Corrosion Protection and The Environment: Galvanizing and Inorganic Zinc Silicate Coatings

INTRODUCTION

The issues of Environment Sustainability and Industrial Toxicity are now significant at all levels of society, no more so than with the Design, Engineering, Construction, and Protective Coating Industry. Architects, Specification Authorities, Standard Authorities, Corrosion Associations, and Project Managers are in the first line of defence in the protection of the environment just to name a few.

The Corrosion Protection, like all other Industries comes at a cost to the Environment. Some of the key issues used in the selection process for Protective Systems include considerations of global warming, greenhouse gas emissions, acid rain, waste management, manufacturing or application impacts, OH&S issues, and long-term environmental sustainability. This article is not all inclusive, the intent is to provide a general insight associated with environmental impact regarding the processes used to facilitate Hot Dip Galvanizing(HDG) & Inorganic Zinc Silicate(IZS) Coatings as related to the Structural Steel Industry.

Environmental Issues Associated with Material Selection:

• What resources are necessary to provide the service?
• How environmentally acceptable is the application process?
• What toxic fumes or waste products are produced. ?
• How environmentally acceptable is the system, what does it contain?
• What airborne emissions and particles if any are generated ?
• What OH&S issues are associated with the process ?
• What are the long-term effects on the environment?

 

Zinc metal in its various forms is the most common material used to combat steel corrosion. Numerous articles over many years have  compared the performance merits of Hot Dip Galvanizing (HDG) and Zinc Coating Systems however, little has been written regarding Sustainability and the Environmental impact that individual corrosion protection systems have on their use.

 

The only materials with a 25 year plus proven anti corrosion performance record in a variety of environments have either been HDG or  Inorganic Zinc Silicate Coatings (IZS). Both provide similar performance (table 1), except for Shipping and Offshore Atmospheric Environments, where HDG is excluded and IZS is exclusively used. HDG performance is 3-5 years maximum whereas IZS is 35 years plus. The application  process of both materials is vastly different and should be viewed in economic terms in the overall context of net present value v net future value. Consideration is needed for such matters as energy, water consumption, greenhouse gas emissions, industrial toxicity, environmental ecological impacts, OH&S issues, and the mineral resources necessary to provide the protection.

There are three levels of environmental impact associated with the Protective Coating Industry. It starts at the mining level of Zinc, the ore body needs to be mined, milled, smeltered then refined to produce metallic zinc. From this point a large preposition will end up in the application process for corrosion protection, either as Zinc slabs for HDG or fine Zinc particles for IZS coatings. From there the impact falls to the application industry as discussed herein. Finally, there is the long-term environmental impact connected to the steel construction industry.

Both HDG and IZS have similar recorded case history listings in a range of atmospheric conditions(Table1). However, the volume of Metallic Zinc necessary to provide the service is quite different, it favours IZS on average by 50% (Table4). The reason for the difference is in the physical make up on how the Zinc is bound into the system and its subsequent sacrificial corrosion rate (ref 12).

Table 1

System Designation	Nominal Coating Thickness microns	AS/NZS 2312 DURABILITY – YEARS TO FIRST MAINTENANCE ATMOSPHERIC CORROSIVITY CATEGORY
		A Very low	B Low	C Medium	D High	EI Very high industrial	EM Very high marine	F Inland tropical
ISO 9223 Equivalent		C1	C2	C3	C4	C5I	C5M	T
HDG	85 125	25+	25+ 25+	25+ 25+	15-25 25+	2-5 2-10	5-15 10-25	25+ 25+
IZS Water based	75 125	25+ 25+	25+ 25+	25+ 25+	15-25 25+	2-5 5-10	10-15 15-25	25+ 25+
IZS Solvent	75 125	25+ 25+	25+ 25+	15-25 25+	10-15 15-25	2-5 5-10	5-10 10-15	15-25 25+

 

With HDG, the Galvanizer has very little or no control over thickness. This is largely dictated by Molten Zinc Temperature, Immersion Time, and Steel Thickness. As thickness increases so accordingly does the final Zinc uptake. For this reason, HDG standards around the world nominate minimum thicknesses for varying steel sections (Table 2 ). For example, 6mm sections and above are listed as 85 microns minimum; however, in reality it is considerably higher due to the nature of the process. Film thickness will average 130 microns and above, well over the nominated minimum.

Table 2

HDG – ENISO 1461- AS/NZS 4680                            Contains 1.4% Lead
STEEL THICKNESS	MINIMUM STANDARD THICKNESS (microns)	ACTUAL AVERAGE DRY FILM THICKNESS (microns)	METALLIC ZINC CONTENT (gm/m²)
3mm plate >	70	99	707
4mm plate	70	110	785
5mm angle	70	110	785
6mm plate >	85	130	928
12mm plate	85	150	1071
200 UB	85	140	1000
310-410 UB	85	150	1071
Heavy Structural Steel	85	180-200	1285-1428
180-200 PFC	85	120	857
125-150 PFC	85	110	785
RHS/SHS (light)	85	85-95	607-678
Thickness to metallic zinc 85 microns x specific gravity (7.14) = 607 gms/m²

 

Steel, and Welding Rod composition is extremely important, if there happens to be high levels of Silicon or Phosphorus, the result is excessively thick HDG, when present in combination can have a disproportional effect producing thicknesses 5 times plus the normal thickness in some extreme cases.

Flaking and brittle delaminating HDG, due to high levels of Silicone and Phosphorous. Average DFT (dry film thickness) 650/Microns equates to.

4641 Metallic Zinc (gm/m²)

 

Metallic Zinc Content at 85microns ranges between 600/1428 grams/m², (Table 2). As zinc consumption (thickness) increases, so does the level of environmental impact. However, there is one advantage, higher HDG thickness increases length of performance providing there is no detachment from the steel surface. It is important the steel fabricator sources members and welding rods that have the appropriate Silicone/Phosphorous levels otherwise they will be charged for the extra zinc consumed which based on the above  example would be considerably well above the normal rate.

The volume of Metallic Zinc in water or solvent based IZS is constant and controlled by the application process there is no relationship to steel thickness or composition on application regarding film thickness (Table 3 ).

 

Table 3:  ZINC SILICATE PAINT SYSTEM – AS/NZS 3750.15   (Contains  < 0.2% lead)

 

Inorganic Solvent		Inorganic Water
Standard Thickness	Metallic Zinc Content gm / m²	Standard Thickness	Metallic Zinc Content gm / m²
75 microns	210	75 microns	300
100 microns	280	100 microns	400
125 microns	350	125 microns	500
		150 microns	600

 

The reason for similar level of protection with lower Metallic Zinc is twofold. 1) Zinc is held within an inert Inorganic Silicate Matrix (sometimes referred to a liquid glass) which has been shown Scientifically to regulate consumption. 2) Together with a Microcrystalline layer which forms on the steel surface during application. This layer has a stifling effect which gives rise to a much slower sacrificial consumption of Metallic Zinc(Ref 11).

THE HOT DIP GALVANIZING PROCESS

The simplest way to describe the HDG process,  is basically one of mechanical handling of steel components through a cleaning process, dipping into a kettle of molten zinc metal and finally followed by quenching.

 

 

1. Degreasing

The process starts with degreasing. This involves dipping into a hot alkali caustic solution             usually at approx.85°C, immersion time ranges from 1-20 minutes depending on the degree of        contamination.

            Environmental Impact

1. Requires the use of vast quantities of potable mains water and heating energy on a continual basis for twenty-four hours seven days a week. (24/7).
2. Environmental surroundings are hot, damp and caustic.
3. Creates degreasing sludge waste.
4. Rinsing

Immediately after degreasing, rinsing takes place in hot ,mains water and in some cases may receive       a final cold rinse.

Environmental Impact

1. Requires large quantities of reticulated mains water and heating energy on a continual basis (24/7).
2. Creates waste scum and sludge which settles to the tank bottom.
3. Waste material requires removal on a continual basis with special disposal requirements.
4. Pickling

Further cleaning takes place by pickling in either Sulphuric or Hydrochloric acid.

Environmental Impact

1. Requires vast quantities of potable mains water and heat energy in the case of Sulphuric Acid use.
2. Environmental work conditions are hot, damp and extremely corrosive. Atmospheric Acid levels are as low as PH1. It is one of the most severe corrosive environments to be encountered.
3. Creates toxic pickling waste and sludge.
4. Rinsing

After pickling, the work is rinsed in reticulated mains water.

Environmental Impact

1. Large use of recirculating potable mains water
2. Fluxing

Following rinsing, items are immersed in a flux solution generally containing approximately 30% Zinc             Ammonium Chloride, together with Wetting Agents at 80-100°C. The steel is then dried ready for dipping into molten zinc.

Environmental Impact

1. Requires high water and energy usage (24/7) , Chemicals, and wetting agents.
2. Work area hot, damp, requires use of environmentally acceptable fume extraction equipment.
3. Atmospheric conditions are corrosive.
4. Dipping into Molten Zinc

Assuming the item has been properly prepared, dipping and removal takes place at a controlled rate.             Average kettle temperature 445-465°C.

 

Environmental Impact

1. Requires very high energy usage ( usually gas 24/7) 365 days x 24hours / day.
2. Contains 1.4 % lead bound within the molten zinc (Ref 6b ).
3. Requires the use of extraction fans to remove molten zinc bath smoke.
4. Requires the use of protective clothing and shielding for worker protection.
5. Higher levels of Zinc usage if the steel chemistry is high in Silicon/Phosphorus which contributes to a higher environmental impact.

Traditionally there is a Lead layer (average 100-130mm), at the bottom of the galvanizing kettle. Its primary function is to insulate the bottom from excessive heat, prevent Dross adhesion to the bottom and aid in the removal of Zinc Dross. Dross is a by-product largely consisting of Iron, Ash and Flux Skimmings which is classified as Hazardous Waste (Basil Convention Ref 12) and is restricted for export. Most of the Dross is reprocessed locally by others and separated into metallic zinc particles or zinc oxide used in a variety of other Industries.

Dross comes in two forms Floating and Bottom layered Dross. Floating Dross occurs when the top layer of the bath is slightly cooler than the deeper areas. Dross is picked up by the  top Galvanizing ETA layer during withdrawal and becomes attached to the Galvanized product. It appears as pimples/blisters inevitably causing excess coating thickness and surface irregularly and has the tenancy to embrittle the HDG. HDG Standards state articles shall be free of Dross, Blisters, Uncoated areas, and Flux Deposits.

Lead can adhere to HDG steel surfaces if the immersion depth is not carefully controlled and penetrates the lead layer. Due to colour, it is very difficult to visually distinguish lead from molten zinc. The accumulation of bottom layered Dross (Ash, Iron, Flux Skimmings) acts as a barrier between Lead and Zinc. However, Dross is removed on a regular basis, thereby exposing Lead to Zinc  therefore diffusion of Lead during this exposure period into Zinc takes place.

Prime Western Zinc which contains 1.4% Lead is the feed stock used by the Galvanizing Industry( 98% Zinc / 1.4% Lead). Solubility of Lead into Zinc is temperature dependent, if HDG temperature increases above the normal 450°C solubility increases to 1.5% at 465°C. Heavy Metals are considered toxic at very low levels, whereas trace elements at low levels of concentrations are not considered Toxic.

HDG consists of inner and outer layers (Ref 7), the inner layers of iron/zinc have lower levels of lead , the outer layer know as ETA has the same lead composition as the kettle 1.4%. The inner layers have approximately 40-50% less lead. Whilst lead is initially metallically contained it does ultimately go into solution as dust powder  being part of the Zinc Corrosion Products produced (i.e., Zinc: Oxide, Hydroxide,& Carbonate) via the Galvanic Corrosion Process from there into the Environment.

Considering the serious nature of Lead related issues , there is no acknowledgement of Lead  (Table 6b) or other classified Toxic Materials involved in the process by the HDG Industry or Standards Australia who were responsible for issuing the Standard. The  product contains  Lead  well above the level of trace elements.

Industry has a Duty of Care responsibility toward their Legal Obligations regarding Health & Safety, and Environment  It is incumbent  to be totally transparent in regard to these matters, so that appropriate policies if required  can be put into place. There is a principle in Law called ‘Conjoined Responsibility’, a shared responsibility by all parties associated with a project, process, or product if anything untoward occurs. It includes “Tacit Approval” by all organizations who indorse any Standard or provide promotional activity directing potential users toward a particular material or process.

 

 

1. Quenching

Quenching completes the process. Following dipping quenching takes place in a solution 0.15%               containing Sodium Dichromate at app 32°C. This is necessary to inhibit early or flash white rusting.

 

Environmental Impact

1. Requires the use of large volumes of heated potable mains water.
2. Chromate solutions are toxic and environmental pollutants. Quenching materials which contain Hexavalent Chrome 6 are classified by the World Health Authority as Carcinogenic. (Ref 8) The solution is changed regularly due to contamination of flux residues, zinc salts and oxides. Waste disposal is strictly controlled by Authorized Agencies and only by licensed operators. Disposal is a major problem and can only be carried out after Certified Neutralizing Treatment.

The use of Chromate Quenching is now increasingly being banned by authorities throughout  the world for Environmental and O.H&S reasons. Nevertheless, Chromate quenching in some quarters is still used. No rinsing follows so no toxic effluent waste is produced at this point. However, there is a potential OH&S issue. The quenching process inevitably steams, due to the hot HDG steel passing through the quenching process.

The Chrome solution steams, splashes, mists, and drips in the work area as jibs carrying steel go in and out of the quench tank, this can led to unacceptable concentrations of airborne Chromates. Hexavalent Chrome 6 has a short life (yet to be determined), then converts to Trivalent chrome when the HDG surfaces are exposed to air.

Whilst it is often stated the amount of Hexavalent Chrome 6  is virtually unmeasurable. This may or not be so, nevertheless one only needs to be reminded of the Science of Nano Technology, matter that cannot be seen or difficult to measure. Nano particles are now acknowledged by the Health and Scientific Community as the delivery vehicle responsible for a wide range of Human Health and Environmental Pollution issues.

Chromate Quenching should be avoided if the articles are to be Painted or Powder Coated. The  alternative is the use of a Phosphate Solution, usually mixtures of Phosphoric Acid and Phosphates with other additions which produces a finer -grained HDG finish.

HDG SUMMARY

HDG has an outstanding corrosion protection record, however the process necessary to provide that level of performance is extremely corrosive to its own and surrounding environment. HDG facilities have wet and dry areas of operation, the wet pre-treatment section is without doubt one of the most severe corrosive environments to be encountered. Airborne acid levels are as low as PH1, largely due to either Hydrochloric or Sulphuric Acid Vapour Mist. Furthermore, Hot Caustic and Pre-Flux tank areas contribute to a continual damp, humid and moist atmosphere.

The severity of corrosion on the production facility is such that on average a typical HDG plant requires structural restoration approximately every 5-7 years. Whilst HDG and Paint coatings are competitors, the HDG industry has to call on the Paint Industry, to provide the corrosion protection necessary for its application facilities. The overall process produces a stream of secondary waste and toxic materials, which are subject to further processing for safe environmental disposal or recycling for use in other industries.

Environmentally the Process accounts for the following:

1. Vast volumes of potable mains water are necessary for the process. A small to medium Galvanizer would consume approximately 2-plus Million litres of water per annum.
2. High energy usage is required for pre-treatment and molten zinc baths, 365days x 24hours = 8544 hours/pa with the general use of either gas or electricity or both.
3. Requirement for use of extraction fans which contribute to acid rain.
4. Generation of large volumes of greenhouse gas emissions and toxic waste.
5. The use of Acid Chemical compounds, some of which are classified Toxic Pollutants and Carcinogens.
6. Contains 1.4% Lead with the possibility of a further increase if the proper dipping procedures are not adhered to, or when the lead layer is exposed to higher molten zinc temperatures.
7. Welding temperatures of 8300°C plus are inflicted on HDG are high enough to vaporise the lead component which starts at 500° The use of respiratory equipment and proper air circulation under these circumstances should be made mandatory, together with the necessary documentation made available to ensure OH&S standards are maintained .
8. Requirement to use of worker protective clothing and barrier equipment.
9. When a facility ceases operation, invariably the site is difficult to sell due to industrial toxic contamination caused by the process. Remedial and clean-up expenditure is well above the normal requirements.
10. Requirement of extensive design detailing for steel sections to facilitate proper galvanizing to ensure adequate venting, filling, and drainage where necessary otherwise explosive forces are generated. These design requirements are also necessary for the safety of the work force (Ref 8), plant, equipment, and ultimately the environment should any untoward accident
11. Requirement of extensive infrastructure, plant, equipment. and tankage to facilitate the process.
12. Dross considered a toxic material can be pickup by the top HDG layer when withdrawn from the bath.
13. Another major concern that needs to be addressed is when HDG requires Abrasive Blasting for Duplex/Topcoat paint applications. The blasting process impacts the ETA layer(1.4% lead), whilst low abrasive pressures are recommended to minimize excess removal, nevertheless what is removed contaminates the Painting Facility and Environment (Ref 9). This application is normally undertaken by the Protective Coating Industry who need to be made aware of this issue so that appropriate actions can be taken.

 

INORGANIC ZINC SILICATE (IZS) – COATING PROCESS

The application of IOZ is basically a conventional cold spray-painting operation which involves two basic steps:

• Surface preparation by abrasive blasting cleaning
• Spray application of water or solvent based IZS Coating that follows.

      A       Surface Preparation

There are two main methods used, to propel the abrasive against the surface to be cleaned:

• Electrically driven centrifugal wheels or, 2) Blasting Pots using compressed air.

Both methods are in enclosed chambers or booths, with the abrasive medium being predominantly steel shot or grit.

Site abrasive blasting also occurs; however, this is generally for maintenance purposes to            existing structures or where the steel sections are too large for a Blasting Chamber. The abrasive medium is mostly Garnet or Ilmenite which are nontoxic and not harmful. Under these circumstances, dust is generated although  authorities stipulate some form of encapsulation to contain any fallout.

Minimum standards for Surface Preparation is Class SA 2.5 AS1624.4 2005 (near white Metal, although SA Class 3 (White Metal) is the preferred standard for exposure where severe environments are to be in counted.

Environmental Impact

1. Requires the intermittent use of either electrical or diesel power for compressed air. On average 5hrs/day x 5days x 52weeks = 1300hours/Pa.
2. Abrasive is recycled within the system. Dust is contained within the booth and extracted via a dust collector; approximate volume is 200litres every 3 months. Dust collected is nontoxic and is predominantly steel mill scale and or iron oxide (rust).
3. No toxic waste or fume emissions are generated by the process.
4. Work environment is totally dry and benign.
5. Protective clothing (hood) needs to be worn during the abrasive blasting process.
6. Steel chemistry has no impact regarding applied dry film thickness.

 

B     IZS Application Procedure

IZS coatings are proprietary manufactured products and are available as either single or two pack materials. Metallic zinc particles are pre-purchased from Zinc Suppliers (contain <0.2% Lead). Commercial Inorganic Silicate Solutions are purchased for a material supplier with further additions added to provide flow characteristics during application. These coatings are cold applied by either conventional or airless spray-painting methods. The lead level is a trace element and not in dust or chemical form as has been widely suggested or implied, no further lead or other heavy metals are used in the manufacturing process.

Environmental Impact

1. Application environment is totally dry, non-corrosive and benign.
2. No toxic fumes or waste is generated by the process.
3. Lead component is <0.2%, with no further possibility of increase, this trace element is contained within the film during the corrosion process associated with IZS.
4. Empty paint containers are benign and recycled.
5. Mains water usage to clean spray equipment is minimal, 40litres/day/operation – (Est) 15000litres p/a. Water component for 12500 litres in manufacturing is 1500 litres.
6. Does not require the use of extraction equipment.
7. No special requirements for safety equipment or clothing during application
8. No VOC associated with water based IZS .
9. No heating involved in the process therefore no energy requirements.
10. No special requirements regarding steel fabrication design, apart from chamfering of steel edges to ensure adequate edge protection.(Ref table 5)
11. Very little infrastructure necessary apart from a steel lined blasting chamber which includes a dust collector and conveyer system to recycle steel shot or grit. together with steel trestles for spray painting activities.
12. Solvent borne IZS- There are relatively small levels of solvent that go to atmosphere during the curing process.
13. No OH&S issues associated with welding of IZS, nevertheless, it would be prudent to err on the side of caution, respiratory masks should always be used when welding.

IZS SUMMARY

The recorded performance is also outstanding. There is virtually no environmental impact apart from the small levels of greenhouse gas emissions associated with the intermittent use each day, of either electrical or diesel generated power, necessary to provide compressed air for surface preparation and painting activities. The water resources required are insignificant as are the O.H&S requirements. There are spray losses associated with spray painting, the level with IZS is low and generally ends up as over thickness on the steel surface or on the factory floor as overspray. This is periodically swept up and sold for recycling. Annual volumes for an average operation are in the order of 2 cubic metres (2m³). Material collected is a mixture of IZS and normal everyday dirt and grime.

OVERALL SUMMARY COMMENT

Zinc is almost exclusively used for the protection of steel from corrosion. Like all processes, it is under constant scrutiny to ensure environmental sustainability. It does come at a cost, be it financial or in Environmental Terms. Financially the cost of providing corrosion protection may only represent a small proportion the total project cost; however, the level of environmental impact will be proportional to the method used.

The Corrosion Industry in Australia and Internationally are aware of the challenges and are actively developing new technologies and safety measures to eliminate or reduce the environmental impact of their particular materials or process to acceptable levels, where necessary.

The Paint Industry in particular has been subjected to very high scrutiny and has had the flexibility to introduce new technologies, for example, environmentally friendly non-polluting water based or solvent free paint materials. In contrast, the HDG industry due to its inherent nature, has limited flexibility within its process. It has not fundamentally changed since conception. It may be that the Galvanizing process  has reached a state of equilibrium. Therefore, as a process needs to be recognised for the protection it provides, along with the environmental consequences of its use.

Environmentally and from a OH&S prospective, there is a marked difference between the two established methods, IZS displays a much lower Environmental and OH&S Impact for the same level of service. Although the recorded performance is similar, IZS has Lead levels only as trace elements, consumes infinitely less energy, water, metallic zinc resources, produces no toxic waste, and has minor emission pollution in comparison to the HDG process.

AS/NZS4680 claims to be aligned with ISO1461 whilst the Standards are similar, there are differences regarding uncoated areas where Zinc is missing (bare areas) from the substrate when it leaves the kettle. ENISO1461 states 10cm² whereas AS/NZS4680 states 40cm².The difference for allowing missing zinc between the two standards is not minor, it could be argued that AS/NZS4680 is allowing Australian and New Zealand Galvanizers a rather larger advantage in not having to repair these areas. IZS does not have this luxury, under no circumstances are uncoated areas permitted as this would be cause for rejection.

ENVIRONMENTAL ZINC RESOURCES COMPARISON ESTIMATE

Table 4

 

METALLIC ZINC COMPARISION
HDG EXAMPLE	1.     10 tonne 200UB30 Steel beams
	2.     Surface area 30.9 m² /Ton = 309 M²
	3.     Minimum Standard Dry Film Thickness 85 Microns/m²
	4.     Actual Dry Film Thickness 140 Microns = 1000 grams Metallic Zinc/m² (Ref Table2)
FORMULA: 309m² x 1000grams/m² = 309000grams = 309 Kilos Metallic Zinc
IZS EXAMPLE	1.     10 tonne 200UB30 Steel beams
	2.     Surface area 30.9 m² / Ton = 309 m²
	3.     Recommended Dry Film Thickness 75 microns /m²
	4.     Actual Dry Film Thickness ( say)  125 microns /m² at 500grams Metallic Zinc/m²  (Ref Table3)
FORMULA: 309m² x 500grams/m² = 154500grams = 154.5 Kilos Metallic Zinc
SUMMARY ZINC ESTIMATE	HDG – 309 kilos  IZS       155 kilos                                           154 kilos (difference 50%)
IZS Coating on average require 50% less metallic zinc for equivalent performance

 

AS/NZS 4680 recommends the use of Organic Zinc Rich Primer (AS/NZS 3750.9) as the repair material for HDG stating these products should contain ‘Not less than 92% Zinc in the Dry Film’. This statement is ambiguous, confusing and is incomplete, there is no reference or qualification of Metallic Zinc, Zinc Oxide, Trace Elements, or whether it be weight, or volume etc. Organic Zinc Rich Primers at 100 microns range between 165-245 grams of Metallic Zinc/m² depending on the particular binder used. The reason,  manufacturers under the Standard, are free to formulate their own products using a variety of generic binders.  There needs  to be a balance between the zinc component  and organic binder ratio to ensure adequate application properties (Ref 11), hence the reason for the range.

Table 5

Cost Comparison Top Coating HDG and Inorganic Zinc Silicate
300/Tonne Structural Steel	HDG & Top coatings	Inorganic (solvent)	Inorganic (water)
Radius / chamfer edges 300 tonne @ $15 tonne	Not required	$4500	$4500
HDG – 300 tonne @ $1350 / tonne Zinc Coating  7500 m²	$405,000	$412,500	$427,500
Removal of imperfections for painting HDG 300 tonne @ $120  / tonne	$36,000	Not required	Not required
Degrease and water wash HDG. 300 tonne @ $125/ tonne	$37,500	Not required	Not required
Abrasive blast preparation for painting 7500m² @ $25.00  m²	$187,500	Not required	Not required
HDG Transportation, Fabricator to Galvanizer / Painter / Site/15 trips x 3	$48,600	–	–
Transportation Fabricator to Painter / to Site/ 12 trips x 2	_	$25,920	$25,920
Total	$714,600	$442,920	$457,920
Extra Zinc pick-up charge due to high Silicon / Phosphorous content if any.	Unknown until HDG Treatment takes place	N/A	N/A
Application paint system	Same	Same	Same
Site repairs Transportation and erection damage if any	Same	Same	Same
First maintenance costs	Same	Same	Same

 

Figures quoted are conservative estimates and typical only.

Inorganic Solvent $55/m²

Inorganic Water $57/m²

Transportation Cost $180/hr / 20 tonne load

Transportation HDG 15 loads x 3 x 6 hrs= 270/hrs.

IZS Zinc Coatings 12 loads x 2 x 6 hrs =144/ hrs.

Excludes Top Coating material costs and GST.

No allowance shown for  extra design detailing and fabrication requirements for HDG.

 

The service life of HDG is directly proportional to Zinc thickness, for example a 6mm plate section  will average 130 microns which equates to 928/grams/m² of metallic zinc (Table2). This raises two questions, why  recommend a repair material that only has a range between 165-245 grams? when there are alternatives that can provide 600 to 800 grams. The second, Organic Zinc Rich Primers are not recommended as single coat materials for any exposures outlined in AS/NZS 2312 (Protection of Steel),they are all part of Multi- Coat Systems.

Thickness, and metallic zinc content is the main feature of the Galvanizing Process. Promotional activity is largely pervaded that all Zinc Coating materials performance is entirely related to thickness. This premise  does not apply to IZS Coatings. The performance evidenced in AS/NZS 2312 clearly shows  IZS have equal performance ( Ref Table 1), in similar environmental conditions with the use of considerably less metallic zinc ( Ref Table 4). To re-emphasize this point, one area not covered by the Standard Marine Shipping and Offshore Oil Industry clearly demonstrates the performance differences between the two systems, HDG 3-5 years maximum compared to IZS 35 years plus.

Table 6A         Inorganic Zinc Silicate AS/NZS 3750.15

Listed Pigment Constituents AS/NZS 3750.15  Inorganic Zinc Silicate Paint Coating
Total Zinc (As Zn)	98% (min)	By Mass
Metallic Zinc (As Zn)	94% (min)
Total lead (Pb)	0.2% (max)        Trace element
Total Cadmium (Cd)	0.1% (max)
Total Iron (Fe)	0.05% (max)
Arsenic (As)	0.0005% (max)
Note the zinc oxide content may be cancelled as the difference between the total zinc content and the metallic zinc content multiplied by 1.25

 

Table 6B         Hot Dip Galvanizing AS/NZS 4680 / 2006

Listed Pigment Constituents Prime Western Zinc (PWG Z5 Grade)
Zinc (Zn)                                    98.00%
Lead (Pb)	1.4% (max)
Copper (Cu)	0.020% (max)	These constituents not shown in the Standard or other documentation
Cadmium (Co)	0.0050% (max)
Iron (Fe)	0.020% (max)
Chemical composition (BSEN 1179.2003(E) Z5 grade) and IS13229 – 2011 (Prime Western Grade Z5 )

 

Technically the inclusion and further absorption of lead by the Galvanizing process, means the material is a Zinc / Lead Alloy. The question here is, why weren’t all the constituents listed in the same manner as with IZS Coatings and what was the reasoning behind this decision? Whist the  IZS standard nominates the maximum Lead level at 0.2%, independent testing of five commercial materials using an Induction Couple Plasma Assay test followed by Spectrum Trace showed level well below the allowable limit, ranging from < 0.01 to 0.04%.

Where Architectural appearance is required the Steel Fabricator together with the Galvanizer need to take extra precautions to achieve uniform appearance which comes at a cost. Specialized dressing for architectural  purposes only occurs as required. AS/NZS 4680 standard is only aimed toward corrosion protection. The Galvanizer needs to ensure adequate quenching treatment  immediately after Galvanizing, otherwise if left to air or slow cool, results in the formation of Kirkendall Voids (ref 7).

The Kirkendall Effect is a metallurgical reaction between the two top Galvanized layers  that continues below the melting point of zinc temperature. This creates voids between the ETA and Zeta Layers which are exposed by abrasive blasting,  adversely effecting  bonding and adhesion of topcoat materials.(Ref 7). The result is peeling (Ref 8A,B &9) which adds to the Environmental Impact . Paint applications over voids further compound the problem with the formation of blisters and adhesion failures (Ref 10 ).If the voids are left undisturbed it in no way detracts from the effectiveness of the HDG in preventing corrosion.

The most effective way of keeping potential problems at bay is through consultation between the Designer, Steel Fabricator, and Galvanizer prior to and after treatment, there is no mention of the Kirkendall effect in any printed literature related to the HDG Industry (9).

HDG Standard only addresses Product Description and a Process Method it does not cover an increasingly significant element of Aesthetic Appearance. If a project requires a multi topcoat system,  it becomes a matter of choice as over coating any Zinc System coverts it to a non-reactive primer. Apart from Environmental and Safety issues, a measure of caution needs to be exercised when considering top coating Zinc materials, under these circumstances particularly where performance is equal. It may be prudent to err on the side of caution and consider the use of IZS coatings for the following reasons:

• Where the use of IZS has an overall lower Environmental impact
• Economic benefits (Table 5) favour IZS in the order of 35%.
• Inter-coat Adhesion is more reliable and permanent.
• Where Degreasing and Abrasive Blasting for topcoat application is not necessary
• Where Silicone/Phosphorous has no effect
• Transportation costs are lower in the order of 45% (Ref 5) resulting in lower Carbon Emissions.
• Where Architectural appearance is better
• Where Performance and First to Maintenance is equal
• Where OH&S exposure is considerably less.
• Finally, where the client can get a Warranty.

The Corrosion, OH&S, and Environmental debate is a long standing one, invariably it highlights the competitive nature of the competing Industries, unfortunately perceptions are created or omitted to gain market acceptance or advantage. Corrosion science and its consequences should be based on evidence, from my perspective having been on both sides of the Industry and now an independent Consultant for many years. It is a question of balance, understanding the facts and knowing when to recommend the appropriate product and procedures for their use.

I may conclude with two Quotes from Standards Australia Web Site:

‘Standards complement the Natural Construction Code which ensures the Public’s Health and Safety, and Amenity.( Clayton James Votand. Building Surveyor)’

‘Standards can ensure quality of a production process that an ordinary consumer can trust.(Sam Hossain. Lawyer)’

References:

1. EN ISO1461 / AS/NZS 4680 After fabrication Hot Dip Galvanizing
2. OCCA S.A. – Q3. 2008
3. General Galvanizing Practice – Galvanizers, U.K.
4. Hot Dip Galvanizing After Fabrication – Galvanizers Association Australia.
5. AS/NZS3750.15 – Inorganic Zinc Silicate Paint .AS/NZS 3750.9 Organic Zinc Rich Primers.
6. AS/NZS2312-2017.The Protection of Steel.AS 1627.4.2005. Surface Preparation.
7. World Health Authority (Research on cancer)
8. Safety Measures in Hot Dip Galvanizing.(T.A Cook, J.W Haffner, MS Williams Sep !997)
9. Painting Galvanized Structures. N Karakasch.
10. Architectural Galvanizing Revisited 2011 Nick Karakasch.
11. Zinc Coating Review 2022/2023 N Karakasch.
12. Galvanizing & Zinc Based Paints. N Karakasch
13. A Szokolik Evaluation Inorganic Zinc Silicate for Oil/Gas Production/Marine Environment
14. ENISO 1461 Hot Dip Galvanizing Standard
15. Basil Convention Dross Waste Part 11, AA010-261900.

 

About the Author

Nick Karakasch is the retired Principal Consultant to Total Corrosion Consultants, Melbourne Australia. Nick’s experience spans 55 years specializing in services to the Protective Coating, Galvanizing, and Fire Protection Industries. He spent many years in a management and technical capacity with the Dimet Coating organisation, the company which invented Inorganic Zinc Silicate Coatings by their founding Director, Victor Nightingall. He has also been the Executive Marketing Manager to the Galvanizers Association of Australia. Whilst living in South Africa in 1970’s he was employed as a Site Contracts Manager by R.J. Southey Pty Ltd. Africa’s largest Corrosion Prevention Contractors.