Nanoparticles: Properties, applications and toxicities:

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.

Preparing large single crystal of metals .

Methods to prepare a large single crystals of metals  has two main ways :

1- Solidification from the melt .

2- Grain growth in the solid state .

1- solidification from the melt :

Czochralski method :

A seed crystal is gradually withdrawn vertically from the surface of a molten metal leads to building a crystal rod with an orientation to the seed and the crystals produced are irregular in the cross section and this result in no contact with the mould , so contamination is reduced. This technique is used to produce germanium and silicon crystals for transistors . It's very useful for silicon which is very reactive at the melting point 1412 degree centigrade . 

Figure 1 : Czochralski method , modern single crystal growing apparatus.

The crystal rod is rotated to ensure uniform cross section and to get homogeneity in the dispersion of the alloying elements. The orientation of the resulting crystal is controlled by a seed crystal of the required orientation . 

In the Bridgeman method , the metal content is lowered through a vertical furnace with a temperature gradient and the crystal is nucleated at the end and grows upward in the content . the metal content i.e. the mould made from pure graphite when using factory metals such as Aluminum , copper , silver , nickel. .But low melting point metals such as tin ,zinc and cadmium are grown up in heat resisting glass tubes . Metals which react with carbon alumina is used .

figure 2 Bridgeman furnace .

Single crystals can be produced in horizontal furnaces and the mould is graphite . 

Another method is the application of the zone melting technique where a molten zone is moved from one end of the bar to another which is also a method of purification .

Crystals can also be produced vertically by the floating zone method in which the metal is not in contact with a container using cold grips to hold the specimen and a molten zone is produced by high frequency heating or electron bombarding from a filament in a high purity argon atmosphere or good vacuum the length of the zone being proportional to the square root of the surface tension to the  square root of the density of the metal  this method is used in high melting point metals like  tungsten, tantalum , molybdenum ,vanadium , nickel , rhenium , etc. 

Grain growth in the solid state:

The most widely used method is the strain-anneal technique.

A fine grained annealed specimen is is strained by 1- 2 percent in tension then annealed at a gradually increasing temperature. Nucleation and growth of a few and often one grain and this grain absorb all strains during annealing . In this way single crystal rods of aluminum and some of its alloys can be grown up to 50 cm long by a temperature gradient 20 degree per one cm for pure aluminum and for some aluminum alloys 1000 degree per cm . Most metals which has phase transformation producing single crystal by slow cooling through the transformation like iron , titanium , uranium and zirconium .

See

 

                                                                       

Corrosion Inhibition of Magnesium by anodizing in safe and unsafe electrolytes :#corrosion inhibition#

Corrosion Inhibition of Magnesium by anodizing in safe and unsafe electrolytes :

to view the article click on the following link below .

https://independent.academia.edu/shaimaaaliabouelela

#corrosion#Magnesium#Anodizing#

#Anodizing#

How to determine corrosion rate using potentiodynamic and linear polarization techniques. #corrosion rate#

Best tips for determining the corrosion rate. 
What are the corrosion specialist doesn't tell you , their secrets in calculating corrosion rates . 
Why most researchers got confused when using potentiodynamic polarization technique for determining the corrosion rate  ?

Author : Shaimaa Ali Abou El Ela .(material science engineer)

Author e.mail : shaimaaaliabouelela@gmail.com

Faculty of petroleum and mining engineer , metallurgy and material science department .

Corrosion is happening every where . I found most of metallurgical engineers doesn't care about learning corrosion and how to protect against corrosion . Usually most of them care about learning about alloys manufacturing . So I decided to put an explanation about the most confusing points about corrosion .
 For example corrosion rate , how to determine it precisely , how to change units from mpy to mmpy or vice versa . Of course I will explain some tips of how to calculate the corrosion rate from different techniques .

 First of all most people think that the best way of calculating corrosion rates by using potentiodynamic polarization technique but NOW I am telling you please don't use this technique if you want to get the accurate corrosion rate , because both cathodic an anodic lines can get interrupted by for example resistance , concentration or activation polarization or the tested material is having a passivation behavior . These factors will really interrupt your Tafel slopes and the tangents will intersect in a wrong point which will show a wrong corrosion current . So to avoid this using linear polarization technique will help you to draw your tangent precisely  . Figure (1)  show that the cathodic and anodic tangents in potentiodynamic curves (V-log I) gave wrong deduction in calculating the corrosion rate . But in figure (2) the same experiment was redrawn using V-I chart (linear) and the corrosion rate was calculated precisely and the results was accurate .  
 But note that you can't use the same sample for both techniques . The sample which is used in potentiodynamic polarization technique experiment shouldn't be used again when holding an another experiment with a linear polarization technique because the surface was highly accelerated to get corroded so the obtained corrosion rate from the second experiment will be so high . So please avoid this mistake . 
The most important step you have to make before starting your experiment is waiting for about 15 minutes . Let your sample after connecting it in your corrosion cell to reach equilibrium this will give you the most accurate corrosion rate .

figure (1)

Figure (2)