galvanized steel is essential to prevent the formation of white rust. Articles should be stored in a way which:
Permits free air flow over the galvanized surface
Allows water to drain off and prevent any ponding
Is free of any plastic wraps or temporary storage
Separates closely packed articles soon after transport
Avoids enclosed humid environments
Prevents continuous contact with wet or damp materials (e.g. soil or grass)
In coastal environments or in areas of high salt deposition, salt should be regularly cleaned off the surface when the galvanizing is exposed (e.g. washing the salt off galvanized steel at the same time as cleaning visible salt off windows in coastal homes).
For more information on wet storage stain (white rust) click here.
What effect does the steel composition have on the hot dip galvanizing process and resulting coating?
A steel’s composition or the elements it contains effects the reaction that occurs between the steel and the molten zinc during the galvanizing process. Silicon and phosphorous concentrations in steel generally have largest effects on the reaction, influencing the structure, appearance and properties of a galvanized coating.
As a guide to the influence of silicon and phosphorous concentration in steels on galvanizing, the following criteria should both be specified for the steel if aesthetics is a critical consideration:
There is a perception that distortion of fabricated steel items is a significant problem, however, in reality, distortion occurs in only a very small number of instances. Distortion has become a rare occurrence as bath sizes and handling facilities have improved.
Galvanizing will not generally cause distortion provided that design and fabrication principles are correct. When steel fabrications do distort during galvanizing, the reasons have usually been ‘built-in’ at an earlier stage. Distortion almost always arises from the relief of stresses as the steel is heated to the galvanizing temperature (usually 445-465℃). Although such stresses may be inherent in the steel and may vary from batch to batch, they are more commonly caused during fabrication. Distortion may also occur if steels of significantly different thicknesses are joined together in a fabrication. Only very rarely is it caused by handling in the galvanizing plant.
Basic design rules for avoiding distortion:
Maximise the uniformity of heat transfer into and out of the steel.
Minimise the effect of stresses while the article is in the molten zinc.
Use symmetrically rolled sections in preference to angle or channel frames. I-beams are preferred to angles or channels.
Ensure assembly and welding techniques minimise stresses in components making up the article.
Avoid designs that require double dipping. It is preferable to build assemblies and sub-assemblies in suitable modules allowing for quick immersion and galvanized in a single dip so the entire article can expand and contract uniformly.
Ensure the structural design of the item is sufficient to support its own weight at 50% of the steel’s specified yield strength.
Avoid using large areas of thin (under 8mm), unbraced flat plate.
Use temporary bracing or reinforcing on thin-walled and asymmetrical designs.
For more information, download our Advisory Note on Distortion and our Design Guide from our Technical Publications page.
Is hot dip galvanizing abrasion resistant?
A typical hot dip galvanized coating consists of various iron/zinc alloys growing out of the steel base which is covered by an outer layer of pure zinc, as shown in the micrograph below. The abrasion resistance of a typical galvanized coating can be likened to a buffer stop. The soft outer zinc absorbs much of the shock of an initial impact and then the underlying iron/zinc alloys, which are harder than the mild steel base, prevent or greatly reduce the penetration of the coating and exposure of bare steel.
Abrasion tests show that if abrasion resistance for epoxy zinc-rich primers and most other conventional paints is taken as unity, polystyrene zinc-rich primers are 5 times better, zinc silicate primers 50 time better and hot dip galvanized steel 400 times better. This comparison was made by comparing the weight of silicon carbide (in grams) to erase 100 micrometres of coating using a Taber Abraser, with results shown below.
Weight of silicon carbide (g) to erase 100µm
Epoxy zinc-rich primer
Polystyrene zinc-rich primer
Zinc silicate primer
Galvanized steel product
The toughness of a galvanized coating makes it particularly suitable in situations where abrasion could be a problem either in assembly or in use. Conveyor systems, including buckets for quarried material and hoppers for coal wagons, are well suited. Galvanizing also limits the damage when spanners or other tools slip or when nuts turn on a galvanized surface during tightening. While toughness of the coating does greatly simplify the handling of large, heavy sections and reduces any remedial measures that are necessary. Even where the base steel is exposed, the sacrificial corrosion protection properties of galvanizing coating will protect these exposed areas and prevent unsightly and damaging rust.
What is bimetallic or galvanic corrosion
Bimetallic or galvanic corrosion is the name for the type of corrosion that occurs when two different metals come into direct contact and an electrolyte (such as moisture) is present. All four components must be present before there is a potential for bimetallic corrosion to take place. The more electronegative or anodic metal, as determined from the electrochemical series (see below), is the one that corrodes preferentially to prevent corrosion of the other metal.
Zinc is one of the most anodic metals and therefore with corrode preferentially to most other metals used in the building industry. Stainless steel and aluminium are commonly used in contact with hot dip galvanized steel, notably as fasteners and are mostly satisfactory. This is due to the small surface area of the stainless steel or aluminium fastener compared to the relatively larger area of the hot dip galvanized article. Best practice includes the use of insulating washers which will electrically isolate the two metals. In very corrosive locations, alternative solutions should be considered. Conversely, hot dip galvanized fasteners should not be used to join stainless steel or aluminium articles. More information is available in Appendix B8 of AS/NZS 2312.2.
In corrosive situations and when in direct electrical contact, copper and its alloys can accelerate the rate of corrosion of hot dip galvanized steel. Corrosion products of copper and its alloys can also accelerate the corrosion of galvanized surfaces.
A guide to compatibility of metals and alloys in contact with hot dip galvanizing is given in our document Reference Manual – The Complete Works, available from our Technical Publications page.
Galvanizing of Fasteners
Hi Team GAA,
I have a few questions for you… As I understand it, AS 1214 requires only a minimum average of 52.5microns for all bolts.
So some questions come to mind:
Is this irrespective of size (bolt diameter)?
Can this be increased, if so, under what conditions?
If not, what is typically done to increase life of HDG bolts considering they may have significantly less coating thickness relative to the steelwork section.