Intercritical heat treatment operation has been in use for the development of Dual Phase Steel (DPS) and has been found to improve the mechanical properties of the steel. In spite of the enhancement a limitation was however observed as to its corrosion susceptibility. In a bid to further enhance the corrosion resistance of the DPS while maintaining its mechanical properties, galvanealing operation was adopted which involving the immersion of the DPS into Al-Zn melt and subsequently subjecting it to annealing operation at 550 °C. Weight loss and linear polarization technique were used to measure or evaluate its resistance in 3.5% NaCl (a simulated marine environment). A minimum of 3 samples was used per immersion time. From the result, it was observed that there is a general sharp decrease in the corrosion rate of the GAS as compared to the control sample. The sample immersed and allowed to dwell in the melt for 20 seconds and further annealed with a soaking time of 20 minutes exhibited the highest corrosion resistance. The polarization curve also shows that the substrate was generally passivated, and this is as a result of the Al-Zn/Fe adhesiveness.
Galvaneal as a process is the result of the combined process of galvanizing and annealing to produce specialized steel substrate. The galvanization is made through the conventional hot-dipping process; and immediately as the stripe exits the coating, the zinc coating is subject to an in-line heat treatment that converts the entire coating to a zinc iron alloy [
Conversion to the alloy results in a non-spangle finish which makes the sheet suitable for painting after fabrication [
DPS has found thorough applications in automotive industries where low carbon steel has been processed to yield high strength to weight ratio [
In the quest for development of steel based materials for applications where high strength, ductility and corrosion are required, a lot of researches have been carried out on low carbon steel with lean alloying elements. This has led to the development of galvanneal steel strip/sheets. The steel compositions conventionally utilized for these purposes are low carbon microalloy steel (high strength low alloy steels) [
In the low carbon microalloy steel, the influence of process parameters on the adherence of the coating elements to enhance the corrosion properties has been well documented. Reumont and associate [
Many other authors have contributed through their findings to a greater understanding and application of galvanneal steel production but none has been reported on the use of medium carbon steel for structural application. This is due to a great extent on the complex nature of the transformation processes occurring during treatment of steels of medium carbon composition in the (α + ᴕ) phase region.
This research will focus on a comprehensive study of the transformation behavior of the structural steel in the dual phase region with a view to establishing optimum conditions for the production of a well defined galvanneal medium carbon with dual phase microstructures as reference.
Materials used for the construction of ship are stored at the dockyard, and from there, steel sample was selected for this experiment. The selected steel was firstly sparked with argon gas using spectrophotometric analyzer to determine the chemistry of the steel. The chemical composition is shown in
Some of the equipments used in the course of this experiment include:
- Hacksaw, Bench vice: This was used to clamp and cut the steel to predetermined sizes.
- Muffle furnace: This served dual purpose in the experiment. Firstly, it was used in the melting of the aluminum and zinc to form Al-Zn melt into which the steel sample was immersed for coating; and secondly, it serves in the immediate subsequent annealing operation.
Element (wt%) | C | Si | S | P | Mn | Ni | Mo | V | Cu |
---|---|---|---|---|---|---|---|---|---|
Composition | 0.3007 | 0.1029 | 0.0064 | 0.0352 | 0.8022 | 0.1120 | 0.0095 | 0.0046 | 0.4605 |
Elements (wt%) | W | As | Sn | Co | Al | Pb | Zn | Fe | |
Composition | 0.0022 | 0.0114 | 0.0083 | 0.0089 | 0.0037 | 0.0061 | 0.0045 | Bal. |
Element (wt%) | Si | Fe | Cu | Mn | Mg | Cr | Ti | Ca | Al |
---|---|---|---|---|---|---|---|---|---|
Composition | 0.4430 | 0.1638 | 0.0041 | 0.0132 | 0.5835 | 0.0024 | 0.0078 | 0.0002 | 98.751 |
- Tong: this aids both the immersion and easy transportation of the samples at high temperature.
- Optical Microscope: This was used in viewing the microstructures of the developed galvannealed steel (GA Steel) prior to analysis, this is essential in order to study the developed structure and the interphase form.
・ Sample preparation: The sample was prepared by sourcing for steel scraps used in the ship dockyard, Lagos and it was machine into a corrosion coupon using hack saw and emery paper on a grinding machine. The grinding was done simultaneously alongside using water as coolant in order to avoid phase transformation of the steel which could ensue from the friction between the sample and the emery grits. The zinc and aluminum were also cut and measured to weight using weighing balance calibrated to 2 decimal places for high accuracy. Grinding of the steel surface was necessary to enhance its adhesiveness during coating.
・ Melting and immersion: 99.7% zinc was melted in 0.3% aluminum at 664˚C in a sequential order of the aluminum preceding zinc. The binary melt was held in the furnace to maintain its state. Hot-dipping was later adopted by immersing the prepared steel into the binary melt and allowed to soak for varied times of 5, 10, 15 and 20 seconds. This is to enhance thorough adhesiveness of the melt on the substrate which has been pre-heated at 400˚C. The immersed samples were immediately transferred to another muffle furnace maintained at 550˚C and allowed to soaked again for 10, 20, and 30 minutes. Four samples were used per treatment for both the microstructural and corrosion tests to be conducted.
・ Corrosion: The corrosion process of a galvannealed coating layer was done by making a solution of 3.5 wt% NaCl solution. The weight was measured at a regular interval of 3 days, before calculating the metal and the corrosion rate of the samples using the formula adapted from ASTM G1-G4 standard.
metal Loss = weight Loss ( g ) × K alloy Density ( cm − 3 ) × exposed Area (A)
Corrosion Rate = Weight Loss ( g ) × K AlloyDensity ( c m − 3 ) × Exposed Area ( A ) × Exposure Time (hr)
Polarization Measurement: The AutolabPotentiostat with Nova 2.0 software and with a potential of −0.2 mv to 1.2 mv at a scan rate of 0.01 mv was used to determine the polarization of the test sample in the solution using platinum counter electrode. The Autolabpotentiostat after carrying a Linear Voltammetry sweep test on the steel samples in different varying environments analyzed and recorded the corrosion parameters, such as the Icorr in area per centimeter square, Ecorr, in voltage and the corrosion rate in millimeter per year among other parameters.
Intercritical heat treatment operation, when carried out on mild steel, has been found to enhance the hardness and strength with reasonable ductility in comparison to other conventional heat treatment [
When the DPS was immersed with a dwell time of 10 seconds prior to annealing (see
A reverse trend was however observed in
This unusual gradual rise could be attributed to the formation of cracks ensued from the heterogeneity of the melt before annealing.
Linear polarization curve technique as shown in
polarization curves indicated that both anodic and cathodic reactions are passivated as a result of the adhesiveness between Al-Zn/Fe-substrate and the inherent corrosion resistance property of the Al-Zn.
In a bid to improve on the corrosion resistance of Dual Phase Steel (DPS) in marine environment, intercritical heat treatment technique was adopted for the development of the steel and further subjected to galvanealing operation.
Weight loss and linear polarization methods were used to evaluate the corrosion behavior. From the result, the following was observed:
・ A good adhesive property between the Al-Zn/Fe-Substrate was feasible as it contributes to the enhanced resistance recorded.
・ Corrosion rate of as low as 0.3 mm/yr was observed in comparison to the control sample which displays about 1.4 mm/yr rate.
・ Galvanealing operation did successfully shift the steel from corrosion to passivation region thus improving the activity life of the GAS when in use.
Thus, dual phase galvanealed steel (DP-GAS) is recommended for use in marine environment due to its unique combined mechanical properties and the current finding of improved corrosion resistance.
The authors of this article heartily appreciate the management of Tertiary Education Trust Fund (TET Fund) for sponsoring this research and to the management of Kogi State Polytechnic, Nigeria for their support.
The authors declare no conflicts of interest regarding the publication of this paper.
Momoh, I.M. and Dongo, I.E. (2019) Corrosion Behavior of Fe-Al-Zn Interphased Dual Phase Steel in Marine Environment. Journal of Minerals and Materials Characterization and Engineering, 7, 71-79. https://doi.org/10.4236/jmmce.2019.72006