Corrosion Inhibition of Magnesium in Safe and Unsafe Electrolytes

 Corrosion inhibition of magnesium in safe and unsafe electrolytes 

Experimental work 

 Materials 

The material used in this study was 99.54 % magnesium.  It was received in blocks. The block was cut into specimens with an area of 1 cm2 cross section and 1 cm height. The analysis of the used magnesium was conducted by X-ray fluorescence (XRF) 

Specimen preparation:    

 Specimens with 1cm height and 1cm2 cross-sectional area was mounted in epoxy resin and the surface of the specimens were grounded with emery paper up to 2000 grit, the specimens were carefully cleaned with distilled water rinsed with acetone and dried under air.

Anodizing electrolytes 

 The electrolyte was prepared at room temperature with distilled water with 3M potassium hydroxide, 1M Sodium Silicate and 0.6M potassium silicate concentrations as PH of 13. 

Anodizing Cell Anodizing process was carried out at room temperature using a two-electrode cell and a DC power supply figure 3.2. The electrolytes of 200 ml volume were contained in a glass cell. The anode was the magnesium specimen while the cathode was a stainless steel bar. The samples were anodized under constant voltage of 5 V for 10, 20 , 30 , 40 , 50 minutes. The film formed on the surface was rinsed with distilled water and dried in warm air. 

Results and Discussion

 Contact angle measurements

 The contact angle was measured at different anodizing times for 3M potassium hydroxide, 1M sodium silicate, and 0.6M potassium silicate solutions. As indicated in the figure  it reaches 114^o,105^o,113^o for 30 minutes anodizing in 3M potassium hydroxide, 1M sodium silicate, 0.6M potassium silicate respectively. That proves that a hydrophobic layer is formed on the surface of the substrate.   

                               The effect of anodizing time on the contact angle

Polarization measurements

Results of corrosion rate determination by linear polarization techniques table (4.1) are in good agreement with contact angle measurements  i.e the condition which gives the highest contact angle showed the lowest corrosion rate. Polarization  curves figure (4.2)  of the specimens anodized in 3M potassium hydroxide, 0.6M potassium silicate, 1M sodium silicate solutions for 10, 20, 30, 40, and 50 minutes were shifted to the positive direction  i.e it became nobler, and at the same time shifted to the left relative to the unanodized specimen indicating  less dissolution.

The electrochemical corrosion parameters which are listed in the table (4.1) indicate that the corrosion rate decreased from 36.8 mpy for pure Magnesium to 7 mpy with an efficiency of 81.02% for 30 minutes anodizing in 3M potassium hydroxide electrolyte and decreased to 2.27 mpy  with an efficiency of 93.84% for 30 minutes anodizing in 1M sodium silicate electrolyte also decreased to 1.38 mpy  with an efficiency of 96.25% for 30 minutes anodizing in 0.6M potassium silicate electrolyte. 

Potassium silicate electrolyte has given us better corrosion resistance than using sodium silicate electrolyte besides having good safe environmentally friendly properties.

Potentiodynamic polarization curves  for pure magnesium and anodized specimens  in  (a)3M potassium hydroxide (b)1M sodium silicate (c)0.6M potassium silicate electrolytes for 10,20,30,40 and 50 minutes. 

Results of linear polarization experiments for anodized specimens in 3M potassium hydroxide,1M sodium silicate and 0.6M potassium silicate electrolytes for 10,20,30,40 and 50 minutes . 

 

X-ray Diffraction  :

Figure (4.3) illustrates an X-ray diffraction patterns results of  formed films after anodizing for 30 minutes using anodizing electrolytes  3M potassium hydroxide, 1M sodium silicate and 0.6M potassium silicate  respectively .

As shown in figure (4.3) MgO and Mg(OH)₂  were the main phases formed during anodizing; It is clear that the MgO amount was larger than that of Mg(OH)₂. The intensity of the peaks of MgO produced in the potassium hydroxide solution was strong owing to the thick film produced. While in the case of 1M Sodium Silicate solution the dominating phase is Mg₂SiO₄ (2MgO.SiO₂).  For 0.6M potassium silicate solution the dominating phase is also  Mg₂SiO₄ (2MgO.SiO₂). Both anodized films using sodium silicate and potassium silicate are partially structured with a glassy morphology as indicated in the figure. The intensity of the peaks of Mg₂SiO₄(2MgO.SiO₂) produced in sodium silicate solution was similar to that produced in 0.6M potassium silicate solution but for 0.6M potassium silicate solution some peaks show amorphous structure.

The occurrence of these phases indicate that the substrate and the solution both contribute to forme the anodic film.

a

b

c

 XRD spectra of 30 minutes anodized specimens (a) 3M potassium hydroxide electrolyte  (b)1M sodium silicate electrolyte  (c) 0.6M potassium silicate electrolyte.

Film thickness measurements

Figure (4.4) shows the variation of film thickness with time using the three different electrolytes 3M potassium hydroxide, 1M sodium silicate, and 0.6M potassium silicate electrolytes. The film thickness for the three solutions increases with the anodizing time rapidly in the first 30 minutes with a slow rate after 30 to 40 minutes. This behavior is due to the increase in the thickness of the anodized layer acting as barrier to the flow of current which decreases the rate of oxidation of magnesium.

Effect of anodizing time on film thickness by using 3M potassium hydroxide, 1M sodium silicate, and 0.6M potassium silicate electrolytes at a constant voltage.

From the results, the optimum deposition time used to obtain maximum thickness was 30 minutes for 3M potassium hydroxide, 1Msodium silicate, 0.6M potassium silicate electrolytes.

The thickness of the anodic film changed according to the type of the electrolyte figure(4.5). The average thicknesses of formed anodic films which achieve the lowest corrosion rate  of 3M potassium hydroxide, 1M Sodium Silicate and 1M Silicate  were measured using an optical microscope and Posi Test DFT Combo are  47 µm, 26 µm, and 48 µm respectively. Films produced in the 0.6M potassium silicate electrolyte were the thickest in comparison with  those  produced in 3M potassium hydroxide solution and 1M Sodium Silicate electrolytes for the same volt 5V.

                                                                  a

b

c

The film thickness of 30 minutes anodized specimens using(a)3M potassium hydroxide electrolyte (b)1M sodium silicate electrolyte (c) 0.6M potassium silicate .

Surface Morphology and EDX results

Figure(4.6 -a) shows the SEM for unanodized polished specimens. The surface morphology of the anodic film figure (4.6 -b) reveals a homogenous distribution of oxides produced in the KOH electrolyte. It contains dark spots which indicate the anodizing process deposited the oxides MgO on the surface of the metal. No cracks were observed in the anodized layer. Film produced in sodium silicate solution in figure (4.6 -c) shows a heterogeneous structure with microcracks in the outer layer of the film. Some domains of the anodized layer have amorphous structures and others have ordinary one.  It was observed that the Layer produced in potassium silicate solution has more domains of amorphous structures than those produced in the layer of sodium silicate solution. The layer consists of a mixture of both amorphous and ordinary domains causes more micro-cracks on the outer layer on the surface figure 

                                                                       a

                                                                       b

                                                                        c

                                                                      d

Surface morphology and spectrum analysis of (a)pure magnesium (b)30minute anodized specimen in 3MKOH electrolyte (c)30mintes anodized specimen in 1M sodium silicate electrolyte (d)30minutes anodized specimen in 0.6M potassium silicate electrolyte.

Microhardness measurements

Shore hardness test indicates that the microhardness for the produced film using 3M potassium hydroxide, 0.6M potassium silicate, and 1M sodium silicate solutions are 97(85.6RHN), 88(83.4RHN), and 99(86RHN) respectively.

Film adhesion testing  

The analysis of the anodic obtained film layers showed that they contain Mg- MgO – Mg₂SiO₄. This indicates that the metallic substrate has diffused into the anodized film layer resulting in further enhancement of the adhesion. Adhesion was measured according to ASTM D 3359 method 13, thickness <125 µm. The adhesion of the anodic films is classified as 3B which denote that the amount of the layer released  by the adhesive tape lies from 5-15% of the formed layer.

Mechanism of the anodizing process:

  Mechanism of the anodizing process could be explained as dissolution and oxygen evolution process. Anions in the electrolyte first need to reach at the anode/electrolyte interface and then enter into anodic coatings.

The general reactions using 3M KOH solution in anodizing process for Mg are as follows:

Mg → Mg⁺² + 2e^-                   (1)

4OH^-→ O₂↑+ 2H₂O +4e^-       (2)

Mg⁺² + 2OH →Mg(OH)₂       (3)

Mg(OH)₂ → MgO + H₂O       (4)

The  general reactions occurring in the anodizing process using 1mole sodium silicate and 0.6-mole potassium silicate for Mg is as follows

 Mg → Mg⁺² + 2 e^-  (1)

 4OHˉ → O₂ ↑ + 2H₂ O + 4 e^-   (2)

  Mg⁺² + 2OHˉ → Mg(OH)₂          (3)

  Mg(OH)₂ → MgO + H₂O      (4)            

 Mg(OH)₂ + SiO₂ → Mg₂SiO₄ (2MgO.SiO₂)    (5)

During the oxidation process, the Mg ions, produced by reaction(1) combine with the OH^- in the electrolyte solution to form Mg(OH)₂ and Mg₂SiO₄(2MgO.SiO₂) reactions(3) and (5) , respectively. The hydroxides change to oxide compounds by the dehydration process, reaction(4). The film formation processes, reactions(3),(4), and(5) may be promoted by a high concentration of the electrolyte, containing more SiO₂ and OH^- ions.

Conclusions

·      The corrosion rate of pure magnesium  decreases from about 37 mpy to be 7, 2,1.4 mpy  with efficiency of 81% , 93.84% , 96.25%  for 30 minutes anodized specimens using   3M potassium hydroxide,1M sodium silicate, and 0.6M potassium silicate electrolytes respectively.

·      Safe electrolytes (sodium silicate and potassium silicate ) used have shown better corrosion resistance than using unsafe electrolytes (potassium hydroxide).

·      XRD diffraction & EDX confirms that for 30 minutes anodized specimens the film is mainly consists of magnesium oxide using 3M potassium hydroxide electrolyte and mainly consists of magnesium silicate Mg₂SiO₄ with a partially amorphous structure using 1M sodium silicate and 0.6M potassium silicate. 

·      The contact angle  of  pure magnesium changed from 70⁰ (hydrophilic) to  114^o,105^o,113^o  (hydrophobic) for 30 minutes anodized specimens using 3M potassium hydroxide, 1M sodium silicate, and 0.6M potassium silicate electrolytes respectively.

·      Film thickness reaches  48, 26, 47µm for   30 minutes anodized specimens in 3M potassium hydroxide, 0.6M potassium silicate, and 1M sodium silicate electrolytes respectively. 

·      Hardness of the anodic films have increased from 38 to 85.6, 83.4, and 86 RHN for 30 minutes anodized specimens in 3M potassium hydroxide, 0.6M potassium silicate, and 1M sodium silicate electrolytes respectively.

·      According to ASTM D3359 method 13 for thickness <125 µm .The adhesion of 30 minutes anodic films using the three electrolytes are classified as 3B which denote that the amount of the layer released  by the adhesive tape lies from 5-15% of the formed layer.