The chemical composition of the zinc bath can strongly influence on the hot-dip galvanized coatings. In this work, the effects of tin addition on the surface morphology, and the corrosion resistance of hot-dip galvanized steel were investigated. The corrosion behavior of steel samples galvanized with zinc and Zn-Sn alloys containing different wt% Sn was analyzed by various corrosion tests such as potentiodynamic polarization Tafel lines and electrochemical impedance spectroscopy (EIS) techniques. Salt spray test was employed in order to study the corrosion products of the specimens. Surface morphology, the composition of coating layers and nature of the corrosion products were also investigated using field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX) and X-Ray diffraction (XRD) techniques, respectively. The results indicated that the addition of small amounts of Sn (0.1 wt%) to the molten zinc galvanizing bath can improve the corrosion resistance of hot-dip galvanized steel.
Metals play an important role in our everyday life. Steel has always been the most favored and widely used material because of their excellent strength, formability, the economics of production, and the ability to recycle indefinitely. However, it tends to react with agents in the atmosphere to form stable bonds―ferrous oxides and salts. The rate of these reactions depends on the nature and concentration of the corrosive agents present in the environment. Hence, most of the energies and research have been involved in reducing corrosion of steel. The most popular and most widely process used for corrosion protection of steel is galvanizing [
In hot dip galvanizing process, several parameters such as chemical composition of carbon steel and chemical composition of coating bath could affect the microstructure and properties of coatings [
The effect of lead on the galvanizing process has been studied by F. C. Porter, et al. [
A great attention is considered with respect to the role of aluminum (Al) in a Zn galvanizing bath. The effect of Al content on the structure and properties of the alloy layers have been investigated by Borzillo A.R et al. [
Additionally, the presence of Sb in the galvanizing bath has also led to improve the intergranular corrosion resistance [
The goal of this research is to develop a new approach to improve the corrosion resistance of the galvanized pieces. To address this issue, different concentrations of Sn were added to the Zn galvanized bath and the effect of Sn addition on the galvanized samples was investigated. The structure of the galvanized layers and corrosion resistance of the Zn deposits in 3.5% NaCl solution has been studied.
Low carbon steel with thickness 0.6 mm was used in the experiment procedure. The chemical composition of the steel used in this study is shown in
Element % | C | Mn | P | Si | Fe |
---|---|---|---|---|---|
Low carbon steel (mild steel) | 0.15% | 0.6% | 0.028% | 0.08% | Balanced |
The steel substrate was received in the form of sheets. Each sheet was cut into small specimens with dimensions 100 × 50 × 0.6 mm. The samples were mechanically treated by emery paper of different grades up to 1200 grit to produce smooth surface without any scratch, rinsed with deionized water, cleaned ultrasonically in acetone, and finally dried in a stream of warm air.
The chemical treatment is used to clean the metal surface from oils, impurities, soap residues and the natural oxide film, if required. This pretreatment includes the following steps:
1) Degreasing: The steel sheets were degreased for 5min. in a hot alkaline degreaser at 70˚C for 5 - 10 min. The chemical composition of the degreaser bath containing 20 g∙L−1 Na3PO4, 20 g∙L−1 NaOH, 20 g∙L−1 Na2CO3 and surfactant. Oil, grease and dirt were properly removed. After this stage, the specimen was rinsed in running water then in distilled water.
2) Pickling: The steel sheets were pickled for 3min. at room temperature in 10% HCl containing inhibitor 50 ppm Cetyl trimethyl ammonium bromide (CTAB) for the protection of metal surfaces and then the steel sheets were rinsed in water. The pickling process was used to remove any oxide from the steel surface.
3) Fluxing: The acid-cleaned steel sample is immersed in a flux solution maintained at about 65˚C for approximately 5 min. in a flux bath consisting of 500 g∙L−1 of ZnCl2, 50 g∙L−1 NH4Cl, and 25 mL of HCl (laboratory grade) to maintain the pH level of 4 - 5, then drying at 120˚C before dipping in the molten Zn bath. The flux layer protects the cleaned metal surface from oxidation.
The zinc bath consists of zinc (purity of Zn 99.93 wt%, from Egyptian Minerals Company) containing also Sn with different wt% ranged from 0.1 to 0.5. The sheets dipped in the zinc bath for 40 sec. and operated at 460˚C ± 5˚C, then withdrawn at a controlled rate (2 cm∙s−1) and carries with it an outer layer of molten zinc which solidifies to form the relatively pure outer zinc coating. When the cleaned and fluxed steel surface contacts the molten zinc of the galvanizing bath, the protective flux layer is removed leaving a clean steel surface which is immediately wetted by the zinc, this results in a reaction between zinc and steel with the formation of zinc-iron alloy layers. The galvanizing experiments were carried out in an electrically heated crucible furnace. The furnace has the following characteristics: 240 Volts, 2.6 kW, 11 Amp, 50/60 Hz and the temperature range is 1200˚C and has a graphite crucible. In order to have a better monitoring and control of the bath temperature, a K-type thermocouple was provided inside the furnace. A temperature-control console was provided with the temperature indicator.
The thickness of coatings was determined by electromagnetic thickness gauge (Thickness gauge 6000-N4). The thickness was measured in five different places on the specimen and the average value was taken.
The structures and the elemental analysis of the galvanized samples at different operating conditions were observed by Field emission scanning electron microscopy (FE-SEM, QUANTA EG) equipped with energy dispersive X-ray system (EDX), respectively.
XRD patterns were obtained over the diffraction ang∙Le range (2θ) of 4˚ - 80˚ using an XRD (X’Pert PRO― PANalytical, Netherlands) diffractometer with Cu Kα (λ = 1.5404 Å) radiation at a generator voltage of 45 kV and a generator current of 30 mA, with a step size of 0.02˚ and a scan speed of 0.05 s−1.
Three different corrosion tests were carried out by potentiodynamic polarization Tafel line; electrochemical impedance spectroscopy (EIS) and salt spray tests.
The electrochemical measurements were performed using an IVIUMSTAT potentiostat-galvanostat operated under computer control. Electrochemical measurements were obtained in 3.5% NaCl solution at 30˚C using a three-electrode electrochemical cell. Before the measurements, the working electrode was degreased with acetone and rinsed with distilled water. The reference electrode that all potentials are referred was Hg/Hg2Cl2/Cl− saturated calomel electrode (SCE) of Eo = 240 mV versus normal hydrogen electrode (NHE) and Pt wire was used as counter electrode. The time?voltage was recorded until the steady state potential was reached which is the open circuit potential (Ecorr), the measurement was started from this potential first in the cathodic direction and then in the anodic direction.
EIS was carried out in the frequency range of 35 kHz - 100 mHz with an amplitude of 5 mV sinusoidal potential using ac signals at the open circuit potential in 3.5% NaCl media. The electrode potential was allowed to stabilize for 60 min before starting the measurement. All experiments were carried out in aerated solutions at a room temperature.
Neutral salt spray test was performed according to ASTM B117 standard (ASTM B117, 2007) using ASCOT cabinet. The salt spray test was used to investigate the corrosion behavior of steel and galvanized steel in 3.5% NaCl solution at room temperature.
The effect of Sn concentrations in the molten Zn bath on the coating thickness and the thickness of the alloyed layers have been investigated as shown in
Sn wt(%) | 0 | 0.1 | 0.3 | 0.5 | |
---|---|---|---|---|---|
Thickness of alloyed layers ( µm) | Gamma (Γ) | - | 3 | 3 | 8 |
Delta (d) | 27 | 20 | 12 | - | |
Zeta(ζ) | 33 | 28 | 24 | 38 | |
Eta (η) | 24 | 35 | 53 | 92 | |
Average coating thickness (µm) | 84 | 86 | 92 | 138.00 | |
Standard deviation (S) | 1.09 | 1.34 | 0.95 | 1.08 |
The third intermetallic layer is a z having 3.76 wt% Sn and 17 wt% Fe. The outer layer h having 1.96 wt% Sn and 8.84 wt% Fe and has a fine structure (
Tafel polarization, electrochemical impedance and salt spray studies were performed to evaluate the corrosion behavior of hot dip galvanized steel.
The corrosion performance of galvanized steel with Zn-Sn alloys containing different Sn wt% and compared with other galvanized steel was represented in
Tin concentration (%) | Ecorr (V) | Icorr (Acm−2) | Rp (Ωcm2) | ba (Vdec−1) | bc (Vdec−1) | Crate (mmy−1) |
---|---|---|---|---|---|---|
Pure Galvanized | −1.32 | 1.727E−05 | 2171 | 0.21 | 0.15 | 1.02 |
+0.2 Al | −1.28 | 8.88 E−06 | 3751 | 0.16 | 0.15 | 0.54 |
0.1 | −1.25 | 4.27E−07 | 5.316E+04 | 0.114 | 0.097 | 0.03 |
0.2 | −1.1782 | 1.686E−06 | 1.594E+04 | 0.116 | 0.133 | 0.10 |
0.3 | −1.3153 | 1.259E−05 | 0.2603E+04 | 0.184 | 0.128 | 0. 74 |
0.5 | −1.3074 | 1.146E−05 | 0.3042E+04 | 0.186 | 0.141 | 0. 68 |
EIS was performed to test the corrosion behavior of all the investigated galvanized steel. The effect of Sn wt% on the corrosion resistance of Zn-Sn coated steel sheet and compared with other galvanized steel was also measured through EIS test as illustrated in
Parameters | Traditional Zn | Traditional Zn + 0.2 Al | Zn + 0.1 Sn | Zn+ 0.2 Sn | Zn | Zn | Zn |
---|---|---|---|---|---|---|---|
+ 0.3 Sn | + 0.4 Sn | + 0.5 Sn | |||||
Rs(Ωcm2) | 4.399E−01 | 5.33E+01 | 7.5E+02 | 7.01E+01 | 7.69E+01 | 8.37E+01 | 1.48E+02 |
R1(Ωcm2) | 1.334E+03 | 9.51E+02 | 3.18E+03 | 5.63E+02 | 5.68E+02 | 3.22E+02 | 1.699E+02 |
R2(Ωcm2) | - | 5.72E+03 | - | 4.02E+02 | - | 1.79E+02 | |
C1(Fcm-2) | 1.27E-04 | 3.08E−05 | 5.76 E−05 | 5.59E-05 | 1.82E−05 | 2.89E−03 | 1.41E−05 |
C2(Fcm-2) | - | - | 1.94E−04 | - | 1.55E−04 | - | 6.34E−03 |
W | 5.25E+03 | - | - | 4.68E−05 | - | - | - |
Error | 1.34E−07 | 5.23E−09 | 6.4E−08 | 1.75E−08 | 9.90E−09 | 5.66E−07 | 5.42E−09 |
n | 0.90 | 0.98 | 0.90 | 0.97 | 0.95 | 0.98 | 0.96 |
time constant on Zn layer is attributed to the compact inner layer of surface oxide (corrosion product) compounds which can be partially protective, R1 and C1 were associated to the coating Layers impedance represented by resistive (Rc) and capacitive (Cc) elements. While, the second time is linked with the double layer capacitance (Cdl) at the electrolyte/coated surface interface and the charge transfer resistance of the coated layer (Rct), respectively [
where, e0 is the permittivity of the free space, er is the relative permittivity or coating dielectric constant, A is the coating surface area and d its thickness. The total polarization resistance (Rp) is the sum of Rc, Rct. In particular, this coating has higher Rp and lower Cdl value, and it follows the same trend as observed in polarization study. Additionally, it’s clearly that, EIS analysis of pure galvanizing coating indicates that a one-time constant is clearly observed as shown in
Salt spray test is the best method to predict the performance of hot-dip galvanized steel in various environments using ASTM B117 specification. The corrosion resistance of galvanized coatings produced from molten baths with different Sn wt% has been investigated using salt spray test. The samples are inserted into 3.5% NaCl salt solution at a constant room temperature, 100% Relative humidity, and 1 atmospheric pressure. Results of the salt spray test reveal that after 700 hrs. holding time, there was white rust (about 95%) and no red rust appeared on the surface for all galvanized samples produced, indicating a good corrosion resistance. This mean that by using salt spray test all the galvanized steel sheets without or with adding Sn exhibited good corrosion resistance and can’t estimate the corrosion rate to show the different between them.
To identify the phase composition of corrosion products of galvanized layer formed with molten Zn and Zn-0.1 wt% Sn alloy hot dip galvanized steel sheet, the XRD was conducted.
The role of the process parameters has been affecting the structural characteristics and the properties of the coating Layers. The results of this investigation suggest some general conclusions like: the presence of Sn in the molten Zn improves bath fluidity and leads to a better wettability of molten Zn-Sn alloy. Coatings obtained by Zn-Sn alloy are characterized by bright and smooth surfaces as traditional galvanizing coatings. The addition of Sn in the composition of the galvanized coating Layers led to producing a fine, uniform and defect free morphology. Moreover, the morphology of the alloyed layers with the addition of Sn has been changed. Sn was used as additive to enhance the corrosion resistance property of hot dip Zn coatings. Sn concentration must be kept at 0.1 wt% in the Zn bath to obtain the desired good corrosion resistance galvanized steel samples.
Z. Abdel Hamid,S. S. Abd El Rehim,A. Abou Shama,M. Ebrahim, (2016) Improvement the Corrosion Resistance for the Galvanized Steel by Adding Sn. Journal of Surface Engineered Materials and Advanced Technology,06,58-71. doi: 10.4236/jsemat.2016.62006