Steel Protection By Hot Dip Galvanizing & Duplex Systems

Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA

Corrosion Resistance of Hot Dip Galvanized Coatings

The life of a hot dip galvanized coating is more or less proportional to its thickness in a given environment. (Table 2).

Hot dip galvanized coatings on steel protect against corrosion in two Ways:

1. Barrier protection is provided by a virtually non-porous film which isolates the steel substrate from corrosion inducing substances in the surrounding environment.

2. Cathodic or sacrificial protection is provided at small uncoated surfaces while corrosion creep under the surrounding coating cannot occur.

The corrosion rate of zinc is low in most environments. This is due to the natural formation of a stable protective film of zinc conversion products which develops on the surface of the coating.

12.1 CORROSION RESISTANCE IN THE ATMOSPHERE

When a hot dip galvanized article is withdrawn from the molten zinc, the coating surface immediately reacts with oxygen and moisture to form combinations of both zinc oxide and zinc hydroxide. Carbon dioxide in the atmosphere rapidly converts these surface conversion products into a stable, tightly adhering, basic zinc carbonate film with very low solubility. This ensures that farther attack of the underlying zinc is prevented. The initial shiny surface with a metallic lustre disappears to be replaced by a matt, light grey appearance (figure 74).

The atmosphere contains greater or lesser corrosive substances such as chlorides. In marine environments and sulphur dioxide associated with industrial pollution. Humidity levels, rain patterns and condensation all influence the degree of corrosion. The different factors can occur in favourable or unfavourable sequences, one after another, alternately. or in combination with each other.

It is normal to differentiate between corrosion rates in:

1. rural environments

2. marine (coastal) environments

3. urban environments

4. industrial environments

(See figure 75 for atmospheric corrosion of zinc).

The atmosphere in cities and industrial areas contains various pollutants. These are able to attack the stable zinc carbonate film producing more soluble products which can be washed away. Consequently the corrosion rate of galvanized steel will accelerate. Modern environmental controls are resulting in lower pollution levels and hot dip galvanizing offers good protection in locations where previously limited coating life was experienced.

Figure 76. Discoloured surface on lighting column. Coating consists mainly of an iron/zinc alloy that grows out to the surface. Iron is freed during corrosion, which leads to rust formation. It is only surface rust and is of aesthetic significance only. The bracket for the traffic sign has a coating of pure zinc as an outer layer.

In marine environments the corrosion of zinc is influenced by the salt content of the air. However, marine air contains small quantities of magnesium salts, with good passivating influences. Corrosion is therefore not as great as may be expected. The salt content of the air usually reduces quickly away from the coast i.e. by 80% over the first 800m from the high water mark.

The colour of corrosion products varies according to the environment in which they are formed. Marine environments give somewhat whiter corrosion products compared with rural and urban environments. Corrosion products are usually darkest in urban environments.

The corrosion of zinc is influenced by many factors. This means that a generally applicable formula for corrosion rates can not be given.

The ubiquitous nature of hot dip galvanizing means that there is always a product such as a lamp post or fence near a proposed future site that can be used to predict future performance.

The Hot Dip Galvanizers Association have frequently been involved in the assessment of the corrosive conditions prevailing at a particular site, prior to the selection of the final coating specifications. Knowledge about the corrosion of zinc, and corrosion rates in different environments, is therefore extensive.

Reddish-Brown Discolouration

Some hot dip galvanized steel can adopt a reddish-brown colour after a period of exposure. After prolonged exposure, particularly in sulphur-rich atmospheres, this discolouration can gradually turn black. The discolouration occurs mainly on coatings of iron/zinc alloy on silicon-killed steels.

The source of discolouration is the corrosion of Fe/Zn alloy to form rust together with humid air or rain water. Rust has a great ability to colour, and even small amounts can cause considerable discolouration.

Sometimes when discolouration is severe it is easy to conclude that rust protection has been greatly reduced, or completely destroyed. However, this is seldom the case. The iron/zinc alloys give better protection (in some environments up to 30-40%) to the underlying steel than pure zinc.

If appearance is important, discoloured surfaces can be painted (figures 30, 31 and 76).

12.2 WET STORAGE STAIN

Sometimes a white, floury and voluminous coating called wet storage stain, or white rust, appears on galvanized surfaces (figure 77).

The coating forms on materials with newly galvanized, shiny surfaces and especially in crevices between closely packed sheets, angles and similar products. A pre condition is that the material is exposed to condensation or rain water in conditions where the moisture cannot evaporate quickly. Zinc surfaces that have already received a normal protective layer of corrosion products are seldomly attacked.

When zinc coatings are exposed to air, zinc oxide and zinc hydroxide are formed. Under the influence of carbon dioxide in the air these are converted to basic zinc carbonates. If air access to the zinc surface is restricted, as in narrow crevices, then the area receives insufficient carbon dioxide to enable the normal layer of carbonates to form.

The wet storage stain layer is voluminous and porous, and attached only loosely to the zinc surface. As a result, protection against continued attack does not exist, Corrosion can therefore continue as long as moisture remains on the surfaces. When wet storage stain has occurred the object should be stacked to enable the surfaces to dry quickly. This will stop the attack and, with free access to air, the normal protective layer will be formed. The wet storage stain is gradually washed away and the coating acquires an appearance that is normal for exposed, hot dip galvanized steel.

Since the product of wet storage stain is very bulky (about 500 times that of the zinc from which it was formed), an attack can appear to be serious. However, an attack of wet storage stain often has little or no significance on the service life of the corrosion protection. In the case of very thin coatings however, e.g. on electroplated objects, a severe attack of wet storage stain can be of significance.

Wet storage stain is best avoided by preventing newly galvanized surfaces from coming into contact with rain or condensate during transportation. Materials stored outdoors should be stacked so that water can run off easily, so that all surfaces are well ventilated (figure 78). Temporary protection against wet-storage stain is obtained through chromating or phosphating. Painting after galvanizing gives very good protection.

Wet storage stain which has already formed can be removed completely or partially by moderate mechanical or chemical treatment. See "Removal of Wet Storage Stain" page 17.

12.3 GALVANIC CORROSION

If two different metals or alloys, completely or partially surrounded by an electrolyte, are connected, a galvanic cell is created, Which metal becomes the anode or cathode is determined by their electrode potentials in the electrolyte in question.

In sea water, which corresponds to the majority of practical conditions, some metals and alloys take up different positions on the electrochemical scale, shown in table 24.

If steel is connected to copper or brass the steel becomes the anode in the cell and corrodes. However, if steel is connected to cadmium, aluminium, zinc or magnesium, it becomes the cathode and is protected against corrosion, while the anode metal is consumed (figure 79).

Galvanic corrosion is also called bimetallic corrosion and is used to protect underwater structures from corrosion, where it is termed cathodic protection.

Table 24. Electrochemical potential scale in sea water at +25C.

For maximum corrosion resistance under conditions of extreme humidity, overlapping galvanized surfaces should be insulated from each other by the application of an inhibitive jointing compound in accordance with SABS 305. Alternatively a suitable paint may be used. Galvanized surfaces in contact with other materials may also require insulation.

Galvanized members in contact with aluminium conductors may require the use of an electrical conducting compound at joint faces, to repel moisture and inhibit corrosion.

Cathodic Protection Afforded by Zinc Coatings

In hot dip galvanized steel, zinc and steel are in good electrical contact with each other. If the zinc coating is damaged in the presence of an electrolyte a galvanic cell is created, The electrolyte could be condensate or rain water. Sometimes the entire structure can be submerged in liquid. In this cell the zinc becomes the anode i.e. corrodes, the exposed steel becomes the cathode and is therefore protected from corrosion (figure 80).

In the initial phase it is possible to see a weak rust formation on the exposed part of the steel surface where the coating has been damaged, but after a while whitish-grey areas form which gradually spread over the entire damaged area (figure 81). The zinc coating corrodes and sparingly soluble zinc alloys descend to the cathode surface where they protect the steel from continued rust attack. This is often called self-healing, which is something of a misnomer since the zinc layer is, of course, not restored.

In case of exposure in water the zinc salts do not always precipitate at the point of damage since they can be flushed away by movement in the water. The protective action remains, however, provided that the steel surface is not too large. The steel is protected by the electrical current generated in the galvanic cell when the zinc corrodes.

Figure 83. Stainless steel fasteners attached to hat dip galvanized plate in immersed conditions, note the sacrificial attack of the zinc coating surrounding uninsulated fasteners compared with the insulated fastener where no attack of the surrounding zinc has taken place.

Owing to the cathodic protection generated by the zinc, rust cannot creep in under the coating at the point of damage in the way that it can creep under films of paint or coatings of metals more noble than steel (figure 82).

Zinc coatings on steel are unusual, since a fairly large area of damage to the coating does not cause catastrophic corrosion. The range of cathodic protection is dependent on the nature of the electrolyte that creates the cell. For structures in normal atmospheres it is usual to expect protective action over several millimeters. However, in sea water significantly greater distances can be expected.

Zinc Coatings in Contact with Non-Ferrous Metals

Aluminium and stainless steel can often be connected directly to galvanized material in air or fairly dry environments without noticeable corrosion taking place. However, in water an insulator should always be used to prevent accelerated corrosion of the zinc (figure 83).

Figure 84. Brass bolt in hot dip galvanized steel on a parking deck.

Copper and copper alloys are more electrically active, and there is often a release of copper ions which spread over large surfaces and cause noticeable attack. For this reason, these metals should never be allowed to come into contact with galvanized steel, and an insulator should always be used (figure 84).

12.4 CORROSION RESISTANCE OF HOT DIP GALVANIZED COATINGS IN AQUEOUS CONDITIONS

General

Zinc carbonate, the protective film formed over a hot dip galvanized coating, is relatively insoluble in water. However, this stability is restricted to an acid/alkali pH range of 6 to 12.5. Zinc is amphoteric in nature; that is, it forms soluble salts at low and high pH values. This is clearly shown in figure 85.

Notwithstanding the above, water contains numerous dissolved salts as well as carbon dioxide and oxygen in solution. Organic matter can be picked up by water as it passes over vegetation. This can also be a major contributor to corrosion in some instances. The effects of water quality on the corrosion rate are summarized in figure 86.

In soft waters, zinc corrosion is accelerated. Also, the tolerance for chloride salts is reduced. A reserve alkalinity level is required to stabilize the zinc carbonate film, This is generally assumed to be of the order of 50 - 75mg/l (as CaCO₃). In hard waters, high chloride levels (> 2000mg/l) can be tolerated. Sulphates, nitrates and phosphates are generally considered to be protective towards hot dip galvanizing. However, when combined with ammonia compounds (such as with fertilizers) soluble zinc compounds may be formed and acid conditions can arise causing attack of hot dip galvanized steel. Organic compounds such as tannins will arrest the corrosion of hot dip galvanized steel but the settling of solids can create conditions for crevice corrosion. Similarly, slime build-up should be avoided as microbially induced corrosion (MIC) can occur, leading to rapid attack.

Flow rates should be maintained at sufficiently high levels to ensure that all debris is held in suspension rather than allowed to settle. It should be considered "good practice" to flush systems on a regular basis. This should be carried out on all fire protection systems although, as the water entering these systems is generally of good quality, corrosion rates tend to be low provided that MIC does not occur in all instances, the corrosion performance of galvanized piping in fire protection systems is far superior to that of bare steel. Crevice corrosion is likely to occur where sediment becomes dense and compacted. This may result in the provision of anaerobic sites suitable for the start of MIC.

Under normal circumstances the amount of dissolved oxygen in a water would be sufficient to ensure that no deleterious effects occur, However, anaerobic or septic conditions can affect hot dip galvanized piping adversely as is the case with other metals. For drinking water purposes some form of chlorination is generally applied. Therefore, in normal distribution systems anaerobic conditions giving rise to MIC, should not occur. It is important when testing water lines that clean water be used and the system drained if it is to be left unused for some time. Chlorination has no effect upon the protection properties of galvanizing. High oxygen levels accelerate the corrosion rate of zinc. Similarly, high carbon dioxide levels tend to produce add conditions, which can accelerate corrosion in flowing systems.

Effect of water temperature

Hot dip galvanized piping has been used for hot water supplies with no deleterious effects in many applications. However, when used above 65C the zinc is no longer protective to exposed steel. It is therefore recommended that hot dip galvanized systems not be used above 65C.

The electricity supply commission (Eskom), advise that with proper pipe insulation, the maximum temperature for hot water cylinders be 60C. For practical purposes therefore, hot dip galvanized piping is acceptable for use in both hot and cold water systems.

In domestic systems copper should only be used downstream of hot dip galvanized piping. This will avoid the possibility of pitting corrosion.

Table 25. Sail aggressiveness at different resistivity levels with hat dip galvanized coatings.

Effect of sea water

Hot dip galvanized coatings perform relatively well in submerged seawater conditions which are severely corrosive to most protective systems. Dissolved salts present in seawater react with zinc to form a protective layer minimizing corrosive action. The pH of seawater tends to be constant worldwide as a result of the buffering action of the hydrogen-carbonate salts present. The presence of pollutants is equally not detrimental provided that levels are within internationally acceptable norms.

A simple nomogram (table 27) has been produced to allow the specifier to determine the suitability of hot dip galvanizing for the protection of steel piping in water. This provides guidance based upon the water quality and general operating conditions likely to be encountered. More detailed information is contained in SABS 0374-1: The suitability of hot dip galvanized steel piping for the transportation of potable water.

12.5 CORROSION RESISTANCE OF HOT DIP GALVANIZED COATINGS IN SOIL CONDITIONS

Soil can contain weathered products, free or bound salts, acids and alkalis, mixtures of organic substances, oxidizing or reducing fungi, micro-organisms, etc. Depending on its structure, soil has different degrees of permeability to air and moisture. Normally, the oxygen content is less than in the air, while the carbon dioxide content is higher. The corrosion conditions in soil are therefore very complicated and variations can be great between different locations, even those in close proximity to each other.

Southern African soils vary from highly corrosive in some regions to moderately corrosive in others.

One method of determining the corrosivity of a soil is to measure its resistivity. Recommendations are given in table 25.

If the resistivity of the soil cannot be determined, the rule-of-thumb method listed in table 26 can give a measure of guidance. Where the exposure of metals to soil is concerned, it is advisable to seek expert advice from suitably qualified sources.

See also "Guidelines for Burled Hot Dip Galvanized Conveyance Piping" - available from the Association.

12.6 HOT DIP GALVANIZED STEEL IN CONTACT WITH BUILDING MATERIALS

Mortar, Plaster and Wood

Damp mortar and plaster attack zinc. The attack ceases when the material dries out. Dry or moderately damp wood, both impregnated and unimpregnated, can be nailed with hot dip galvanized nails to good effect, However, in the case of nails or threaded unions that are constantly exposed to water an acid-resistant material is preferred. Other dry building materials, such as mineral wool, do not attack zinc.

Wood with acidic properties should not come into contact with galvanized steel.

Concrete

Unprotected reinforcement can corrode in certain environments when moisture penetrates the concrete through cracks and pores. Since rust has a greater volume than the steel from which it was formed, the covering layer over the reinforcement can crack and spall (figure 88).

Steel components such as bolts and edge guards that have been partly grouted in are often poorly protected against rust. Apart from crack formation and scaling, a problem occurs with unsightly rust staining on the concrete surfaces below.

Table 26. Corrosivity of different soil types.

Table 27. Probability of performance.

This kind of damage can be avoided if the reinforcing steel is hot dip galvanized (figure 87). Hot dip galvanized reinforcing steel or mesh can therefore be used in grouted facade sections. One of the advantages of this is that there is no risk of rust runs discolouring the facade.

Figure 87. Flat dip galvanized reinforcing bars prior to casting concrete, marine conditions.

According to the Building Research Establishment in the UK, the average adhesion for smooth reinforcement steel in concrete is as follows:

hot dip galvanized steel	3.3-3.6 MPa
black steel	1.3-4.8 MPa

The large range for black steel stems from different degrees of rust and compositions of oxide scale.

According to work done in Finland, the stress for 0.1 mm of slip in reinforcement bars in concrete is approximately as follows:

black steel	150 MPa
hot dip galvanized steel	160 MPa
hot dip galvanized and chromated steel	190 MPa

When concrete is cast its pH value is around 13. At this high pH, fresh zinc is attacked and hydrogen is produced, which could give rise to poor adhesion. However, the attack ceases as soon as the concrete has hardened and any residual pores are not harmful.

In order to avoid fresh zinc surfaces coming into direct contact with wet concrete it is advisable to allow the galvanized steel to age for several weeks. The cover layer of basic carbonates which then appears will minimize both attack and the production of gas, and will also promote adhesion. Another common way of preventing attack from fresh concrete is to chromate the galvanized steel. A further alternative is to add about 40 ppm (by mass) of chromates, to the water when concrete is mixed.

12.7 ABRASION RESISTANCE OF HOT DIP GALVANIZED COATINGS

Pure zinc is a soft metal, even though it is harder than most organic coating materials. The iron/zinc alloys produced in hot dip galvanized coatings are, however, very hard. In fact, they are harder than ordinary structural steel (figure 89).

The alloys are therefore more resistant to abrasion than pure zinc and experiments have shown that the alloy layer has a resistance to abrasion 4-5 times that of pure zinc.

Hot dip galvanized articles are often used when the surface is to be subjected to abrasion. Examples of this include stairs, floor hatches, hand railings, grid flooring and walkways (figure 5).

12.8 HOT DIP GALVANIZED COATINGS EXPOSED TO ELEVATED TEMPERATURES

Conventional zinc coatings can be exposed continuously to temperatures up to about 200C and non-continuously to temperatures of up to 350C.

At sustained temperatures in excess of 200C a diffusion reaction begins inside the coating and causes the outer layer to split-off from the underlying iron/zinc layer. However, the iron/zinc layer has a very good resistance to corrosion and can, depending on its thickness, protect the steel from rust for a very long time.

Aluminium-alloyed zinc layers on thin sheet can resist even higher temperatures. Aluzinc and galvalume for instance, can withstand sustained temperatures up to 315C.