Galvanized Steel

The galvanised steel wires are applied next and then a final outer sheath of a material such as PVC or polyethylene.

Processes

John L. Clarke, in Advanced Concrete Technology, 2003

26.2.1Galvanized steel reinforcement

Galvanized steel reinforcing bars have been successfully used in several countries over the past 50 years (Australia, Bermuda, Netherlands, Italy, the UK, and the USA) and consumption is increasing. The main advantages of galvanized steel are:

it delays the initiation of corrosion and cracking

it has very good performance in carbonated concrete

it tolerates higher chloride migration levels than uncoated steel

it provides protection to the steel during storage

it has longer life in cracked carbonated concrete than uncoated bar.

Hot-dipped galvanized steel is produced by dipping clean and fluxed steel into a bath of molten zinc. The layer formed on the surface of the steel usually consists of a thin outer coating of pure zinc on a series of layers of zinc/iron alloys with increasing iron content.

The performance of galvanized steel in concrete as reported in the literature (Andrade et al., 1995) is contradictory. Although it has been used successfully in practice, laboratory studies suggest that its performance would not be cost-effective. The factors behind this divergence of views, currently the object of discussion, are:

the pH of the cement paste

the bond between the reinforcing bars and the concrete

chromate passivation of the galvanized steel

the structure and thickness of the zinc coating

the resistance of the zinc coating to corrosion induced by chloride ions

Zinc is passive in most cement pastes as the pH of uncarbonated cement pastes is 12–13.5. A passive layer would be formed when pH < 13.3, the upper limit for passivation, due to the formation of a layer of calcium hydroxyzincate, inhibiting further corrosion. The passivating process results in an homogenous zinc depletion of about 10 μm. A more protective film is produced from pure zinc than from an iron–zinc alloy. It is recommended that an external pure zinc layer of at least 10 μm and a total galvanized layer of at least 80–85 μm are needed to provide suitable protection when embedded in concrete.

In concrete made from a cement with exceptionally high soluble alkali, film formation could be inhibited during the setting period and corrosion of the zinc in the hardened concrete will depend on the environment (humidity, chloride penetration).

During the formation of the passive layer, hydrogen is evolved. Although the evolution of hydrogen raises the spectre of embrittlement, the reinforcing bars normally used in construction are not susceptible to hydrogen embrittlement. Similarly, the hydrogen evolved during the pickling process (pickling is part of the preparation of the surface prior to the application of the zinc, using a weak acid) before galvanizing does not cause a problem. Galvanizing is not generally recommended for steels with a tensile strength above 700–800 N/mm2, i.e. not for prestressing steels here the risk of hydrogen embrittlement is more severe than for unstressed reinforcement.

Several reports (Andrade et al., 1995) compare the reduction in bond strength of galvanized and uncoated steels, both plain and deformed. Reduction in bond is attributed to the formation of hydrogen bubbles at the interface between the bar and the concrete. It has been suggested (Andrade et al., 1995) that this can be overcome by adding chromate to the concrete mix or giving the bars a chromate passivation treatment. On the other hand, the zincates produced – which are less expansive and more soluble than iron corrosion products in the cement environment – could diffuse into the pores of the concrete and make the concrete more dense locally, increasing the bond strength above what would be expected for uncoated bar.

In practical terms, most construction is carried out with deformed bar and it is probable that the evolution of hydrogen will not affect the bond strength of galvanized deformed steel reinforcement. However, the use of a passivation agent is still debated. The most effective is a chromate but its use as a concrete admixture raises a number of serious environmental and health questions on-site and would certainly be rejected by cement manufacturers and contractors. It would be more appropriate to use chromated bars as, in the first instance, it would restrict the amount of chromate used and ensure it was where it was needed. It would furthermore provide additional corrosion protection before use and ensure that poor storage would not lead to white rust on the reinforcing bar.

Zinc coatings remain passive in carbonated concrete and the rate of corrosion is much lower than for uncoated steel. This makes galvanized steel reinforcement ideal for use in concrete which is at risk from carbonation.

As regards corrosion resistance in chloride-contaminated concrete, the distinction has to be made between cast-in chloride and that which penetrates from the outside. Cast-in chloride may attack the zinc coating before and during the formation of the passive calcium hydroxyzincate whereas chlorides penetrating from the outside will find the passive layer already formed and so may be less dangerous.

Though zinc can be depassivated and attacked in the presence of chloride ions, the tolerance of galvanized steel to chloride is higher than that of uncoated steel. Galvanizing protects the steel against chloride ingress because it is more tolerant to chloride, requiring a higher concentration for depassivation and it corrodes more slowly in chloride-contaminated conditions.

URL: https://www.sciencedirect.com/science/article/pii/B9780750656863503128

Corrosion Atlas

In Corrosion Atlas Case Studies, 2020

Contributed By: Mascha van Hofweegen and Frank de Vos

Case History 02.09.25.001

 Material Galvanized steel System Fan coil units in the ceiling Part Pipes to fan coil units in the ceiling (cooling water) Phenomenon Corrosion under the insulation (CUI) Appearance Water leakage from the insulation. Outer surface of the pipes affected Time in Service 3 years Environment Office space Cause Wrong material choice. Galvanized steel cannot withstand condensation. Poorly applied insulation and many cold bridges. The cooling water temperature was lowered. Due to the construction of the cooling water pipes, the condensate formed was retained in the insulation Remedy When replacing the pipes, replace the galvanized steel with, for example, copper, plastic, or coated steel. Furthermore, the insulation must be fitted better, and the pipes must be designed in such a way that no condensation can enter the insulation but can be removed adequately Additional References
URL: https://www.sciencedirect.com/science/article/pii/B9780128187609000027

Environmental and Solubility Issues Related to Novel Corrosion Control

W.J. van Ooij, P. Puomi, in Thermodynamics, Solubility and Environmental Issues, 2007

2CORROSION OF INDUSTRIALLY IMPORTANT METALS

Steels, galvanized steels and aluminum alloys are industrially important metals that are produced in large quantities. Of these materials the corrosion protection of steel is most challenging, even if iron is more noble than zinc or aluminum [2]. Fig. 1 shows a schematic of the corrosion process on iron along with the possible half-cell reactions depending on the type of environment [12].

The hydrated ferric oxide (Fe2O3 · 3H2O) that is formed in the reactions between iron, water and oxygen is the red rust that is usually referred with the generic term, rust. Pretreated and painted cold-rolled steel (CRS) when scratched is particularly susceptible for red rust bleeding especially in acidic corrosive conditions [12]. Therefore, steel is often galvanized to protect the steel with a sacrificial zinc alloy coating, which will corrode instead of steel, if the galvanized steel is cut or scratched [13].

The reactivity of metals in different conditions, can be anticipated to some degree by studying Pourbaix diagrams, which provide information on the stability of the oxides/hydroxides on metals. These diagrams illustrate areas of conditions where the metal is passive, corrosive or immune [14]. These regions are, however, only indications, actual corrosion rates cannot be derived from the diagrams. From the Pourbaix diagram of aluminum, shown in Fig. 2a, it can be concluded that aluminum is an amphoteric metal for which the protective oxide film dissolves at low (below pH 4) and high pH (above pH 8–9). Also for aluminum there is a large potential difference (driving force) between the lines representing the cathodic and anodic half-cell reactions. However, aluminum is an excellent example of the fact that the corrosion rate is relatively low due to kinetic limitations, despite the large driving force for the corrosion reactions [2].

Iron is similar to aluminum in that a protective oxide forms in nearly neutral solutions. However, for iron the field of oxide stability is substantially greater at elevated pH, and iron is far more resistant to alkaline solutions compared with aluminum. Contributing to the overall resistance of iron, is the generally more noble half-cell electrode potential for the anodic dissolution reactions which lower the driving force for corrosion reactions. It is, however, apparent from Fig. 2b that this resistance disappears in more acidic solutions [2].

For zinc the passive region is even narrower than for aluminum. Zinc has soluble forms of compounds under pH 9 and above pH 11 (Fig. 2c). The figure also shows that in moderately alkaline solutions (pH 9–11) Zn(OH)2 will precipitate, but in a strongly alkaline solution, the solid hydroxide will dissolve as zincate ions, $\text{Zn}{\left(\text{OH}\right)}_{4}^{2-}$.

It is well known that aluminum as such is fairly passive, because a very dense and uniform aluminum oxide Al2O3 layer is formed onto the metal to protect the metal from corrosion. Highly ductile light weight aluminum alloys that are passed through specific heat treatments can, however, make aluminum susceptible to corrosion. These materials may contain alloying elements such as magnesium and/or copper, which alter and complicate the corrosion behavior of aluminum. Typical forms of corrosion for the alloys are localized and pit corrosion. Due to the dense structure of the aluminum oxide layer, the corrosion rate of aluminum alloys is, however, substantially slower compared with corrosion/dissolution of CRS or HDG steel [15].

Regular HDG steel coatings corrode by dissolving and re-precipitating as zinc-oxide crystals over the surface. Thus, the surface attains an irregular appearance. The corrosion products of zinc are Zn(OH)2, ZnO (dehydration of Zn(OH)2) and basic salts formed as a result of compounding of Zn(OH)2 with ZnCl2 or ZnCO3. The corrosion products are porous and allow free access of corrosive electrolytes to the zinc coating [18]. This mechanism usually creates preferential pathways through areas of higher porosity and causes locally accelerated corrosion. Therefore, the rate of corrosion of HDG steel has been established to be essentially linear [16].

URL: https://www.sciencedirect.com/science/article/pii/B9780444527073500082

Metal Decorative Materials

In Building Decorative Materials, 2011

4Galvanized Steel Stud

It is a framework material made by rolling and stamping galvanized steel sheet and thin cold-rolled annealed steel coiled strip with a cold-bending machine. It has features such as light self-weight, high stiffness, high fire-resistance and impact-resistance, shock-resistance, easy and convenient processing and installation etc. Finishing material made up of galvanized steel stud and thistle board not only meets the requirement of fire resistance, but also is convenient for construction and suitable for massive assembling and construction. Moreover, it allows other facing decorations on its surface layer, such as coating or papering etc. Metal frame has gradually replaced traditional wooden framework material in interior suspended ceilings and partitions, and is widely used in decoration projects.

According to material, metal framework is classified to galvanized steel stud and aluminum alloy stud; according to different application areas, galvanized steel stud is classified to partition wall used and suspended ceiling used etc.

(1) Partition Galvanized Steel Stud (See Figure 9.3)

According to application, partition galvanized steel stud is classified to: along-top stud, along-floor stud, vertical stud, reinforced stud, thorough cross-stay stud and accessories etc; according to shape, classified to U- stud and C- stud etc.

According to national standard “Building Galvanized Steel Stud” (GB/T 11981–2001), partition galvanized steel stud mainly has these series: Q50, Q75, Q100 and Q150. Q50 series is applied to partitions with storey height less than 3.5 m; Q75 series is for partitions with storey height ranging 3.5-6.0 m; Q100 and above series is for partitions with storey height more than 6.0 m. The names, product codes, specifications, applications and producers of galvanized steel studs are given in Table 9.2.

Table 9.2. Names, Product Codes, Specifications, Application Scopes and Producers of Partition Studs

Name Product code Mark Specification (mm) Steel consumption (kg/m) Application scope Producer
Width Height Thickness
Along-top and along-floor stud Q50 QU50×40×0.8 50 40 0.8 0.82 Storey height less than 3.5 m Beijing Building Light Steel Structure Factory
Vertical keel QC50×45×0.8 50 45 0.8 1.12
Thorough cross-stay stud QU50×12×1.2 50 12 1.2 0.41
Reinforced stud QU50×40×l.5 50 40 1.5 1.5
Along-top and along-floor stud 075 QU77×40×0.8 77 40 0.8 1.0 Storey height 3.5-6.0 m
Vertical stud QC75×45×0.8 75 45 0.8 1.26 Storey height 3.5-6.0 m
QC75×50×0.5 75 50 0.5 0.79 Storey height less than 3.5 m
Thorough cross-stay stud QU38×l2×l.2 38 12 1.2 0.58 Storey height 3.5-6.0111
Reinforced stud QU75x40xl.5 75 40 1.5 1.77
Along-top and along-floor stud Q100 QU102×40×0.5 102 40 0.5 1.13 Storey height less than 6.0 m
Vertical keel QC100×45×0.8 100 45 0.8 1.43
Thorough cross-stay stud QU38×l2×l.2 38 12 1.2 0.58
Reinforced stud QU100×40×1.5 100 40 1.5 2.06

Partition galvanized steel stud is mainly applicable for the partition walls and corridor walls in office buildings, restaurants, hospitals, entertainment places and theaters, especially is suitable for the partitions of multi-storey buildings and additional stories as well as for the light-weight partitions of multi-storey factory buildings and clean workshops etc. After combined to each other with relevant joint pieces, partition galvanized steel studs form a wall framework. With both sides covered with different faceplates (such as plaster board, asbestos cement panel or colored profile steel sheet etc.) and facing layers (such as wallpaper, wood faceplate or paint coating etc.), partitions with different properties are created.

The joining forms of galvanized steel stud and faceplate are shown in Figure 9.4.

(2) Suspended Ceiling Galvanized Steel Stud

According to load supporting capacity, suspended ceiling galvanized steel stud is classified to accessible and inaccessible studs; according to the sectional shape of section material, classified to U-stud, C- stud and L- stud; according to applications, to main stud (also named bearing stud), sub- stud (medium, small stud also called cladding stud) and joint accessories, refer to Figure 9.5.

Suspended ceiling galvanized steel stud mainly includes four series, D38, D45, D50 and D60; as to the product codes, specifications and producers, refer to Table 9.3.

Table 9.3. Product Codes, Specifications and Producers of Suspended Ceiling Galvanized Steel Studs

Name Product code Specification (mm) Steel consumption (kg/m) Spacing of suspension centers (mm) Type of suspension ceiling Producer
Width Height Thickness
Main stud D38 38 12 1.2 0.56 900-1200 Inaccessible Beijing Building Light Steel Structure Fatory
D50 50 15 1.2 0.92 1200 Accessible
D60 60 30 1.5 1.53 1500 Accessible
Sub- stud D25 25 19 0.5 0.13
D50 50 19 0.5 0.41
L- stud D35 35 35 1.2 0.46
T16-40 unexposed suspended galvanized steel stud D-l suspended ceiling 16 40 0.9 kg/m2 1250 Inaccessible
D-2 suspended ceiling 16 40 1.5 kg/m2 750 Inaccessible, fireproof
D-3 suspended ceiling DC+T16-40 2.0 kg/m2 900-1200 Accessible
D-4 suspended ceiling T16-40 1.1 kg/m2 1250 Inaccessible
D-5 suspended ceiling DC+T16-40 2.0 kg/m2 900-1200 Accessible
Main stud (galvanized steel) D60 (CS60) 60 27 1.5 1.37 1200 Accessible Beijing New Type
Main stud (galvanized steel) D60 (C60) 60 27 0.63 0.61 850 Inaccessible Beijing New Type Building Material Main Factory
Aluminum alloy T-shape main stud D32 25 32 900-1200 Inaccessible
Aluminum alloy T-shape sub- stud D25 25 25
Aluminum alloy T-shape side- stud D25 25 25

The structure of U-stud suspended ceiling is shown in Figure 9.6.

Suspended ceiling galvanized steel stud is mainly applied to the construction or reconstruction of restaurants, office buildings, entertainment places and hospitals etc. Inaccessible suspended ceiling is only capable of bearing its self-weight, and its stud has small sectional surface area. Accessible suspended ceiling not only bears its self-weight, but also bears the weight of moving people, generally is able to bear a concentrated load of 80-100 kg/m2, therefore, it is often applied to the construction of suspended ceilings in large space theaters, concert halls, conference centers or ceilings equipped with center conditioning system.

URL: https://www.sciencedirect.com/science/article/pii/B9780857092571500097

Nanoparticles incorporated in silane sol–gel coatings

2.2Ceria

To improve the corrosion protection of galvanized steel, Montemor and Ferreira [14] used a bis-1,2-[triethoxysilylpropyl]tetrasulfide silane film containing ceria (CeO2) nanoparticles. Through taking advantages of EIS technique, they investigated the electrochemical behavior of the silanized metal and concluded that the barrier properties of the film depend on the concentration of nanoparticles. Fig. 23.13 shows the comparison between the silane coatings containing CeO2 and those with silica nanoparticles in 0.005 M NaCl solution. The results indicated that, for both the nanoparticles, there is an optimum concentration in which best protection is provided, and the SiO2 nanoparticles are not as effective as the CeO2 nanoparticles. This might arise from the low stability of SiO2 under increased alkaline conditions generated at the cathodic sites. The SiO2 nanoparticles decompose in the alkaline pH, leading to the formation of an expansive gel that accelerates the silane film degradation and delamination. Unlike SiO2, the CeO2 particles are very stable in the alkaline conditions. When the silane network degrades, the CeO2 nanoparticles can leach out and precipitate together with the zinc corrosion products, producing a more stable and protective layer of corrosion products [14].

Moreover, these researchers investigated the effect of activation of nanoparticles (silica and ceria) with the cerium ions on the protective function of the hybrid silane film. After 24 h of immersion, the EIS spectra (Fig. 23.14) show that the activation of the nanoparticles with cerium ions resulted in higher impedance or, in other words, most effective inhibition.

URL: https://www.sciencedirect.com/science/article/pii/B9780128193594000234

Influence of surface materials on biofilm formation

Inês B. Gomes, ... Manuel Simões, in Viruses, Bacteria and Fungi in the Built Environment, 2022

4.2.1Metallic materials

Typical metals used in plumbing systems include galvanized steel or iron, and copper. Metal alloys, such as brass, can also be used and far exceed the performance specifications of their respective parent materials (WHO, 2006). For all of them, corrosion is a concern, and the World Health Organization (WHO) has guidelines and standards for maximum acceptable levels of metals in the DW supplies to prevent health risks, undesirable taste, and structural damage (pinhole leaks) (Jin et al., 2020). Materials should be chosen according to the application, having in mind corrosion and other factors. For instance, a suitable product for cold water system may not be ideal for a hot water system (WHO, 2006).

Regarding galvanized steel or iron, its popularity is declining due to concerns of high corrosion and resulting undesirable taste. Furthermore, it is heavy to handle, inflexible, and is joined by threading and screwing, which makes it time-consuming (WHO, 2006). On the other hand, copper is extremely flexible, smaller in overall diameter, and less susceptible to corrosion than the equivalent galvanized steel pipes. However, contact with dissimilar metals should be avoided for both materials since it is typically where electrolytic corrosion is enhanced (WHO, 2006).

The potential for biofilm formation is dependent on several factors that have led to different results for the same tested materials. For instance, metal corrosion is known to increase the surface area of the material for bacteria attachment and to provide protection from disinfection, decreasing their efficiency (Assaidi et al., 2018; Lehtola et al., 2005). Corrosion can follow reactions between the pipe material and chlorine in the water, although to different extents, depending on the material. Metallic materials increase disinfectant decay compared to plastic materials. However, iron is known to react more rapidly with disinfectants leading to higher corrosion in comparison to copper and the attendant loss of residual disinfectant (Lehtola et al., 2004; Simões & Simões, 2013). Therefore, a lot of iron pipes are cement-lined to add an extra layer of protection (Assaidi et al., 2018; Simões & Simões, 2013). This is a determining factor in biofilm formation, with studies repeatedly reporting higher biofilm growth on iron or steel pipes (Yu et al., 2010), with one reporting 10 to 45 times more bacterial biomass on grey iron pipes than plastic (Assaidi et al., 2018), and another showing 100 times more growth on steel pipes than copper and plastic materials (Lehtola et al., 2005).

Another factor that increases surface area is the pipe roughness. Materials with similar porosity and roughness have shown to support similar densities of fixed bacteria, but not all papers reported this relationship (Assaidi et al., 2018; Gulati & Ghosh, 2017). For example, galvanized steel surfaces support higher numbers of bacterial cells than stainless steel, which can be attributed to the lower surface roughness of stainless steel. Copper has a much smoother surface than both and plastic materials even more, helping to understand low biofilm formation in these materials (Gulati & Ghosh, 2017). Moreover, other characteristics of the material have impact on microbial interactions, such as the hydrophobicity and surface charge. For example, stainless steel is hydrophilic and can have a slightly negative surface charge, which reduces bacterial adhesion, as described by Lehtola et al. (2005). These authors found lower numbers of bacteria on stainless steel in comparison to steel, PVC, and copper. Furthermore, microbial community and biofilm morphology have also shown to be influenced by the pipe material (Buse, Lu, Lu, Mou, & Ashbolt, 2014; Liu et al., 2014; Yu et al., 2010; Zhang et al., 2017). For example, stainless steel and steel coated with zinc have shown potential to harbor different bacterial communities (Yu et al., 2010). Moreover, Zhang et al. (2017) reported higher microbial diversity as well as the occurrence of opportunistic pathogens in biofilms formed on ductile iron than on stainless steel. The authors further described Enhydrobacter, Propionibacterium, and Acinectobacter as the main genus found on ductile iron biofilms. On the other hand, the most commonly found microorganisms in stainless steel belong to Flavobacterium, Arcicella, and Acidovorax.

Another determining factor is the pipe age, with older pipes having increased corrosion, leading to more disinfectant decay and consequently facilitating microbial proliferation. Time has shown to affect pipes in the following order: cast iron > steel > cement-lined cast/ductile iron (Simões & Simões, 2013).

Regarding copper, this metal has antimicrobial properties and a slow corrosion process, leading to biofilm formation occurring much slower than in other pipe materials (Jang et al., 2011). As a result, experiments done in a short period of time have shown copper to be the best pipe material, resulting in lower biofilms development; in some cases, copper performance is comparable to plastic materials (Learbuch, Smidt, & van der Wielen, 2021; Moritz et al., 2010; Waines, Moate, Moody, Allen, & Bradley, 2011; Yu et al., 2010). As incubation time increases, so does biofilm growth (Table 4.2). For example, Lehtola et al. (2005) showed increased biofilm development in copper only after 200 days, which led to a similar final number of microorganisms to the value shown in PE. Wingender and Flemming (Wingender & Flemming, 2004) showed higher or similar number of CFUs levels in copper after 540 days when compared to stainless steel, PE, and PVC.

Table 4.2. Examples of different studies on the influence of materials in biofilm formation potential.

Materials tested Biofilm formation potential (BFP) Refs.
Metals: CU; SS, steel coated with zinc;
Plastics: PE; PB
90 days experiment: CU—lowest BFP, steel—highest BFP (Yu et al., 2010)
Metals: CU alloys (0%, 57%, 96%, 100%); 7 days experiment: reduced number of nondamaged microorganisms for 96% alloy with chlorine treatment (Gomes, Simões, &amp; Simões, 2020)
Metals: CU, SS, steel;
Plastic: PVC;
15 months experiment: steel—highest BFP, CU—initially lowest biofilm growth, SS—best performing material overall (Jang et al., 2011)
Metal: CU;
Plastic: PE;
308 days experiment: CU—initially lower BFP for 200 days. PE—lowest BFP overall (Lehtola et al., 2004)
Metal: CU;
Plastic: PE;
Chlorine treatment effect weaker on CU’s BFP (Lehtola et al., 2005)
Metals: CU, SS;
Plastic: PEX;
2 years experiment: CU and SS similar BFP (van der Kooij et al., 2005)
Metals: cemented steel; cemented cast iron, tarred steel, grey iron;
Plastics: PVC, PE
8 months experiment: PE and PVC lowest BFP. Grey iron highest BFP (Assaidi et al., 2018)
Metals: CU, galvanized steel, SS;
Plastics: PVC, PPR, PEX
45 days experiment: galvanized steel highest BFP, CU no BFP (Niquette, Servais, &amp; Savoir, 2000)
Metals: CU, Al;
Plastics: PVC, PP, PE
90 h experiment: CU no BFP (Gulati &amp; Ghosh, 2017)
Metals: CU; SS
Plastic: PE, PVC
540 days experiment: higher or similar BFP in CU compared with SS and plastics (Wingender &amp; Flemming, 2004)
URL: https://www.sciencedirect.com/science/article/pii/B9780323852067000137

Application of Microbial Cleaning Technology for Removal of Surface Contamination

3.6Types of Substrates

Substrates such as carbon and stainless steels, galvanized steel, brass, copper, aluminum, plastics, ceramics, fiberglass, glass/quartz, sterling silver, nickel, titanium, and concrete have been successfully cleaned. Not only is the cleaning solution effective on metal parts, it will not damage nonmetal components that may be attached to the parts being cleaned such as rubber or plastic fittings. As with all parts cleaners, some surfaces will be cleaned at different rates than others due to the degree and type of contamination present on the surface. Because the cleaners operate at a near-neutral pH and lower temperatures, metal parts can be cleaned without etching. Metal, plastic, and fiberglass parts will keep their original finish.

URL: https://www.sciencedirect.com/science/article/pii/B9780128155776000153

Atmospheric Corrosion of Zinc and Zinc Alloyed Coated Steel

D. Thierry, ... N. Le Bozec, in Encyclopedia of Interfacial Chemistry, 2018

Zn-Mg-Al and Zn–Mg coated steel

As already mentioned in previous articles, hot dip galvanized steel (HDG) is widely used for applications in the automotive, building, and appliance industry. This is linked to the corrosion properties of HDG providing both a barrier and a galvanic protection of the steel substrate. In the automotive industry, there is a need to move to new coatings with improved corrosion resistance in order to (1) reduce the thickness of the coating and (2) eliminate the need of additional corrosion protection in confined areas such as hem flanges.

In the last 2–3 decades, a large body of literature has been published on further enhancements of the corrosion properties of HDG by additions of magnesium, aluminium, or a combination of both.61–67 New coating alloys containing aluminium and magnesium have been developed for continuous galvanized steel sheet.61–69 The zinc magnesium aluminium coated steel will be refered as ZM in this review. These coatings normally contain 0.2%–11% of magnesium and 0.1%–3% of aluminium which are added to zinc. The addition of magnesium and aluminium results in a complex microstructure containing primary zinc, MgZn2, binary eutectic MgZn2–Zn, binary eutectic Zn–Al, and a ternary eutectic depending upon the composition of the coating. This is illustrated in Fig. 24 for a coating containing 2% of magnesium and 2% of aluminium. In addition to this, there are recent reports on the corrosion of zinc’magnesium coatings produced by physical vapor deposition (PVD).70,71 It should be mentioned that the microstructure may differ according to the exact composition of the coating. However, the phases mentioned earlier are generally included. In this case, the coating is mainly formed by an inner layer of zinc and an outer layer of MgZn2. Alternatively, Zn and Mg are codeposited simultaneously and coatings containing from about 10 to 16 wt% Mg are formed. As PVD and the hot-dip galvanizing process result in materials with very different structures and surface concentrations of magnesium and aluminium, it is often difficult to compare the results.

The corrosion performance of these novel coatings have been studied both under laboratory and field conditions. Fig. 25 shows the results obtained after 6 weeks exposure in different cyclic corrosion tests. For more details on the different cyclic corrosion tests use reference.72 The relative performance of the ZM coated steel is dependent upon testing conditions and it is highest for tests that involve a high chloride load (e.g., neutral sat spray at 5% and VDA 621-415). It should be noticed that this generally corresponds to conditions for which very high corrosion on HDG and zinc are observed. Similar results have been observed in the literature when comparing the time to red rust of zinc’magnesium coated steel produced by PVD and submitted to different accelerated tests.64 Although, the corrosion performance of ZM coated steel is always superior to HDG in chloride environments, it exists few reports with an opposite situation. LeBozec et al.14 and Persson et al.43 reported that ZM coated steel showed poor corrosion behavior in CO2 free and SO2 environments, respectively. One possible explanation provided by the authors was that under these conditions the surface pH became very alkaline (e.g., CO2 free environments) or acidic (SO2 containing environments), resulting in a much higher dissolution of the zinc and aluminium rich phases.

Long-term data on the corrosion behavior of ZM coated steel compared to HDG are very scarse. Salgueiro and coworkers compared the corrosion behavior of ZM to HDG in different environmental conditions including marine, rural, and urban locations.73 A ratio of improvement between 2.4 and 3.3 was found after 2 years of exposure. Similar results were found by Tomandl et al. after 1 year exposure in marine and marine tropical field sites74 and by Thierry et al. after 4 years exposure at 15 different field station worldwide.75 It should be noted that a good performance of ZM coated steel compared to HDG was also measured in industrial environments that contain moderate amount of SO2. It should be noticed that as already mentioned for HDG, the corrosion of ZM materials was highly localized after field exposure with a preferential attack of the binary or ternary eutectics.75

Despite the large body of literature on the subject, there is still no clear single explanation of the role of magnesium and aluminium in the enhanced corrosion properties of Zn–Mg and Zn-Al-Mg coated steel compared to HDG. However, several mechanisms for the improvement are suggested in the literature.

It is rather well accepted that the binary and ternary eutectics containing magnesium and magnesium and aluminium, respectively, are dissolved preferentially. Some authors have attributed the better behavior of these coatings to the formation of Mg containing corrosion products.76–78 Possible hypothesizes on the effect have been an enhanced insulating character of the zinc corrosion products due to magnesium and a limited charge transfer reaction at the grain boundaries.79,80 The role of magnesium has also been explained through a reduction of the surface alkalinity by the formation of Mg(OH)2 which favors the formation of simonkolleite and zinc hydroxy sulfate.76,81–83 The results observed in Refs. 76–80 imply that magnesium is present in the inner layer of the corrosion products formed on Zn-Al-Mg coated steel, although no corrosion products associated with magnesium has been clearly identified. The presence of aluminium in zinc-aluminium’magnesium coated steel seems to result in different mechanisms with the formation of layer double hydroxides (LDH). These compounds have the general formula Zn(or Mg)xAly(A)m(OH)n·zH2O (where A is an anion and in atmospheric corrosion often carbonate). Although the formation of LDH has been well documented (see for instance Refs. 43, 81,82), the protective character of these compounds is still a subject for further investigations. The mechanisms of formation of LDH and ZnO have been studied by Salgueiro in different electrolytes.84 This is schematically illustrated in Fig. 26. It was found that the formation of LDH is delayed in NH4+ and HCO3 environments due to their buffering effects.84 It should be noticed that as demonstrated by LeBozec and coworkers, LDH are not formed in CO2 free environments resulting in poor performance of these coatings.14

Finally, it should be noticed that although these coatings present a very complex microstructure, no systematic investigation has been performed on the effect of microstructure on the corrosion behavior of ZnAlMg coated steel. However, the results obtained by Prosek et al. on model alloys having composition close to that used for coated steel have shown that microstructure can play an important role. Indeed, it was shown that alloys with finer microstructure led to better corrosion performance probably due to a blocking of the cathodic sites.85

URL: https://www.sciencedirect.com/science/article/pii/B9780124095472134316

New Applications

Reinhold Schwalm, in UV Coatings, 2007

10.2.3Coil Coatings

Coil coating is a technology where “prefinished” sheets of galvanized steel or aluminium are produced, which are stamped, deep drawn, bent or otherwise formed into the final shape by the processing industry. The coating process (see Figure 10.26) and with it the environmental regulations are shifted to the coil coaters. Four major market segments are served by coil coated materials. Each market segment is mainly using one or two specific resins types. The market segments are construction (roofing, panels, garage doors), transportation (truck and bus body panels, automotive exterior trim components, gas tanks, engine components), consumer products (appliance “white goods”, like refrigerators or washing machines, office furniture, shelving, computer components, signs, industrial equipment) and packaging (cans, containers, crowns, barrels, drums). The biggest market in terms of coating usage is the construction sector, followed by consumer products and transportation. The coatings used are either thermoplastic or thermosetting. With existing technology the line speeds are in the range of >200 m/min, the curing times in the range of 10 to 60 s, in which the coating must have good levelling properties and exhibit good adhesion to the metal substrate as it is bent, punched or drawn during fabrication. The coatings are almost all solvent-based. Up to now, water-based, powder and UV-curable systems have not succeeded to gather a significant market share for a variety of reasons. The water-based systems are difficult to clean when switching from one colour to another and increased use of gas to fuel the incinerators reduces economy, whereas powder coatings suffer from the difficulty to obtain the smooth, low coating thicknesses commonly demanded. UV-curable coatings show poor adhesion due to high volume shrinkage. The resin types used shifted in their market prominence from alkyd, vinyl and epoxies to polyesters. For exterior metal buildings the polyesters were not durable enough, whereby siliconized polyesters and fluorocarbons are rapidly emerging.

The basic resin systems, polyesters (60% share), PVC plastisols (30%), PVDF (5%) and others, as well as their performance profile regarding a few key requirements, such as flexibility, surface hardness, metal adhesion, corrosion protection, weathering resistance and heat resistance have been compared.39

The high curing speed of the coil coating process and the two-dimensional application should be well suited for UV-curable systems, however, the technical problems, mainly adhesion, flexibility and through cure issues in pigmented coatings, have to be solved. Despite the fact, that a complete shift from the high temperature curing (up to 230°C) to the low temperature UV curing should be should be an important economic driver for change, the use of UV-curable coatings is developed in order to substitute one layer (primer) or introduce a third layer in the conventional two-layer (primer, topcoat) coating assembly.

Earlier developments are cationically curing systems based on hydroxyl terminated polyesters and epoxide crosslinkers (Cyracure UVR 6110, Dow).39

Further evaluations of UV-curable systems for direct metal use41–44 and the through cure of pigmented UV-curable coatings45 have been reported.