Effect of Mg Addition (in Zinc Bath) on Galvanized Sheet Quality

In order to improve the corrosion resistance of the conventional hot dip galvanized sheets, a new type of Zn-Mg coating has been developed. Experiments were conducted in HDPS by varying zinc bath Mg level, bath temperature & dipping time. It has been found that steel sheet galvanized in zinc bath composition of 0.50%Mg, 0.25%Al, 0.08%Si & 0.08%Sb and processed at bath temperature of ~460 C and dipping time : 2.0 sec. results in best combination of properties in terms o f corrosion rate : 4.0 mpy (~1/3 w.r.t conventional GI sheets : ~12 mpy), formability of the composite (Ev : 10.4 mm for sheet thickness of 0.8 mm) at par with substrate material, coating adherence as per Lock Forming Quality (LFQ) standard and appearance bright and smooth. The improved corrosion resistance of newly developed coating is attributed to the presence of Mg-Zn intermetallic phases along the grain boundary inhibiting the cathodic reaction as well as formation of protective MgO & protective corrosion products of zinc. The presence of Sb in the galvanizing bath has also led to improved intergranular corrosion resistance. Superior formability, adherence and appearance of the coating is due to presence of adequate Al & Si level in the galvanizing bath, which led to suppression of brittle Fe-Zn phase formation at the coating-steel interface.


Introduction
Zinc coated sheets are widely used in Construction, Automotive and appliance industry. The demand by the customers for the perfo rmance of the coated sheets with respect to corrosion protection, formability, jo inability and paintability is steadily increasing. The demand fo r extended longevity of the galvanized sheet is increasing to meet the demand for maintenance free products. To meet this demand the corrosion resistance of the galvanized sheets need to be improved. Corrosion resistance of the zinc coated products can be improved by alloying the galvanizing bath with alu min iu m, n ickel, magnesium and other alloying elements. Addition of alu miniu m to zinc bath is one of the most useful development so far. Nevertheless, there is scope for further develop ment in terms of coating parameter, composition and lower bath temperature to improve corrosion resistance and other properties, and at a lower cost so that it can be economically explo ited.
As regards to alloying elements, in addit ion to Al, several reports have published the effect of Mg [1,2]. Mg has the best corrosion resistance, lowest density among metals, superior specific strength and high electro-negativity in emf series. Also, Zn-Mg alloys can offer low bath temperature [3,4]. Further, It has also been reported that small amount of Si addition in zinc bath improves the coating ductility [5]. In view of the above, the aim of the present work was to find out the optimu m level of Mg in zinc bath as well as optimised galvanizing parameters such as bath temperature, dipping time etc. for achieving improved properties in terms of corrosion resistance, formability properties along with adequate peel-off resistance of the zinc coating. In addition to above, for improving the intergranular corrosion resistance of the Zn-Mg-Al-Si coating, possibility of replacing Pb with Sb in zinc bath has also been explo red. This will also help in addressing the environmental issues related with Pb.

Experimental
During coating simulat ion experiments, Co mmercial Quality Co ld Rolled (CQCR) sheets (thickness : 0.8 mm) were galvanized in HDPS by varying zinc bath Mg level (0.25-0.75%), bath temperature (420-460 o C) & dipping time (2.0-4.0 sec.), while A l, Si & Sb levels were maintained at ~0.25%, ~0.08% & ~0.08% respectively. Chemical composition of the steel substrate used in simulation studies has been shown in Table I. Process variables and their ranges have been shown in Table II.

Results & Discussion
Simu lated galvanized sheets were characterized in terms of coating thickness, Corrosion rate, formab ility properties and coating adherence. The results have been shown in Tab le III.

Corrosion Resistance of Zn-Mg Coated Sheets
Corrosion resistance of the Zn-Mg-A l-Si coated sheets, measured through electropolarisation test using Taffel p lot, varied in the range of 4.0 to 10.5 mpy (co mpared to 12-15 mpy observed in conventional zinc coated sheets), depending on the processing condition. For the coated sheets, galvanized in 0.25% Mg bath, corrosion rate varied fro m 8.0 to 10.5 mpy., while the corrosion rates of the galvanized sheets of 0.50 & 0.75% Mg bath was found in the range of 4.0 to 7.6 mpy. In Fig.1, variation of corrosion rate with Mg content is shown.  It can be seen that with increase in Mg content fro m 0.25 to 0.75% for a bath temperature of 460 o C and d ipping time of 2.0 sec., corrosion rate of the Zn-Mg coated sheets decreases fro m 10.5 to 4.0 mpy. Fro m the above it can be observed that increase in Mg content fro m 0.25 to 0.50% improves the corrosion rate by more than 50%, however, further increase of Mg level fro m 0.50% to 0.75% doesn't results in any further improvement in corrosion rate of the coated sheets. Corrosion rates of the coated sheets galvanized in 0.75%Mg bath was found more o r less same as that of the 0.50%Mg bath. High corrosion resistance of the material coated in ternary Zn-Mg-Al bath has been attributed to the segregation of Al and Mg at grain boundaries which forms Zn/Mg/Al ternary eutectic phase mixture [6]. It has been reported that zinc forms protective corrosion product with Al and Mg [6] and Al rich phase first corrodes, wh ile surrounding ternary eutectic phase mixture remains stable [7,8]. Format ion of this protective layer seems to be responsible for better corrosion resistance of Zn-Mg-Al coated sheet. In SEM micrographs (shown in Fig.2) of 0.25% Mg coated sheets, formation of uniform and strong Zn/Mg/Al ternary mixture along the grain boundary was not observed. Though in some isolated regions, ternary mixture of Zn/Mg/Al was observed. This must have led to inferior corrosion resistance properties in 0.25% Mg coated sheets as compared to 0.50 & 0.75% Mg coated sheets. However, in the SEM micrographs (shown in Fig.3&4) of the coated sheets, galvanized in 0.50 & 0.75% Mg bath, it can be clearly seen that there is very prominent and uniform format ion of Zn/Mg/Al ternary eutectic phase mixtu re (co mposition; Mg : 0.77%, Al : 1.73, Zn : 92.79%) at the grain boundaries due to segregation of Al and Mg at these sites leading to slower anodic d issolution of the Mg-Zn intermetallic phases and format ion of protective MgO, which results in enhanced corrosion resistance of Zn-Mg-Al coated sheets [9,10]. Further, corrosion resistance of the Zn-Mg coated sheets was also measured through salt spray test. The results are shown in Table IV. It can be seen that the corrosion resistance of the coated sheets galvanized in Zn-Mg-Al-Sb bath is much superior compared to those galvanized in Zn-Al-Pb, Zn-Al and pure zinc bath.   In case of pure Zn, Zn -0.20Al and Zn-0.20-0.10Pb coated sheets, 100% white rust format ion takes place within 6 hrs, 14 hrs and 18 hrs. respectively, wh ile in case of Zn-Mg-Al-Sb coated 100% white rust coverage on the sheet surface happens only after 32 hrs. of exposure. This happens because Zn-Mg coatings inhibit the cathodic reaction and thus improve the corrosion resistance. The usual zinc carbonate hydroxide and zinc o xide observed in Zn-0.20Al and pure Zn coatings are suppressed in the presence of magnesiu m in the coating, and the whole surface is covered with zinc chlo ride hydro xide, which is protective for the coating layer.

Formability property of Zn-Mg Coated Sheets
The formability characteristics of the newly developed coated sheets were quantified in terms of erichsen cup value of the composite. In the simulated GI sheets, erichsen cup value (Ev) of the co mposite varied in the range of 8.8 mm to 10.4 mm depending on the bath composition and processing conditions of the sheet. Effect of bath temperature and dipping time on the Ev value of the coated sheets has been shown in Fig.5.
It can be seen that with increase in dipping time (for a particular bath composition and bath temperature), Ev value decreases slightly. This may be attributed to the fact that with increase in residence time, the thickness of alloy layer (Fe-Zn intermetallics), increases slightly. However, with increase in bath temperature (for a particular bath composition and dipping time), Ev value imp roves. Generally, formability of the galvanized sheets decreases with increase in bath temp., as increase in the bath temperature is generally associated with enhancement in the kinetics of the formation of Fe-Zn IM Cs. However, in this case, due to presence of adequate Si (0.08%) and A l (~0.25%) in the zinc bath, tendency for the format ion of Fe-Zn IMC gets significantly reduced (as shown in Fig.6). and this leads to improvement in formab ility characteristics of the coated sheets.   In order to assess the forming capability of Zn-Mg coated sheets in actual stamping operation, grid marking of coated sheets were done and subsequently these sheets tested in Erichsen cup tester. The coated sheets were deformed till fracturing of the co mposite. St rain level near the fracture zone was measured through transparent scale. It has been found that newly developed Zn-Mg coated sheets can be Coating Peel-off No Peel-off safely deformed upto 30% strain. No cracks were visib le even at fracturing of the co mposite. However, in conventional zinc coated sheets, even at 25% strain level cracks can be seen in the coating before the fracturing of the composite (as shown in Fig.7.).

Assessment of Coating Adherence
Coating adherence of the Zn-Mg coated sheets was assessed through Lock Forming Tester. Most of the sheets man ifested excellent coating adherence (as shown in Fig.8), no cracks were observed near the bend portion. However, in some cases, minor cracks were observed. On examining these samples, it has been found that most of these samples were processed at lower bath temperature (420 o C) for higher d ipping time (4.0 sec.). Due to lower solubility of Si at this temperature and higher residence time of the strip at this temperature, some formation of Fe-Zn IMC does take place and this leads to poor coating adherence. In some of the samples, poor coating adherence was observed due to entrapment of Sb containing exogenous compound (having co mposition Sb : 75.38%, Mg : 18.85%, Zn : 5.76%), as shown in Fig.9. Th is must have happened due to improper mixing of Sb (due to its higher melt ing point) in the zinc bath.

Conclusions
Based on the work carried out, following conclusions can be drawn : • Addition of Mg (0.50-0.75%) and Sb (~0.08%) in the zinc bath results in substantial improvement in corrosion resistant properties of the zinc coated sheet due to formation of Zn/Al/Mg eutectic mixture at grain boundaries and distribution of Sb within the coating.
• Si (~0.08%) addition along with Al (~0.25%) in the bath leads to complete suppression of the format ion brittle Fe-Zn intermetallics at the coating-steel interface, thereby resulting in significant imp rovement in the coating adherence and formability properties of Zn-Mg coatings.
• Zn-0.50%Mg-0.25%A l-0.08%Si coated steel sheets with imp roved corrosion resistance (1/3 rd w.r.t conventional Zn coated sheets), formab ility properties (at par with substrate material) and coating adherence (as per LFQ standard) with bright and smooth appearance have been successfully developed in the laboratory. constant support and encouragement during the course of work acco mplished under this project.