1

Relationship between corrosion performance and microstructure of

Zn-Al and Zn-Al-Mg model alloys

Tomas Prosek, Dominique Thierry, Institut de la Corrosion / French Corrosion Institute, Brest/France; Joacim Hagström, Dan Persson, Nuria Fuertes, Fredrik

Lindberg, Claes Taxén, Swerea KIMAB AB, Stockholm/Sweden; Jan Šerák, ICT Prague/Czech Republic

Summary

Zn-5wt.%Al and Zn-3wt.%Al-2wt.%Mg model alloys were cast and heat treated in

order to obtain specimens with distinct microstructure and identical chemical

composition. The microstructure was characterized in detail using scanning electron

microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX) and x-ray diffraction

spectroscopy (XRD) to identify chemical composition, size and distribution of present

phases. Mass loss of samples with different microstructure and identical chemical

composition subjected to a cyclic corrosion test and a test under non-rinsing

conditions differed by a factor of up to two. Some model alloys showed better

corrosion performance compared to commercial coatings with similar composition

tested in parallel. This indicates that microstructure modification can improve the

corrosion performance of industrial zinc alloy coatings. The mechanism is discussed

based on measurements of corrosion stability of individual phases, preferential

corrosion, composition and stability of corrosion products, dc electrochemical

characteristics and infrared absorption spectroscopy (IRRAS) experiments.

1. Introduction

In order to increase the corrosion durability and reduce costs of zinc coated steel

products, great efforts are invested into the optimization of the zinc coating

composition by alloying. Successful examples of the development are coatings

alloyed with aluminium. At present, the main development activity is focused on Zn-

Al-Mg coatings for building and automotive applications.

A large body of scientific literature is available on the effect of the microstructure of

aluminium and magnesium alloys on corrosion. It is believed that the microstructure

of zinc alloy coatings is strongly affecting their corrosion properties as well. Recent

publications indicate that the microstructure induced by alloying especially with Mg

might play the key role in the formation of protective layers [1,2]. It was suggested

that a finer microstructure induced by the alloying prevents extreme local pH values

by averaging the pH in anodic and cathodic sites, which in turn leads to formation of

layers of higher protective quality. Elvins et al. reported for Zn-5Al coatings that

different microstructures at identical composition can have an effect on the corrosion

behaviour [3]. Although the volume fraction of primary zinc dendrites remained

unchanged at higher cooling rate from the melt, the larger number of smaller non-

interconnecting dendrites had a positive effect on the corrosion resistance. Elvins et

al. also investigated the effect of very low Mg additions in the range to 0.05 wt.% [4].

The additions caused an increase in the zinc dendrites volume fraction from 6 to

22 %. The authors found by scanning vibrating electrode technique (SVET)

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measurements during 24-hour exposure to 5% NaCl solution that this microstructural

modification boosted cut edge corrosion.

As demonstrated for Zn-5Al coatings, there is a large space for improving the coating

corrosion stability and overall product durability by tailoring the coating

microstructure [3,4]. This is of utmost practical importance since the market volume

for alloy coatings of different compositions is growing due to their superior corrosion

and other functional properties. The aim of this study was to get first idea on the role

of the Zn-Al(-Mg) microstructure in atmospheric corrosion. Further field exposures

and mathematical modelling are in progress and will be reported later. The subject

will get detail attention in a forthcoming RFCS MICROCORR project [5].

2. Experimental

2.1 Sample preparation and microstructure characterisation

Model alloys of Zn-5Al, Zn-3Al-2Mg, Zn-6Mg and Zn-16Mg, as well as Zn and Al

were prepared by induction melting in a graphite crucible under protective

atmosphere of 99.5% argon. Purity of raw Zn, Mg and Al was 99.99, 99.99 and

99.5 wt.%. After introduction to a crucible, the metals were heat up to 750 °C and

kept at the temperature for 6 hours to dissolve and homogenize. For Zn, the

temperature was 600 °C and holding time 1 hour. Then, still under the argon

atmosphere, the melt was poured into a brass mould and solidified at cooling rate

about 3 K/s. Cylindrical casts were of 50 mm in diameter. In order to obtain distinctly

different microstructures, casts of Zn-5Al and Zn-3Al-2Mg were heat treated under

different regimes. Chemical composition measured by x-ray fluorescence (XRF), heat

treatment parameters and hardness for the prepared materials are given in Table 1.

The castings were cut to round samples of 50 mm in diameter and about 3 mm thick.

Table 1: Denomination, composition, heat treatment (HT) and hardness of cast materials

Den. Al [wt.%]

Mg [wt.%]

HT T [°C] / time [hour]

Hardness HV5

Den. Al [wt.%]

Mg [wt.%]

HT T [°C] / time [hour]

Hardness HV5

ZA1 5.1 – 340 / 16 70 ± 3 ZAM1 3.1 2.0 340 / 0 141 ± 3

ZA2 5.1 – 370 / 0 79 ± 4 ZAM2 3.0 2.1 340 / 16 128 ± 5

ZA3 5.2 – 370 / 72 76 ± 5 ZAM3 3.2 2.0 340 / 72 114 ± 3

Z – – As cast 58 ± 5 ZM6 – 6.1 360 / 0 272 ± 8

A 99.6 – As cast 23 ± 2 ZM16 – 16.2 360 / 72 288 ± 12

As reference, line hot-dip coated steel panels of HDG (Zn-0.2Al), G (Zn-5Al) and

ZAM (Zn-2Al-2Mg) with the coating thickness of 20 μm were used.

The alloy microstructure was studied after grinding and polishing by scanning

electron microscopy (SEM) with back scattered electron (BSE) imaging. The phase

composition was analysed by x-ray diffraction spectrometry (XRD) and energy

dispersive x-ray spectroscopy (EDX).

2.2 Corrosion testing

Flat round samples cut from the casts were subjected to accelerated corrosion tests and mass loss measured. The samples were polished down to 400 grit and their backsides masked with tape. Reference panels of HDG, G and ZAM were cut to 50×50 mm and their backside and edges masked with tape.

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Renault ECC1 D17 2028 test (ECC1), is a cyclic accelerated corrosion test developed and used by the automotive industry. Four parallel samples were exposed to 42 daily cycles with total duration of 6 weeks. The test is conducted at 35 °C and comprises wet (90% relative humidity, RH) and dry (55% RH) phases. Water solution containing 1 wt. % NaCl at pH 4 is sprayed over the samples during 30 minutes once a day. Besides mass loss measurements after removal of corrosion products in saturated glycine, NH2CH2COOH, solution following the ISO 8407 standard, run-off water collected under the exposed samples and solutions obtained by the corrosion product dissolution were analysed for Zn, Al and Mg. Chromic acid (100 g/L CrO3, 10 g/L AgNO3) was used to remove corrosion products from aluminium, material A. Samples pre-contaminated with 1.4 g/m² chloride as NaCl dissolved in methanol [6] were exposed to a 4-week non-rinsing corrosion test (static test) at 80% RH and at 20 °C. Mass loss was measured after removal of corrosion products.

2.3 Electrochemical measurements

Free corrosion potential Ecorr, corrosion rate from polarisation resistance (–20 to 20 mV / Ecorr) and anodic and cathodic polarisation curves for freshly polished model alloys were measured by dc electrochemistry in aqueous NaCl solutions at different pH and at 20 °C under stirring. 4.6M NaCl solution formed in equilibrium between NaCl and humid air at 80% RH [7] was prepared and the pH adjusted with HCl and NaOH to 4, 7 and 10. In addition, experiments were carried out in a diluted 10 mM NaCl solution at pH 7. The solutions were in contact to air before the measurements to get saturated with oxygen. Saturated calomel electrode (SCE) and platinum wire were used and reference and counter electrodes. The working electrode was made of a piece of model alloy embedded in epoxy resin with an area of about 500 mm².

2.4 In situ IRRAS measurements of corrosion product formation

Initial formation of corrosion products was followed in situ by infrared reflection absorption spectroscopy (IRRAS). The experimental cell consisted of a stainless steel body with KRS-5 (thallium bromo iodide) windows. The cell is built to fit a Harrick’s multi-reflection accessory providing an angle of 78° vs. the surface normal of the incident light. The in situ IRRAS measurements were performed with a Varian FTS 7000 spectrometer, equipped with a KBr beam splitter and a DTGS detector, by adding 500 scans at a resolution of 8 cm–1. IRRAS spectra were recorded as single beam spectra before and after introduction of humid air at 85 % RH and then after different times during the exposure in humid conditions. Spectra were obtained by taking the ratio of a single beam at a certain exposure time vs. a single beam spectra obtained after 5 minutes exposure to humid air or in dry air. The in situ studies were performed on samples which were cut to a size of 19×22 mm. A small amount of a saturated solution of NaCl in ethanol was deposited on the surface with a pipette leading to a surface concentration of approximately 70 mg/m² Cl–.

3. Results and discussion

3.1 Microstructure of model alloys

Four principal phases have been identified in studied alloys: η phase with hexagonal close-packed (HCP) structure containing beside zinc up to 1.5 wt.% Al; α and β solid solutions with face-centred cubic (FCC) structure composed of zinc and aluminium; and Mg2Zn11 phase with 6–7 wt.% Mg and up to 1 wt.% of dissolved Al. The β phase

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has a theoretical Al content of 22 wt.% but our measurements showed 17–30 wt.% Al. The α phase typically contained between 40 to 70 wt.% Al. Zn-5Al, ZA1–ZA3. In the Zn-5Al alloys, a small number of primary dendrites of dark α/β FCC phases and primary dendrites of light η phase was found. Eutectic crystallisation led to formation of continuous η phase with particles of β phase further transforming in the eutectoid point to η and α phases. Micrographs of Zn-5Al alloys documenting distribution of particular phases are in Figure 1.

Figure 1: Left, right: BSE micrographs of Zn-5Al with identified phases; Centre: EDX scan for Al and Zn by the yellow line shown on left; 1: η phase, light; 2: α phase, black; 3: β phase, grey; 4: Primary α/β dendrite, partly transformed

Microstructures of differently heat treated materials ZA1, ZA2 and ZA3 are seen in

Figure 2. After the heat treatment at elevated temperature of 370 °C for ZA2 and ZA3,

the microstructure coarsened significantly. The dark β phase has transformed to α and

η. Although the total area of Al-rich particles did not differ substantially between the

materials, it was mostly composed of the β phase for ZA1 and a mixture of η, α and β

phases for ZA2 and ZA3. The mean size of α/β particles was 2, 5 and 10 μm for ZA1,

ZA2 and ZA3, respectively. Their distance increased too, see a summary in Table 2.

Small precipitates, sub-micron laths or needles 0.1–0.5 μm long found in ZA1 were

almost not present in ZA2. They were much coarser and more two-dimensional of 0.5–

1 μm in diameter and oriented in specific crystallographic directions in ZA3.

Figure 2: BSE micrographs of Zn-5Al alloys; width 300 μm

Table 2: Phase structure of Zn-5Al model alloys ZA1–ZA3

Mean α/β phase size

[μm]

Mean α/β phase

distance [μm]

Primary α/β dendrites

[vol.%]

Primary η dendrites

[vol.%]

Secondary α/β phase

[vol.%]

η phase [vol.%]

ZA1 2 4 1 3 20–25 [1] Rest

ZA2 5 10 1 3 25 [2] Rest

ZA3 10 15 0 0 25 [2] Rest

[1] Mostly β phase; [2] Including η phase after the eutectoid transformation within original sections of β phase

Zn

Al

ZA1 ZA2 ZA3

5

Table 1 shows that ZA2 and ZA3 were somewhat harder than ZA1. It is expected that

the heat treatment at 370 °C led to maximal solution hardening of both β and η

phases because they got saturated with Zn and Al, respectively.

Zn-3Al-2Mg, ZAM1–ZAM3. Primary dendrites of η phase and ternary eutectics

composed of particles of Mg2Zn11, α/β Zn-Al solid solution and η phase can be

identified in the structure of Zn-3Al-2Mg materials, see Figure 3. In ZAM2 and ZAM3

heat treated for prolonged period of time, eutectoid transformation led to partial

decomposition of the β phase to η and α situated on rims of the former particles. The

transformation was almost complete for ZAM3. Elemental maps showed clear

separation of Mg to the Mg2Zn11 phase. Al was present beside the α/β phases also in

the η and Mg2Zn11 phases in a small quantity of about 1 wt.%.

Figure 3: Left and centre: BSE micrographs of Zn-3Al-2Mg with identified phases; Right: EDX scan for Al, Zn and Mg by the line shown in the inlay; 1: η phase, light; 2: Ternary η – Mg2Zn11 – α/β eutectics; 3: Mg2Zn11 phase, grey; 4: partly transformed α/β phase, dark grey/black

The heat treatment led to coarsening of the microstructure, see Figure 4 and Table 3. This is particularly evident for the ternary eutectic. The typical particle size was 0.5–1.5, 1–5 and 3–9 μm for ZAM1, ZAM2 and ZAM3, respectively. In ZAM1, there was 35 and 10 vol.% Mg2Zn11 and α/β phases. It corresponds to the chemical composition considering the phases contained about 6 and 20 wt.% Mg and Al, respectively. With prolonged heat treatment, the volume fraction of the α/β phases decreased. It is in line with the observation that the β phase transformed to the Al-rich α-phase. It can be estimated that a volume fraction of α and β was about identical in ZAM3. The volume fraction of Mg2Zn11 apparently decreased too. This is surprising since the Mg concentration was identical and Mg was not detected in any other phase. XRD measurements detected absolutely lower (9–16 wt.%) but about constant amounts of Mg2Zn11 in the alloys. It is expected that the volume fraction of this phase did not change during the heat treatment, only the phase size.

Figure 4: BSE micrographs of Zn-3Al-2Mg; width 300 μm

ZM6 was composed of Mg2Zn11 with remnants of less than 10 vol.% of the η phase. Hexagonal MgZn2 phase was expected to dominate in ZM16. However, only

Al

Zn

Mg

ZAM1 ZAM2 ZAM3

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monoclinic Mg4Zn7 phase and minor remnants of η phase were detected by XRD. Commercial line galvanized material ZAM contained 12 wt.% of α/β phases, 77 wt.% of η phase and 12 wt.% of MgZn2 according to XRD. For G with 5 wt.% Al, there was 92 wt.% of η phase and 8 wt.% of α/β phases. These data generally agree to previously published results [3,8]. Table 3: Phase structure of Zn-3Al-2Mg alloys ZAM1–ZAM3

Mean η

phase

size [μm]

Mean η phase

distance

[μm]

Mean eutectic

phases size

[μm]

η phase

dendrites

[vol.%]

Eutectic

η phase

[vol.%]

Mg2Zn11

[vol.%]

α/β phase

[vol.%]

ZAM1 15 30 1 30 25 35 10 [2]

ZAM2 20 30 3 40 [1]

33 18 9 [3]

ZAM3 20 10 6 55 [1]

25 14 6 [3]

[1] Including growth during heat treatment in ZAM2 and ZAM3; [2] Mostly β phase; [3] Including η phase formed by eutectoid transformation of β phase

3.2 Corrosion performance and role of microstructure

Mass loss data after a cyclic corrosion test ECC1 and a static test under non-rinsing conditions with pre-deposited NaCl are given in Figure 5. An identical effect of the alloy microstructure on corrosion stability was revealed in both tests. The lowest mass loss of Zn-5Al materials was observed for ZA2. It was only slightly higher for ZA3 but strongly elevated for ZA1. In conditions of the ECC1 and static test, ZA1 was over 2-fold and 55% more corroded than ZA2, respectively. ZAM1 was the most stable Zn-3Al-2Mg material with 60% and 50% lower mass loss compared to ZAM3 in the respective tests. These observations clearly demonstrate the importance of Zn-Al and Zn-Al-Mg alloy microstructures in corrosion processes. Comparison of Z, ZM6 and ZM16 shows the inhibiting effect of magnesium on atmospheric corrosion as demonstrated earlier [1,2,6,9,10]. In a previous study using testing conditions similar to the static test in this work, a material with composition identical to ZM6 outperformed ZM16 [10]. It is probable that it is also related to somewhat different phase composition and microstructure because of different casting and heat treatment regimes. The current data agree to reference [11].

Figure 5: Mass loss of model alloys and reference materials in corrosion tests

Mass loss of HDG panels was slightly lower than that of pure zinc (Z). It is most probably due to presence of a small quantity of aluminium in HDG. Line Zn-5Al (G) gave mass loss comparable to that of the least stable model Zn-5Al, ZA1, in the

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ECC1 test. In contrary, it performed as well as the most stable ZA2 in the static test. Mass loss of line-produced ZAM was higher than for the best performing Zn-3Al-2Mg alloys. Although ZAM and ZAM1–3 cannot be directly compared due to somewhat higher Al content in the model alloys, the data suggest that even the microstructure of commercial coatings might be improved in view of their corrosion stability. As seen in Figure 4 and in data in Table 3, the most corrosion resistant ZAM1 had the finest microstructure. Similar experience has been acquired in studies of Al, Mg and other alloys. The detrimental effect of micro-galvanic coupling between phases with different potential drops with their decreasing size and the corrosion attack becomes more uniform. For Zn-5Al, the same effect can explain the difference in corrosion stability of ZA2 and ZA3. ZA2 was less corroded and had a finer microstructure. However, ZA1 does not fit into this picture. Figure 2 and Table 2 show that the microstructure of ZA1 is the finest of the Zn-5Al alloys whereas is was most corroded in both accelerated tests. It is possible that the β phase that dominated in the structure of ZA1 was less advantageous in view of atmospheric corrosion than the α phase mostly present in ZA2 and ZA3 as a result of the eutectoid transformation. For example, the oxygen reduction ability, which is of strong importance in atmosphere, could be higher on the β phase. A study of isolated α and β phases is planned to answer this question.

3.3 Stability of alloy components

Stability of Zn-5Al and Zn-3Al-2Mg model alloys as well as those representing pure phases, i.e. Z (η phase), ZM6 (Mg2Zn11 phase) and ZM16 (MgZn2 or Mg4Zn7) was studied in water electrolytes. No alloy representative of the Zn-Al α and β phases was available. Instead, pure aluminium (A) was used although it does not contain any zinc and cannot thus model the behaviour of the α/β phases properly. Polarisation curves of alloys simulating pure phases are plotted in Figure 6, left. Due to more electronegative nature of magnesium, Zn-Mg phases had more active Ecorr than zinc (Z) in 10 mM NaCl. Mg-Zn phases can thus be expected to corrode preferentially forming initial anodic locations in the Zn-Al-Mg structure. Ecorr of pure Al (A) was nobler than that of Z and Zn-Al phases would thus form cathodes although the efficiency of oxygen reduction on A was lower than on the other materials.

Figure 6: Left: Cathodic polarisation curves of selected model alloys simulating pure phases in 10 mM NaCl at pH 7; Right: Free corrosion potential in different NaCl solutions

A

Z

ZM6

ZM16

8

Values of Ecorr obtained for all materials in solutions with different NaCl concentrations and pH are in Figure 6, right. In can be seen that in contrary to 10 mM NaCl, corrosivity of 4.6 M NaCl towards Z, Zn-Al, Zn-Mg and Zn-Al-Mg was similar with almost identical Ecorr values in the whole pH range. Aluminium was nobler than the other materials in 4.6 M NaCl at pH 4. At pH 7, it could have either more negative or more positive Ecorr depending on the surface state. Even very minor variations in sample preparation led to a large spread of the potential as documented by the error bars. At pH 10, aluminium was systematically less noble than Zn-based alloys. Although the Al-rich phases detected in Zn-Al and Zn-Al-Mg alloys were not composed of pure Al, the data suggest that their Ecorr and relative stability to the alloy matrix might strongly depend on pH. At slightly acidic and neutral pH, Al-rich phases can be nobler than the matrix and serve as cathodes [12]. After alkalisation due to oxygen reduction reaction, they can activate and dissolve readily. The corrosion rate calculated from polarisation resistance measured 10 minutes after sample immersion was about identical for most materials at 0.1, 0.6, 0.8 and 1.0 mm/year in 10 mM NaCl / pH 7, 4.6 M NaCl / pH 10, 4.6 M NaCl / pH 4 and 4.6 M NaCl / pH 7, respectively. The initial corrosion rate thus increased with the NaCl concentration and it was maximal at pH 7. Lower corrosion rates were measured for ZM16 and in particular for A. The cathodic activity was maximal at pH 10. The fluctuating stability of Al-rich phases in Zn-Al was documented in another experiment. A sample of ZA1 was exposed to 10 mM NaCl for 14 hours and then inspected by SEM. As seen in Figure 7, left, the light η phase corroded preferentially. However, close analysis of the eutectic particles containing η and α/β phases revealed that it was often the Al-rich phases that dissolved from these structures, see Figure 7, right. It can be explained by their low stability in originally cathodic locations where a high pH environment formed.

Figure 7: SEM BSE images of a corroded zone in ZA1 after 14 hours of exposure to 10 mM NaCl at pH 7 at 20 °C; η phase is lighter

Solutions running off the exposed samples during the ECC1 test were collected and analysed for the amount of dissolved metal ions by inductively coupled plasma mass spectrometry. Corrosion products formed on the samples were dissolved in glycine or chromic acid and also analysed. Thus, information on the total amount of corroded elements and their solubility from corrosion products was obtained, see Figure 8. There was systematically more Mg present in corrosion products than in base alloys clearly indicating the Mg2Zn11 phase dissolved preferentially. In contrary, the Al content in corrosion products was lower compared to the Al concentration in alloys,

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i.e. the α/β phase was more stable than the matrix during the exposure [6,13]. The run-off data prove low solubility of Al-based corrosion products. Except for the most corroded ZA1, no dissolved aluminium was detected. Because the β phase dominated in the ZA1 structure over the α phase, this observation may suggest that the former phase less alloyed with Al dissolved more readily than the latter one. Magnesium tended to form relatively well soluble corrosion products. Whereas only about 0.1 wt.% of oxidised zinc dissolved to water solution, it was about 10 wt.% for magnesium. Thus, Mg-based corrosion products were three orders of magnitude more soluble than Zn-based ones. Solubility of aluminium corrosion products was negligible [6,13]. Both Al and Mg alloying reduced the absolute and relative zinc run-off.

Figure 8: Left: Amount of metals detected in run-off water collected under samples exposed to the ECC1 test for 6 weeks; Right: Comparison of the content of alloying metals in base non-corroded metal (BM) and in corrosion products (CP) formed in the ECC1 test

The electrochemical measurements in NaCl solution did not reveal any dramatic difference in the initial corrosion stability of zinc and zinc alloys with Al and Mg. Long-term stability of the materials during immersion to 4.6M NaCl at pH 7 was assessed at 20 °C by measuring mass loss after 4 weeks of exposure, see Table 4. The Mg2Zn11 phase (ZM6) was less stable than zinc (Z) in immersion conditions, which is in line with the more active Ecorr of the former phase. It shows that the improved corrosion stability of Mg-alloyed phases in atmospheric conditions is related to formation of more protective corrosion products and not intrinsic corrosion stability. Zn-Al and Zn-Al-Mg alloys were significantly less corroded than zinc. Table 4: Mass loss after 4-week immersion to 4.6 M NaCl at pH 7 and at 20 °C; in g/m²

ZA1 ZA2 ZA3 ZAM1 ZAM2 ZAM3 Z ZM6 ZM16 A

0.5±0.6 1.0±0.0 1.0±0.0 2.7±0.1 2.3±0.0 1.6±0.4 8.7±0.2 12.7±0.3 13.6±1.0 0.6±0.1

3.4 Corrosion products and protection mechanism

Corrosion product formation on surfaces of selected alloys pre-contaminated with 70 mg/m² chloride as NaCl was followed in situ in humid air at 85% RH and at 20 °C. The main corrosion products on zinc were hydrozincite, Zn5(OH)6CO3; simonkolleite, Zn5(OH)8Cl2·H2O; and zincite, ZnO. These corrosion products were also formed on the Zn-Al materials but the amount of ZnO was significantly lower. Layered double hydroxide (LDH), Zn2Al(OH)6(CO3)1/2·H2O, Mg6Al2(OH)16CO3·4H2O or similar [8], formed on Zn-Al and Zn-Al-Mg as the initial corrosion product. No ZnO was detected on Zn-Al-Mg. Hydrozincite and simonkolleite appeared after a few hours of exposure. The amount of corrosion products was estimated by comparison with simulated spectra of the pure corrosion products, see Figure 9.

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Figure 9: Estimated thickness of a layer of corrosion products formed on Z, ZA1 and ZAM3 with pre-deposited 70 mg/m² chloride during exposure to air at 85% RH and at 20 °C

The rapid formation of LDH on Zn-Al and Zn-Al-Mg has been previously reported [13–14]. It proves that the Al-rich phases start to dissolve rapidly in originally cathodic zones, especially at high chloride load [15]. It has been shown in the previous chapters that Zn-Mg phases were more electronegative than zinc and tended to dissolve preferentially from the matrix. The phases were generally less stable than zinc when immersed to bulk NaCl solution (Table 4) but they showed good stability in atmospheric conditions (Figure 5). The discrepancy in Zn-Mg phase stability in immersion and cyclic atmospheric conditions can be explained by volume of the present water electrolyte. In contrary to bulk solution, a thin electrolyte film forming in humid atmosphere is easily enriched with Mg2+ with strong effect on the surface chemistry. Mg2+ ions buffer the pH of the surface film bellow about 10.4 where magnesium precipitates form [16]. Since the environment cannot get strongly alkali, formation of zincite is inhibited [6,13,17,18]. In addition, Salgueiro Azevedo et al. proposed that Mg2+ kinetically limits further transformation of basic zinc salts to zincite during wet/dry cycling [17]. The rate of oxygen reduction that controls the rate of the corrosion process in atmospheric conditions was shown to be high on zincite due to the relatively high electric conductivity and inhibited on surfaces protected with the other corrosion products [17,18]. The improved corrosion resistance of Zn-Al-Mg alloys in atmosphere is thus connected to the presence of more insulating corrosion products.

0 200 400 600 800 1000

0

5

10

15

20

25

30

35Zn

5(OH)

6(CO

3)

2

Zn5(OH)

8Cl

2*H

2O

ZnOthic

kness n

m

exposure time min

0 200 400 600 800 1000

0

2

4

6

8

10

12

14

16

18

20

LDH

Zn5(OH)

8Cl

2*H

2O

Zn5(OH)

6(CO

3)

2

ZnO

thic

kness n

m

exposure time (min)

A

za1

za12

za13

0 200 400 600 800 1000

0

2

4

6

8

10

12

14

16

18

20

22

24

26

LDH

Zn5(OH)

6(CO

3)

2

Zn5(OH)

8Cl

2*H

2O

thic

kness

exposure time min

Z

ZA1

ZAM3

11

Indeed, due to the rather high solubility of most magnesium compounds, such effects would be short term in rinsing conditions. However, it can inhibit the rapid onset of corrosion in harsh exposure conditions and allows for formation of stable zinc-based corrosion products as observed by Hosking et al. [9]. In the presence of aluminium, zinc oxide can be replaced by LDH with low solubility and expected long-term protection ability [19]. For further discussion on the protection mechanism of Zn-Al-Mg coatings, see references [1,6,9,10,13,14–18].

4. Conclusions

Atmospheric corrosion of Zn-5Al and Zn-3Al-2Mg alloys was significantly affected by their microstructure in terms of phase composition, size and distribution. Mass loss varied up to 2-times as a function of the microstructure. In most cases, finer structures were more corrosion resistant. The results suggest that even the microstructure of commercial coatings might be improved in view of their corrosion stability. Zn-Mg phases were more electronegative than zinc matrix, formed anodic locations and dissolved preferentially. Al-rich phases were nobler than zinc in acidic and neutral electrolytes but activated at alkali pH formed at originally cathodic sites. Alloying of zinc with aluminium and magnesium reduced zinc run-off due to stabilisation of less soluble and more protective zinc corrosion products. Formation of undesirable zinc oxide was strongly reduced in presence of Mg and Al.

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