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    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 hum