Corrosion Science 52 (2010) 2565–2570

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Corrosion Science

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Scratch-resistant anticorrosion sol–gel coating for the protection of AZ31magnesium alloy via a low temperature sol–gel route

Heming Wang, Robert Akid *, Mohamed GobaraMaterials & Engineering Research Institute, Sheffield Hallam University, Howard Street, Sheffield S1 1WB, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 March 2010Accepted 5 April 2010Available online 11 April 2010

Keywords:A. AZ31A. Magnesium alloyA. PolyanilineA. Sol–gel coatingC. Corrosion protection

0010-938X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.corsci.2010.04.002

* Corresponding author. Tel.: +44 (0) 114 225 4062E-mail address: r.akid@shu.ac.uk (R. Akid).

A novel hybrid sol–gel/polyaniline coating has been developed for application onto an AZ31 magnesiumalloy for corrosion protection. Electrochemical Impedance Spectroscopy in 3.5 wt% NaCl and diluted Har-rison’s solutions, along with salt spray tests showed that the coating possesses excellent corrosion resis-tance. The hybrid coating was modified by doping with silica nanoparticles (for scratch resistance) andcured at a low temperature of 75 �C. Whilst conventional sol–gel methods tend to limit the coating thick-ness values up to 10 lm, the new hybrid sol–gel/polyaniline system presented here allows thick coatingsto be deposited, in this case, around 50–60 lm.

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1. Introduction Sol–gel technology is fast becoming a recognised approach for

Magnesium is an exceptionally lightweight metal, with a den-sity of 1.74 g/cm3, being 1.6 and 4.5 times less dense than alumin-ium and steel, respectively [1]. Mg and Mg alloys are considered tobe promising materials for a wide range of applications allowingweight reduction in many components, particularly those used inthe automotive and aerospace sectors, providing lower fuel con-sumption and reduced CO2 emissions. Magnesium and its alloyshave good mechanical properties, especially those containing man-ganese, aluminium, zinc, zirconium and rare earths, good castabil-ity, and can be recycled [2]. However, the application ofmagnesium and its alloys has been fairly limited with the majordrawback for many engineering applications being its poor corro-sion resistance, especially in chloride-containing aqueous electro-lytes. Hence the need for surface treatments or coatings toimpart long-term corrosion protection [3–5].

Historically passive metals such as Mg, Al and Zn have relied onthe use of passivation treatments or primer coatings containing, Cr(VI) compounds to impart corrosion resistance. Unfortunately,these treatments are toxic and carcinogenic and now face aworld-wide ban [6]. Alternative Cr (VI)-free surface treatments orcoatings such as, laser cladding, thermal spraying and physical va-pour deposition [7–9]; or anodizing, conversion coating and organ-ic coatings [10] have been explored, each having their ownadvantages and disadvantages.

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producing an alternative environmentally acceptable anticorrosionor functional coating, where the major advantage of hybrid sol–gelsystems over conventional ceramic-based sol–gel coatings is thatcrack-free thick coatings (above 10 lm), with controlled composi-tion and morphology, can be formed at low cure temperatures(down to room temperature) [11]. In addition, sol–gel technologyalso provides the advantages of low cost, simple formulation chem-istry and flexibility of application method for coating complexgeometries, e.g., dip, spray, etc. To this end several attempts to applysol–gel films on Mg-based alloys have been recently reported, par-ticularly for the highly reactive structural alloy ‘AZ31’ [12–19].

Electrochemical Impedance Spectroscopy (EIS) has become oneof the most widely used techniques for understanding the mecha-nism of corrosion protection of coatings allowing ‘semi-quantita-tive’ information to be obtained regarding coating performance.Several articles [16–19] have presented the anticorrosion proper-ties of sol–gel coatings on Mg alloys based upon EIS measurements.Lamaka et al. [19] recorded very good impedance results (around1 X cm2 at 0.01 Hz, after 14 days (336 h) immersion) for a sol–gel coating cured at a high temperature of 160 �C. Whilst thisimpedance value is acceptable for a coating system, the use of sucha dilute test solution (0.005 M NaCl) would not normally be con-sidered to represent an industrial environment; a more typicalchloride concentration being 3.5 wt% (0.6 M), hence in this respectsuch a coating system would not normally be used without theaddition of a topcoat in order to achieve any acceptable level ofperformance.

In the present study, we report on a novel organic–inorganic hy-brid coating based on a silica matrix combined with a polyaniline

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(PANI) polymer. Polyaniline (PANI) is a conductive polymer whichhas unique electrical and optical properties; is relatively cheap,easy to synthesise and stable under a wide variety of experimentalconditions. Since DeBerry reported that polyaniline provided cor-rosion protection to stainless steel, there have been a large numberof investigations using polyaniline as an additive in corrosion pro-tection systems [20] for ferrous alloy substrates. Few studies[21,22] have been found that report on the application of polyani-line for the corrosion protection of the light alloys, e.g., aluminium.Furthermore to our knowledge there are no studies where a sol–gel/PANI hybrid system has been used for the corrosion protectionof magnesium and its alloys, particularly where the coating offers a‘‘self-healing” capacity at damaged sites in the coating.

2. Experimental

2.1. Preparation of hybrid sol–gel coatings

Chemical oxidation of aniline was performed by oxidation withammonium persulphate in 1 M hydrochloric acid solution. 5.0 mlof 0.107 M aniline is dissolved in 300 ml of 1 M HCl at 0–5 �Cand stirred for 1 h. A solution of 5.6 g ammonium persulphate in1 M of 100 ml HCl was added dropwise to aniline solution over aperiod of 15 min with vigorous stirring. The solution is then con-tinually stirred at 0–5 �C for 6 h. The precipitate was collected witha Buchner funnel and washed with four portions of 50 ml of 1 MHCl. The precipitate is dried at 60 �C for 24 h, the HCl doped PANI(Emeraldine Salt, ES) is obtained as a green powder. The basic formof PANI (Emeraldine Base, EB) is obtained by stirring the ES with1 M NH4OH for 3 h followed by filtration and drying. Organic–inorganic matrix silica-based sols, catalysed by HNO3 acid, wereprepared and used in this study. Further details are provided inprevious papers [11,23,24]. The mixture of the PANI-doped solwas prepared in two ratio levels, denoted low and high. To increasethe scratch resistance of the hybrid sol–gel/PANI coating, 1.0 wt%silica nanoparticles (10–20 nm) were added in the final sol.

Die-cast magnesium alloy AZ31 (containing approx. 3 wt% Aland approx. 1 wt% Zn) was immersed in an acidified IndustrialKleen IKB 501 cleaning solution (Concoat Limited Company) forabout 30–60 s to clean the surface. The coating was then appliedto one side of the AZ31 alloy surface using a spray method. Thecoated magnesium alloy was oven dried in air at 75 �C for 5 minand then spray-coated again. Finally the coated AZ31 sample wasleft in the oven to be dried in air at 75 �C for no less than 3 h. Forcomparison, one commercially available pretreatment/coating(‘Alodine 1200’ pretreatment solution (Henkel)) and three differentsol–gel samples were investigated; i.e. sol–gel coatings withoutPANI (labelled as SGO), sol–gel coatings with high doped ratio ofPANI (labelled as SGH) and sol–gel coatings with low doped ratioof PANI (labelled as SGL).

2.2. Experimental techniques

Coatings were immersed in either 3.5 wt% NaCl solution or di-lute Harrison’s solution (0.35 wt% (NH4)2SO4 and 0.05 wt% NaCl)at 20 �C. Corrosion tests were carried out using a conventionalthree-electrode cell arrangement with the sample (area: 41 cm2)as the working electrode, a ‘saturated’ calomel reference electrodeand a platinum counter electrode. Electrochemical impedancemeasurements were obtained at the measured open circuit poten-tial (Eocp) values applying ±10 mV perturbation, in the frequencyrange 105–10�2 Hz. EIS data were recorded for samples immersedin Harrison’s or 3.5 wt% NaCl solution.

Salt spray testing (ASTM B117) was carried out at 35 �C in a5 wt% NaCl spray at 100% humidity. Images were recorded every

120 h to compare the performance of samples throughout the testperiod. To assess adhesion a sellotape test was carried out as fol-lows: the samples were scratched using a razor blade to form across-hatch. Sellotape tape was applied to the cross-hatch andfixed well before peeling it from the surface. The sellotape wasthen examined for signs of detachment of the coating. The thick-ness of the coating was estimated through the cross-section imageof the coated sample using a scanning electron microscope, (PhilipsXL40), at 20 kV. Thin sputtered carbon films were applied to coat-ings in order to prevent surface charging during SEM examination.

3. Results and discussion

3.1. Corrosion assessment of bare and coated AZ31

3.1.1. Corrosion characteristics of AZ31 magnesium alloysBare AZ31 showed significant corrosion activity within several

minutes after immersion in the 3.5 wt% NaCl solution as seen bythe generation of hydrogen bubbles. Characteristic filiform corro-sion was observed on samples subject to a commercial surface pre-treatment, notably ‘Alodine 1200’ chromate conversion coating.Optical photos of typical surfaces pre-treated with the ‘Alodine’conversion coating are shown in Fig. 1, following immersion timesof 3 and 24 h, respectively, within Harrison’s solution (a signifi-cantly less aggressive environment than 3.5 wt% NaCl). Here, itcan be seen that filiform corrosion, several millimetres in length,is spread widely over the whole surface after 3 h immersion, seeFig. 1 (left). Filaments propagated in a random direction followingprolonged immersion time up to 24 h, see Fig. 1 (right).

3.1.2. Corrosion behaviour of the sol–gel coating without PANI (SGO)A non-doped (without PANI) sol–gel coating on AZ31 showed

significant corrosion activity, particularly at defect sites in thecoating where hydrogen bubbles were observed at the surface ofthe coating. This behaviour was typical when inhibitor-free sol–gel coatings were immersed in 3.5% NaCl solution for just 1–2 h.For this reason it is not possible to carry out EIS testing in 3.5%NaCl, hence a dilute Harrison’s solution was chosen to assess thecorrosion behaviour of this particular coating system.

On immersion in Harrison’s solution filiform corrosion is ob-served beneath the SGO coating after only 2–5 h, as shown inFig. 2 (circled sites). The filiform filaments are seen to propagatesporadically in a random direction following further immersionup to 24 h. The electrochemical response of the sol–gel coatedSGO sample was then evaluated within Harrison’s solution usingEIS, as shown in Fig. 3. Here, the coated AZ31 showed an initialhigh impedance (2 MX cm2 at 0.05 Hz) after 30 min of immersiondue to the barrier properties of the coating. At this stage the elec-trolyte has not fully reached the surface of the alloy. However, theimpedance dramatically decreased to about 0.005 MX cm2 after24 h immersion. This decrease in impedance correlates with theappearance of filiform corrosion, as shown in Fig. 2.

3.1.3. Corrosion resistance of the hybrid sol–gel coating withhigh-ratio of PANI (SGH)

It was noted that the high PANI ratio SGH coating suffered frompoor adhesion of the coating to the magnesium alloy substrate, seeFig. 4, resulting in delamination after 24 h immersion in 3.5 wt%NaCl solution. Despite this poor adhesion, it was considered worth-while exploring the ‘‘self-repairing” functionality of the dopedPANI using dilute Harrison’s solution. Here, the SGH sample wasdamaged by scratching the surface before immersion. The scratchwas prepared using a razor blade giving a scratch width of around100 lm, which penetrated down to the substrate. After immersionin Harrison’s solution for 3 weeks, the sample was taken out and

Fig. 1. Filiform corrosion on Mg alloy AZ31 treated by an ‘Alodine 1200’ conversion coating.

Fig. 2. Filiform corrosion on Mg alloy AZ31 coated with a sol–gel coating withoutcorrosion inhibitors.

Fig. 3. Impedance behaviour of the sol–gel coating (without corrosion inhibitors) inHarrison’s solution.

Fig. 4. Scratched surfaces of Mg alloy AZ31 with a high-ratio-doped sol–gel/PANIcoating after 21 days immersion in Harrison’s solution.

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then washed in tap water and dried. Whilst delamination was seenaround the scratched area, no pitting or filiform corrosion occurredwhere the surface was exposed to the solution; and it was notedthat a passive film had formed, see Fig. 4, as seen through a colourchange of the surface, at the delaminated and scratched sites. Thispassive film has protected the alloy from corroding, exhibiting a‘‘self-healing” capability, as provided by the PANI. EIS measure-ments were also carried out on a non-scratched sample atEocp = �1610 mV (SCE). The impedance curves of the SGH coatingare shown in Fig. 5. The impedance data showed very unstablebehaviour in the first 20–24 h reflecting the fact that the electro-lyte had not fully penetrated the coating. This is due to the initialhydrophobic coating property. After this period, a relatively lowimpedance of 0.8 MX cm2, at 0.01 Hz frequency, is observed. Thereason for this may be due to the highly porous structure ofthe coating, being related to the high PANI concentration withinthe sol–gel coating and the ‘conducting’ nature of the PANI. The

electrolyte reached the surface of the substrate and some activityoccurred, see Section 3.1.4.1. It was observed that the impedancerecovered after 3 days immersion reaching the highest value of

Fig. 5. Impedance behaviour of the high-ratio-doped sol–gel/PANI coating inHarrison’s solution.

Fig. 6. Bode plots of a ‘‘relatively” low-ratio-doped sol–gel/PANI coating in 3.5%NaCl solution.

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3.0 MX cm2 at 0.01 Hz after 11 days immersion. This is due to theformation of a passive layer on the surface reflecting the ‘self-healing’ functionality of polyaniline in the sol–gel coating. On fur-ther immersion, the impedance of the coating gradually decreasedto 1 MX cm2 after 63 days (1500 h) immersion. Due to the pooradhesion performance of the SGH sample, detailed interpretationof the EIS data for the SGH samples has not been conducted.

In summary, the SGH coating provides good corrosion protec-tion when immersed in Harrison’s solution; however, it exhibitedpoor adhesion particularly when immersed in a severely corrosiveenvironment such as 3.5 wt% NaCl solution, indicating that thehigh content of PANI in the composition of the coating hindersthe formation of a chemical bond to the substrate which has beenobserved through the network of the silica-based sol–gel matrix[25]. The reason for this is one related to a decrease in the forma-tion of Si–O–metal bonds with increasing PANI content. On this ba-sis, the concentration of PANI in the coating was reduced andadjusted to an appropriate ratio in order to obtain a satisfactorychemical bond to the substrate.

Fig. 7. Equivalent circuits used for numerical fitting of the EIS data from (A) to (D).(A) Before 72 h, (B) 72–120 h, (C) 120–240 h and (D) 240–648 h. Note: not to scale.

3.1.4. Corrosion performance of the sol–gel coating with relativelylow-ratio-doped PANI (SGL)3.1.4.1. EIS analysis. Evolution of Bode plots for the SGL hybrid sol–gel coating is shown in Fig. 6. Very high impedance (close to10 MX cm2) is observed after 30 min immersion in 3.5 wt% NaClsolution with Eocp = �1620 mV (SCE). On extended immersion inthe electrolyte the impedance slowly decreases as the electrolytebegins to reach the substrate surface. However, the impedance va-lue remains above 1 MX cm2 even after 648 h (27 days) immersiontime, see Fig. 6A. This type of hybrid sol–gel/PANI coating providesvery good long-term corrosion protection to the AZ31 with no vis-ible signs of corrosion after 648 h immersion in 3.5 wt% NaCl.

Detailed interpretation of the EIS spectra was performed bynumerical fitting of the Bode plots using the equivalent circuitspresented in Fig. 7. As previously reported [11] and more recentlythrough observation by TEM cross-section images [25], two layersare initially formed after the sol–gel is applied to a substrate; i.e.the layer of the sol–gel coating itself (outer layer) and a bondingnano-layer (interfacial layer) of the order of 20–30 nm arising fromthe reaction between the sol and the substrate.

Following immersion up to 72 h in 3.5% NaCl solution, the elec-trolyte diffuses through the sol–gel layer, eventually reaching theinterfacial layer. During this period, a relatively large decrease ofthe impedance of the coating occurs in the mid frequency rangefrom 10 to 1000 Hz. However, there was only a small decrease inthe impedance at 0.01 Hz with the value close to 10 MX cm2. This

change can be assigned to water uptake within the sol–gel coating.At this stage, the corrosion protection of the substrate is mainlyattributed to the good barrier properties of the sol–gel coating.The equivalent circuit depicted in Fig. 7A (up to 72 h immersion)can be used to fit the EIS data. This equivalent circuit was proposed

Fig. 9. Evolution of the interfacial layer sol–gel coating resistance (Ri) and coatingcapacitance (Ci or Qi) following immersion in 3.5% NaCl solutions. Values wereobtained by numerical fitting using the equivalent circuits given in Fig. 7.

Fig. 10. Evolution of the passive layer sol–gel coating resistance (Rp) and coatingcapacitance (Cp or Qp) following immersion in 3.5% NaCl solutions. Values wereobtained by numerical fitting using the equivalent circuits given in Fig. 7.

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and established assuming the following: (i) an electrolyte resis-tance (Rs); (ii) a sol–gel layer; where the pure capacitor graduallychanges to a constant phase element (Co or Qo); (iii) Ro which rep-resents the resistance of the pore or defects in the sol–gel layer inparallel with Co or Qo and (iv) the properties of the interfacial layerare described by a resistance Ri in parallel with a constant phaseelement Qi. After 72 h immersion and up to 120 h, the electrolytereaches the substrate and generates what might be described asmetastable active–passive activity which occurs on a scale that isnot visible during the test. This activity is represented by the addi-tion of a Warburg impedance (Wi) to the circuit, see Fig. 7B. Forimmersion periods beyond 120 h, a new passive film betweenthe interfacial layer and the substrate forms as described abovein Fig. 4 (Section 3.3.1), being represented by a pure capacitor(Cp), a resistance Rp in parallel, and a diffusive Warburg impedance(Wp) as shown in Fig. 7C. On further immersion in 3.5 wt% NaClsolution, the pure capacitor (Cp) transforms to a constant phaseelement (Qp). The Warburg impedance disappears after 240 h asthe passive layer becomes stable, as shown in Fig. 7D.

The results of fitting the experimental data, using the circuitsgiven in Fig. 7, are presented in Figs. 8–10, respectively. Afterimmersion for about 120 h, the resistance of the sol–gel outer layerdecreased from 2 to 0.02 MX cm2 while the resistance of the inter-facial layer remained stable at about 40 MX cm2. During this per-iod, the electrolyte had not fully reached the substrate and theAZ31 magnesium alloy was protected via the barrier propertiesof the sol–gel coating. From 120 to 240 h immersion time, a diffu-sive Warburg impedance (Wp) is included in the circuit and thisrepresents the metastable active–passive activity when the elec-trolyte reaches and interacts with the substrate. Interestingly, dur-ing the immersion time between about 120 and 350 h, theresistance of the sol–gel outer layer recovered from the low value(0.02 MX cm2) and became stable at about 1 MX cm2, suggestingthat the sol–gel outer layer continued to provide corrosion protec-tion to the substrate through its barrier properties. During thistime, the resistance of the interfacial layer dropped just below0.1 MX cm2. This is considered to occur due to a reaction takingplace at the interface. Eventually, a new passive layer forms pro-viding protection to the magnesium alloy AZ31 with a change inthe impedance value as seen by the impedance change from 0.1up to 1 MX cm2. This would suggest that a ‘self-repair’ process istaking place, in this case due to the doped polyaniline in the sol–gel coating [26]. After 648 h immersion, the resistance of the outerlayer and the ‘‘passive” layer has reduced to 0.1 and 0.003 MX cm2,

Fig. 8. Evolution of the outer layer sol–gel coating resistance (Ro) and coatingcapacitance (Co or Qo) following immersion in 3.5% NaCl solution. Values wereobtained by numerical fitting using the equivalent circuits given in Fig. 7.

respectively. However, the resistance of the interfacial layer in-creased to 20 MX cm2, maintaining good protection to the sub-strate. These results demonstrate that the corrosion protectionmechanism provided by the hybrid sol–gel/PANI coating is some-what complicated and further studies are currently being per-formed to identify the exact corrosion protection mechanism ofAZ31.

3.1.4.2. Visual observation from salt spray tests. The corrosion resis-tance of this sol–gel coating has also been demonstrated by con-ducting salt spray tests at 35 �C within a 5 wt% NaCl spray at100% humidity. Fig. 11 shows a sample after 500 h salt spray test-ing. Examination of the sample revealed no filiform corrosion orcreepage along the scratch, further confirming the good perfor-mance of this hybrid sol–gel coating.

3.2. SEM characterisation

A back-scatter electron image of the cross section of the hybridsol–gel coating is shown in Fig. 12. The thickness of the coating wasmeasured using a cross-section image and found to be around50–60 lm. Elemental maps of Mg and Si contained within the

Fig. 11. Photo of low-ratio sol–gel/PANI coated samples after 500 h salt spray test.

Fig. 12. Back-scatter electron image of a cross section of the hybrid low-ratio-doped sol–gel/PANI coating.

2570 H. Wang et al. / Corrosion Science 52 (2010) 2565–2570

AZ31 alloy and the sol–gel coating, respectively, were obtained asshown in Fig. 12.

3.3. Adhesion and scratch resistance

The adhesion of the sol–gel coating was assessed following500 h immersion in 3.5 wt% NaCl solution by using the sellotapetest previously mentioned. No signs of detachment of the coatingwere observed, indicating good adhesion of the coating to the sub-strate. The coating also showed a pencil hardness greater than 6Hwith good scratch resistance. This is comparable with a commer-cial fusion bonded epoxy powder coating.

4. Conclusions

A novel organic–inorganic hybrid sol–gel coating incorporatingpolyaniline in the composition was successfully produced and ap-plied on AZ31 magnesium alloy for the application of corrosionprotection. The hybrid sol–gel coating was cured at a low temper-ature of 75 �C. EIS and salt spray tests confirmed the coating pro-vided excellent corrosion resistance and has very good scratchresistance due to the incorporation of silica nanoparticles in thesol–gel. Analysis of the EIS inferred a stable passive layer formedon the surface of AZ31. The change of resistance in three layersduring the immersion period would suggest a ‘‘self-repairing”mechanism which results from the incorporation of polyanilinein the coating. Changing the ratio of sol–gel to conducting polymerin the coating identified that the corrosion resistance and adhesionperformance of the coating are highly dependent upon the ratio ofthese two components.

Acknowledgements

The authors thank the Materials and Engineering ResearchInstitute at Sheffield Hallam University for the provision of facili-ties. Acknowledgement is also given to the Egyptian governmentfor the scholarship for M.G.

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