Corrosion behavior of Graphene reinforced Al-12Si coated 7075 Aluminium alloy

3.1. Ball milling

The Al-Si powder mixed with 0.5 wt.% GNS and 1 wt.% GNS separately were ball milled using Planetary Mono mill Pulverisette 6 (Classic line). The powder mixture of Al-Si powder with GNS was first poured in a steel jar of 500 ml capacity and steel balls of 5 mm diameter weighing 300 g were adopted. The Ball-to-Powder ratio (BPR) was set at 5:1 and ball milling was carried out in dry media without any process agent. To obtain the homogeneous mixture, powder was first milled at 180 rpm for 0.5 hour and then subsequently till 5 hours. In order to avoid overheating and sticking of powders to ball, every milling cycle consisted of 5 minutes of milling and 10 minutes of halt. The microstructure was obtained for the samples collected at 0.5 hour, 1 hour and 5 hours of milling for both 0.5 wt.% GNS and 1 wt.% GNS. Also, the elemental composition was mapped to visualize the Al-Si and GNS mixing.

3.2. Plasma spray coating

The Al-Si powder reinforced with 0.5 wt.% and 1wt.% GNS was deposited on AA7075 substrate. Initially, the specimens were cleaned using acetone solution to decontaminate the same. Then, the grit sand blasting was followed with air pressure of 3 -5 kg/sq.cm to deoxidize and make the surface rough for better adhesion of the coating. Argon (Flow rate- 100-150 standard cubic feet per hour (scfh); pressure-100-120 psi) and hydrogen (Flow rate- 10-13 scfh; pressure < 50 psi) were the secondary gases used for spraying. The powder was fed at a rate of 100-120 g/min and at a spray distance of 150 mm.

3.3. Potentiodynamic polarization study

The potentiodynamic polarization studies were conducted using CHI660E-CH-instrument and with thoroughly cleaned AA7075 alloy specimen. The uncoated Al 7075 (1), Al-Si coated (without reinforcement) Al 7075 (2), 0.5 wt.% GNS (3) and 1 wt.% GNS (4) reinforced Al-Si coated 7075 aluminium alloy substrates acted as the working terminal with uncovered area of 1 cm² in natural sea water electrolyte having conductivity of 5.6 S/m (siemens per meter). The electrode set-up comprised of 7075 aluminium alloy as anode, platinum electrode as counter terminal and standard Calomel electrode as reference cathode. The polarization estimations were carried out and polarization bends were testified by changing the potential of cathode naturally with a scan rate of 0.01 V/s from –1.045 V to –0.645 V potential range. The corrosion potential, $E_{corr}$(V) against Current density, $I_{corr}$(A/m²) was plotted (Tafel-plot) using Origin Pro software. In order to obtain the corrosion potential ($E_{corr}$) and current density ($I_{corr}$) for each of the working terminal specimen, the anodic and cathodic branches were extrapolated in the Tafel Plots. The corrosion potential, current density and anodic-cathodic slope obtained from polarization measurements and Tafel plots were further adopted in atmospheric galvanic corrosion simulation.

3.4. Corrosion model

A one-dimensional numerical model was developed for bimetallic galvanic couple of AA7075 aluminium with AISI4340 steel with cathode to anode area ratio 1:1 using COMSOL Multiphysics 5.3 finite element package. The model geometry is as shown in Fig. 3.

Fig. 3Numerical model for Al70705-AISI4340 galvanic couple with governing equations and boundary conditions

The following assumptions were made with respect to electrolyte:

1) Electrolyte film solution is in equilibrium with salt particles

2) Electrolyte film at a given load is spread uniformly over the surface

In the current model, a one-dimensional Bezier curve was adopted to make the galvanic couple of AA7075-AISI4340. The thickness of the electrolyte film changes with respect to relative humidity and salt load density. The maximum film thickness was found at 100 % relative humidity [12].

The governing Nernst-Planck equation for transport of species $i$, is as shown in Eq. (1):

where, $N_i$ is the flux of $i$ species, $D$ is the diffusion coefficient, $c_i$is the concentration, $z$ is the charge, $u_i$is the mobility of species $i$, $F$ is the Faraday constant, $Ø$ is the electric potential and $V$ is the solvent velocity. As seen in the above Eq (1), species flux $N_i$ is function of diffusion, migration and convection.

For specifying the current distribution in electrolyte, two boundaries (1 and 2) were selected and electrolyte thickness and conductivity were defined. Electrolyte conductivity defines the dependence of current on potential gradient. The governing equation for electrolyte as per Ohms law was given by Eq. (2) and (3):

Aluminium alloy 7075 with electrode potential of –0.893 V is found to be less noble than AISI 4340 steel with electrode potential of –0.535 V of steel and hence acts as an anode. The oxidizing cell of AA7075 follows Butler-Volmer equation as given by Eq. (4) for electrode reaction kinetics:

where, $i_{loc}$ is instantaneous or local current density (A/m²), $i_0$ exchange current density (A/m²⁾, ${\alpha }_a$ is the anodic transfer coefficient, ${\alpha }_c$ is cathodic transfer coefficient, $R$ is ideal gas constant (1.986 cal/mole K), $T$ is temperature in Kelvin, $F$ is Faraday’s constant (96,500 C/mol) and $\eta$ is electrode over potential in volts

In case of AISI4340 steel which acts as cathode, oxygen reduction takes place on the electrode surface. This reaction is restricted by absorption of oxygen through electrolyte film. The limiting current density, $i_{lim,O_2}$ (A/m²) is a function of film thickness, diffusivity and solubility of oxygen as per Eq. (5):

The reduction cell of AISI 4340 steel follows Tafel equation. Local current density, $i_{loc}$ is determined as per Eq. (6):

A normal mesh size was adopted with maximum element size of 1.61, minimum element size of 7.2 µm and curvature factor 0.3. The total number of elements was 50 with element ratio of 10. After developing the mesh, the current distribution study was simulated with different parameter levels of load density and relative density. Relative humidity was varied from 80 % to 98 % while the salt load density was varied from 0.0005 to 0.007 kg/m². In the simulation, the maximum anode current density and average anode current density was analyzed as a function of relative humidity and salt load density.

4.1. Characterization of ball milled Al-Si/GNS powder

The SEM images of ball milled powders (Al-Si with 0.5 wt.% GNS reinforced; Al-Si with 1 wt.% GNS reinforced) is as shown in Fig. 4 and Fig. 5. It can be observed that Al-Si powder reinforced with grahene has undergone flattening and fracture with increase in milling time. This can be attributed to brittleness developed in silicon and graphene. Also, the shearing effect with the kinetic motion of balls and powder causes the change is shape and size of the powders. The average size of the powders was observed to be 45 µm before ball milling process and it reached to 200 µm by the end of 5 h of milling time. Fig. 6(a) shows EDAX of aluminium, silicon and GNS dispersed in Al-Si after 1 hour of milling time. It is observed that graphene is uniformly dispersed in the matrix.

Fig. 4Microstructure of Al-SI with 0.5wt.% GNS reinforced: a) Premixing for 0.5 h at 180 rpm; b) 1 h of milling; c) 5 h of milling

a)

b)

c)

Fig. 5Microstructure of Al-SI with 1wt.% GNS reinforced after a) 1 h of milling; b) 5h of milling

a)

b)

Fig. 6EDAX mapping of Al-SI with 1 wt.% GNS reinforced: a) Al dispersion; b) Si dispersion; c) GNS dispersion

4.2. Potentiodynamic polarization study

The potentiodynamic polarization study was conducted for coated and uncoated specimen in order to analyze the effect of coating on the corrosion behavior of the specimen in natural sea water medium. The Tafel plot is as shown in Fig. 7. It was observed that both cathodic and anodic branches were influenced by the GNS/Al-Si coating on AA7075 alloy. The presence of coating causes the changes in anodic dissolution of the metal and also the cathodic hydrogen reduction states of the metal. Also, the GNS/Al-Si coated specimen exhibited a significant shift towards positive state (noble state). The $E_{corr}$ for uncoated specimen was obtained as –0.893 V while Al-Si with 0.5 wt.% GNS reinforced specimen exhibited –0.761 V. Also, the current density ($i_{corr}$₎ of AA7075 uncoated specimen got reduced from 1.3×10^-4 to 9.78×10^-6 with the presence of GNS/Al-Si coating.

The polarization parameters obtained from Tafel curve is as shown in Table 2. It was noted that higher corrosion potential lowered the tendency of the metal corrosion. Thus, GNS reinforced coatings improved the corrosion potential of AA7075. Even though the coating with 1 wt.% GNS did not show a significant improvement, 0.5 wt.% GNS reinforced Al-Si exhibited improved corrosion resistance. Also, with the decrease in corrosion current density, the corrosion dynamicity got decreased. A minimum corrosion current density of 9.78×10^-6 was shown by 0.5 wt.% GNS reinforced Al-Si coatings.

Fig. 7Tafel Plot for coated and uncoated specimens

4.3. Atmospheric galvanic corrosion

4.3.2. Electrode potential vs. reference electrode

The electrode potential ($V$) for uncoated and GNS/Al-Si coated specimens at 0.007 kg/m² load density and 98 % relative density is depicted in Fig. 9(a-d). The electrode potential obtained as a function of load density and relative humidity is as shown in Table 3. It was observed that the electrode potential increased with increase in load density and relative humidity. While, the highest electrode potential was observed for Al-Si with 0.5 wt.% GNS reinforced specimen at 0.007 kg/m² load density and 98 % humidity, the least electrode potential was observed for uncoated specimen at 0.0005 kg/m² load density and 80 % relative humidity.

Fig. 8Electrode potential of a) uncoated specimen, b) Al-Si coated specimen, c) Al-Si with 0.5 wt% GNS reinforced coated specimen, d) Al-Si with 1wt.% GNS reinforced coated specimen

a)

b)

c)

d)

Fig. 9Electrode potential of a) uncoated specimen, b) Al-Si coated specimen, c) Al-Si with 0.5 wt% GNS reinforced coated specimen, d) Al-Si with 1wt.% GNS reinforced coated specimen

a)

b)

c)

d)

Table 3Electrode potential (V) vs reference electrode (coated and uncoated)

Load density (kg/m2)	Relative humidity	Electrode potential vs. Reference Electrode (V)
		Uncoated		Al-Si		Al-Si with 0.5 wt.% GNS reinforced		Al-Si with 1wt.% GNS reinforced
		Anode	Cathode	Anode	Cathode	Anode	Cathode	Anode	Cathode
0.0005	0.8	–0.85	–0.52	–0.71	–0.52	–0.67	–0.52	–0.78	–0.52
0.0005	0.9	–0.84	0.53	–0.71	–0.53	–0.66	–0.53	–0.77	–0.53
0.0005	0.98	–0.84	–0.53	–0.7	–0.53	–0.66	–0.53	–0.77	–0.53
0.001	0.8	–0.84	–0.53	–0.7	–0.53	–0.66	–0.53	–0.77	–0.53
0.001	0.9	–0.84	–0.53	–0.7	–0.53	–0.66	–0.53	–0.77	–0.53
0.001	0.98	–0.83	–0.54	–0.69	–0.53	–0.65	–0.53	–0.76	–0.53
0.002	0.8	–0.83	–0.53	–0.69	–0.53	–0.65	–0.53	–0.76	–0.53
0.002	0.9	–0.83	–0.53	–0.69	–0.53	–0.65	–0.53	–0.76	–0.53
0.002	0.98	–0.82	–0.54	–0.68	–0.54	–0.64	–0.54	–0.75	–0.54
0.0035	0.8	–0.82	–0.54	–0.69	–0.54	–0.65	–0.54	–0.76	–0.54
0.0035	0.9	–0.82	–0.54	–0.69	–0.54	–0.64	–0.54	–0.76	–0.54
0.0035	0.98	–0.82	–0.61	–0.68	–0.55	–0.64	–0.54	–0.75	–0.56
0.007	0.8	–0.82	–0.63	–0.68	–0.55	–0.64	–0.54	–0.75	–0.56
0.007	0.9	–0.82	–0.6	–0.68	–0.55	–0.64	–0.54	–0.75	–0.55
0.007	0.98	–0.81	–0.75	–0.68	–0.61	–0.63	–0.57	–0.74	–0.68

4.3.3. Maximum anode current density

Fig. 10(a-d) depicts the maximum aluminium anode current density as a function of load density and relative humidity for the galvanic couple of Aluminium alloy and AISI4340 steel. It was observed that the maximum anodic current density decreased with increase in load density and increased with increase in relative humidity from 80 % to 90 %.

Fig. 10Maximum anode current density (A/m2) for a) uncoated specimen, b) Al-Si coated specimen, c) Al-Si with 0.5 wt.% GNS reinforced coated specimen, d) Al-Si with 1 wt.% GNS reinforced coated specimen

a)

b)

c)

d)

However, increase in relative humidity beyond 90 % lowered the maximum anode current density at 0.0005 kg/m² load density. The maximum current density for uncoated specimen was obtained to be 380 A/m² while for Al-Si coated, Al-Si with 0.5 wt.% GNS reinforced and Al-Si with 1 wt.% GNS coated were obtained as 115, 42, 242 A/m² respectively. It was also observed that the anode current density was least for Al-Si with 0.5 wt.% GNS. Hence the dynamic rate of corrosion rate will be least with Al-Si with 0.5 wt.% GNS reinforced coated specimen.

4.3.4. Average anode current density

Fig. 11(a-d) shows the average anode current density in the galvanic couple of aluminium alloy and AISI 4340 steel. The average anode current density is the current density when the anode has stabilized and provides a measure of total corrosion rate of the sample. Lower the average current density, lower is the corrosion rate of the specimen [25]. Table 3 describes the average current density obtained during the parametric study with respect to load densities and relative humidity.

Fig. 11Average anode current density (A/m2) for a) uncoated specimen, b) Al-Si coated specimen, c) Al-Si with 0.5 wt.% GNS reinforced coated specimen, d) Al-Si with 1 wt.% GNS reinforced coated specimen

a)

b)

c)

d)

It was observed that the average current density increases in load densities up to 0.0035 kg/m² but decreases significantly later towards 0.007 kg/m² for as received AA7075 specimen. Also, the average current density increased with increase in relative density till 95 % humidity for load density up to 0.0035 kg/m² and decreases later for uncoated specimen. When the load density is 0.007 kg/m², the current density increases up to 90 % humidity and then decreases significantly. For coated specimen, the average current density increases with increase in load density. Also, the current density increases with increase in relative humidity from 80 % to 95 % and decreases significantly then.

The maxiumum average current density for uncoated specimen was observed to be 1.95 A/m² at 0.0035 kg/m² and 95 % humidity. In comparison, for Al-Si and Al-Si with 0.5 wt.% GNS, the maximum average current density were 1.23 A/m² and 0.95 A/m² respectively at 95 % humidity and load density of 0.007 kg/m². It is observed that for Al-Si with 0.5 wt.% GNS reinforced coated specimen exhibited lower current density. In case of Al-Si with 1 wt.% GNS reinforced coated specimen, the maximum average current density was 1.93 A/m² at 0.0035 kg/m² load density and 95 % humidity. It follows similar trend as that of uncoated specimen.