Formation of brucite-Mg(OH)2 films by electro-deposition process and their evaluation properties
Byung Gu, Kim
Department of Marine System Engineering
Graduate School of Korea Maritime University
Sea water which occupies three quarters of the earth is the strongest corrosive environment containing various kinds of chemical ions. For instance, Cl- ion in seawater accelerates metallic corrosion, on the other hand, other ions such as Mg2+ and Ca2+ can be very useful to protect metal from corrosion by means of electrochemical techniques. The cathodic protection is one of such methods. The interesting feature of cathodic protection is that Mg and Ca ions in seawater have capability to form calcareous deposits on metal surfaces. In general, cathodic current on metal applied by electro-deposition as cathodic protection-application principle is assumed to increase the OH- in the solution neighboring to the metal surface and the increase pH at metal / seawater interface cause precipitation of brucite crystal structure-Mg(OH)2 as following formula The properties of electro-deposition films change because the chemical reaction process is affected under various solution conditions. Here, the films which was made according to various dissolved gases in natural sea water were analyzed and evaluated. The study was tried to find optimum current density for making electro-deposition films by the AZ31(Mg alloy) anode which was selected through the above mentioned experiments.
- Electro-deposition films according to various gases The formation of electro-deposition material affect the stability of product according to sacrificial anodes by free-energy for cathodic protection. Therefore the study was carried out how does the various anodes make effects on the electro-deposition film.
- Electro-deposition films according to various current densities Mg2+ + 2OH- → Mg(OH)2. This is typically the main calcareous deposits-compound in electrodeposited coating films. The Mg(OH)2 film on metal are believed to promote a physical barrier against oxygen diffusion. Especially, it is considered that Mg(OH)2 films on steel lower the corrosion rate in seawater environment. However, both the slow depositions rate and a weak adhesion between deposit films and metal substrate remains a challenging task. Based upon this assumption, environment friendly electro-deposited Mg(OH)2 films were formed by applying cathodic protection on steel substrate submerged in various seawater conditions. The effect of anode and current density on deposition rate, composition structure and morphology of the deposited films were investigated by Scanning Electron Microscopy(SEM) and X-ray Diffraction(XRD), respectively, The adhesion and corrosion resistance of the coating films were evaluated in accordance with JIS code and by natural potential test respectively. From an experimental result, the electro-deposited film under using AZ31(Mg alloy) anode has the best physical properties because Mg(OH)2 layers are coordinated octahedral by O-H groups with the hydrogen pointing in the direction of the next layer. In addition, it is found that as cathodic current density goes up, the weight gain of electro-deposit films increases. However, the adhesion of electro-deposit prepared at 5 A/cm2 current density is better than under 8~10A/cm2 conditions. Finally, it is found that the formation time is reduced more than others in the condition that oxygen gas is added to the substrate.
Concludingly, the study for the improvement of adhesion, corrosion resistance and economic feasibility were proceeded as follows.
- Electro-deposited films according to the various sacrificial anodes