Corrosion properties of 316L stainless steel coated with polyelectrolyte multilayers of varying anionic acidity M. Khaled*1, B. Abu-Sharkh2, E. Amr2, B. S. Yilbas3, A. Manda1 and A. Abulkibash1 Polyelectrolytes have been proposed as promising systems for the potection of stainless steels and for biomedical and drug release applications. Multilayer nanofilms with varying anion acidity were deposited on AISI316L stainless steel. The cationic polyelectrolyte was polyallylamine hydrochloride (PAH) whereas the anionic polyelectrolytes with increasing acidity were polyacrylic acid (PAA), polystyrene sulphonate-co-maleic acid and polystyrene sulphonate (PSS). Potentiodynamic polarisation showed an increase in corrosion potential Ecorr upon coating with multilayer nanofilms and a corresponding decrease in corrosion current. Transient currents were observed upon application of PSS due to its high acidic strength although it showed better pit recovery characteristics as shown in cyclic polarisation experiments. Constant potential experiments at 700 mV v. Ag/AgCl for 12 h showed a suppressed current by 50% for the PAH/ PSS coated steel compared to the uncoated specimen. The SEM images showed the existence of agglomerates, uncovered areas and corrosion products underneath channels on the coating. Keywords: Nanofilm, Nanotechnology, Stainless steel, Corrosion protection, SEM, AFM, Polyelectrolytes
Introduction Steel is the most widely used metal in industry but it suffers from weak resistance to corrosion in aggressive media. The addition of nickel and chromium, however, results in a corrosion resistant alloy, stainless steel, which has much better corrosion resistance properties. However, stainless steels are not immune to corrosion attack. Localised corrosion (pitting corrosion, intergranular corrosion or stress corrosion cracking) can be developed mainly in chloride containing environments.1 Many methods have been developed to minimise the corrosion of metals in general. These include corrosion inhibitors, surface modification, alloy substitution and organic, inorganic or metallic coating.2–6 Recently, it has been suggested that polyelectrolyte multilayers can act as effective coatings for the protection of stainless steel. Polyelectrolyte multilayers (PEMs) are a new class of organic thin film made by exposing a substrate to charged polymers, i.e. cationic and anionic polyelectrolytes, in an alternating fashion using layer by layer self-assembly. The film thickness is typically 1–2 nm for each layer.7,8 Polyelectrolyte systems have also been 1
Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 3 Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 2
*Corresponding author, email
[email protected]
proposed for biomedical applications, e.g. tissue engineering, and for controlled drug release.9 In the present paper, the electrochemical corrosion properties have been investigated of AISI316L stainless steel rods coated with PEMs of cationic polyallylamine hydrochloride (PAH) and anionic polyelectrolytes of varying acid strength. The weak acid was polyacrylic acid (PAA) and the strong acid was polystyrene sulphonate (PSS). A polyelectrolyte of intermediate acid strength was polystyrene sulphonate-co-maleic acid (PSS-co-MA). The chemical structures of these PEs are shown in Fig. 1. The coating layer, the corrosion products and the morphology of the surface were examined by scanning electron microscopy after electrochemical tests.
Experimental AISI316L stainless steel wires (1?4 mm in diameter) were polished with SiC papers of 1000 and 1200 grit successively, rinsed with hexane and then, washed with deionised water before coating and electrochemical experiments. The polyelectrolyte multilayer films were deposited using the alternating layer by layer (LBL) deposition method with the aid of the NanoStrata robot using 1?00 mM polyelectrolyte solutions calculated based on the molecular weights of the repeat units. The multilayers were always deposited starting with the cationic PE according to Refs. 7 and 8. The starting aqueous solution (PAH) was the cationic polyelectrolyte, whereas the anionic polyelectrolytes ß 2007 Institute of Materials, Minerals and Mining
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Published by Maney on behalf of the Institute Received 15 March 2007; accepted 22 July 2007 DOI 10.1179/174327807X234714
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pH of the solution above 10 to ensure complete dissolution of PAA in water. Potentiodynamic experiments were carried out in 0?7M NaCl solution (pH55?78) at room temperature using a Gamry potentiostat. The obtained Tafel (polarisation) diagram was analysed using Echem Analyst software. Scan rates of 0?166 mV s21 were applied. The area of the stainless steel wire immersed in the chloride solution was 0?78 cm2 for all samples under investigation. An Ag/AgCl reference electrode was used. A platinum wire with large surface area was used as the auxiliary electrode. The experiments were performed in triplicate to ensure reproducibility. A Jeol JDX-3530 scanning electron microscope (SEM) was used to obtain photomicrographs of the coated surfaces. The film thickness was measured using a Stokes LSE ellipsometer (Gaertner Scientific Corporation). It was found that 10 and 20 nanolayers of PAH/PSS on a quartz substrate resulted in thicknesses of 13?7 and 29?1 nm respectively. The values were the statistical mean of six measurements on different locations on the substrate.
Results and discussion
a PAH; b PAA; c PSS; d PSS-co-MA 1 Chemical structures of polyelectrolytes
comprised PAA, PSS and PSS-co-MA. The stainless steel wires were immersed in the PE solution for 5 min while spinning at a rate of 300 rev min21. The wires were subsequently rinsed for 90 s using deionised water in three different beakers, 30 s in each. Varying multilayer thicknesses comprising 20, 40 and 60 layers of each system were deposited. In preparing the PAA solution, drops of 0?1M NaOH were added to keep the
Tafel polarisation experiments were carried out for the PAH/PAA system in 0?7M NaCl at a scan rate of 0?166 mV s21. Figure 2 shows the Tafel plots for the uncoated 316 stainless steel and for the coated specimen with 20, 40 and 60 multilayers. The corresponding electrochemical corrosion parameters are shown in Table 1. It is noted that the corrosion potential Ecorr shifted anodically from 2183 mV for the uncoated stainless steel to close to 0 mV for the specimen coated with 60 layers of PAH/PAA. The corrosion rate similarly dropped drastically from y34 mm year21 for the uncoated stainless steel to y11 mm year21 for the 60 layers coated substrate. It is also noted from Fig. 2 that transient currents are observed at the high range of the anodic branch especially for the coated substrates, which are reminiscent of metastable pitting transients. The anodic branch of the 60 layers coated specimen showed a more suppressed current and lower current densities in
2 Tafel plots of uncoated AISI316L stainless steel and of specimens coated with 20, 40 and 60 multilayers of PAH/PAA in 0?7M NaCl
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3 Cyclic polarisation scan plots of uncoated AISI316L stainless steel and of specimens coated with 40 layers of PAH/ PAA in 0?7M NaCl
4 Tafel plots of uncoated AISI316L stainless steel and of specimens coated with 20, 40 and 60 multilayers of PAH/PSS in 0?7M NaCl
the semipassive region, indicating an improved surface coverage by the nanofilms. Cyclic polarisation experiments were carried out for the uncoated stainless steel and for substrates coated with 40 multilayers subsequently. Figure 3 shows the cyclic polarisation curves and the corresponding electrochemical parameters are shown in Table 2. It can be noted that the breakdown Table 1 Electrochemical corrosion parameters deduced from Tafel plots of multilayer nanofilms of PAH/ PAA in 0?7M NaCl
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Number of layers
Ecorr, mV
I, nA m22
Corrosion rate, mm year21
0 20 40 60
2185 242 237 2
57 49 39 18
33.5 28.9 22.6 10.8
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potential of the stainless steel coated with 40 layers of PAH/PAA shifted anodically to y790 mV compared to y720 mV for the uncoated steel substrate. The corresponding breakdown currents were similar at about 1?0– 1?1 mA cm22. However, the repassivation potential of the 40 layers coated substrate shifted cathodically to y275 mV compared to y390 mV for the uncoated specimen. Furthermore, the repassivation current for the Table 2 Electrochemical corrosion parameters deduced from cyclic polarisation scans of multilayer nanofilms of PAH/PAA in 0?7M NaCl Number of layers Ep, mV Ip, mA cm22 Erep, mV Irep, mA cm22 0 40
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725 790
1.0 1.1
390 275
0.54 0.032
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5 Cyclic polarisation scan plots of uncoated 316 SS and of specimens coated with 40 layers of PAH/PSS in 0?7M NaCl
coated substrate was 0?033 mA cm22, much lower than the 0?54 mA cm22 for the uncoated steel. Tafel experiments carried out for the 316 stainless steel specimen coated with 20, 40 and 60 layers of PAH/PSS nanofilms are shown in Fig. 4. The PSS was more acidic than PSS-co-MA and PAA. The corresponding electrochemical corrosion parameters are shown in Table 3. It is noted that the transient currents for this system are much more intense and spread out over the anodic branch compared to the PAH/PAA system. The corrosion potential shifted from 2183 mV for the uncoated specimen to 250 mV for the 60 layers coated substrate which is y50 mV more cathodic than the 60 layers coated PAH/PAA nanofilms. It is also noted from Table 2 that there is no significance difference in the corrosion current and corrosion rate after .40 layers are deposited. Cyclic polarisation experiments shown in Fig. 5 shows a significant decrease in the breakdown potential Eb upon coating with 40 multilayers of nanofilms. The Eb shifted cathodically from y725 mV for the uncoated specimen to y525 mV for the 40 layers coated specimen, which is a decrease by y200 mV in the breakdown potential. Furthermore, the area underneath
the reversed current was much less for the coated substrate, indicating a significant enhancement of the ability of the surface to repassivate compared with the PAH/PAA nanofilm. Further investigations were carried out on the PAH/PSS-co-MA where the PSS-co-MA was of intermediate acidity as compared to PAA and PSS. Figure 6 shows the Tafel polarisation curves of the latter system and Table 5 the electrochemical corrosion parameters. It is noted that the anodic branch showed small transient currents which were more severe for the 20 layers coated stainless steel and become less pronounced upon coating with 40 and 60 layers. This behaviour is similar to that observed for the PAA/PAH system in Fig. 2. Furthermore, the Ecorr shifted from about 2180 mV for the uncoated specimen to about 245 mV for the 60 layers coated stainless steel which is comparable to the corrosion potential for the more acidic PAH/PSS system. The corrosion rate of y8 mm year21 for the 60 layers coated substrate was comparable to the PAH/PSS system and somewhat less than the PAH/PAA nanofilm. Apparently, the type of coatings mainly affects the corrosion potential, which varied from y0, 245 to 250 mV respectively, as the
Table 3 Electrochemical corrosion parameters deduced from Tafel plots of multilayer nanofilms of PAH/ PSS in 0?7M NaCl
Table 5 Electrochemical corrosion parameters deduced from Tafel plots of multilayer nanofilms of PAH/ PSS-co-MA in 0?7M NaCl
Number of layers
Ecorr, mV
I, nA cm22
Corrosion rate, mm year21
Number of layers
Ecorr, mV
I, nA cm22
Corrosion rate, mm year21
0 20 40 60
2185 250 293 250
57 18 17 14
33.5 10.3 10.3 8.4
0 20 40 60
2185 2120 260 245
57 32 16 14
33.5 18.7 9.2 8.2
Table 4 Electrochemical corrosion parameters deduced from cyclic polarisation scans of multilayer nanofilms of PAH/PSS in 0?7M NaCl
Table 6 Electrochemical corrosion parameters deduced from cyclic polarisation scans of multilayer nanofilms of PAH/PSS-co-MA in 0?7M NaCl
Number of layers
Eb, mV
Ip, nA
Erep, mV
Irep, nA
Number of layers
Eb, mV
Ib, nA
Erep, mV
Irep, nA
0 40
725 525
1020 315
390 455
540 195
0 40
725 775
1020 800
390 460
540 175
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6 Tafel plots of uncoated 316 SS and of specimens coated with 20, 40 and 60 multilayers of PAH/PSS-co-MA in 0?7M NaCl
7 Cyclic polarisation scan plots of uncoated 316 SS and of specimens coated with 40 layers of PAH/PSS-co-MA in 0?7M NaCl
8 Current v. time for 316 SS uncoated and coated with 40 layers of PAH/PSS: both samples were held at 700 mV v. Ag/ AgCl reference electrode at room temperature
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9 Scanning electron microscopy images for coated sample after applying 700 mV for 12 h in 0?1M NaCl solution
acidity increased. Cyclic polarisation experiments of the PAH/PSS-co-MA nanofilm are shown in Fig. 7 with the corresponding parameters shown in Table 6. It is noted that the breakdown potential shifted slightly anodically from y725 mV for the uncoated specimen to y780 mV for the 40 layers coated stainless steel. The repassivation potential occurred at y460 mV which is y70 mV more anodic than the uncoated specimen. The area underneath the reverse current was comparable in both cases and resembles the behaviour of the PAH/ PAA nanofilm. This indicates that at lower acid strength, the ability to repassivate is less than the PAH/PSS nanofilm at higher acidity. To investigate the long term performance of the nanofilm, 40 layers of the PAH/PSS system were polarised at 700 mV v. Ag/AgCl for y11 h. The current–time curve is shown in Fig. 8. It is noted that the surface was resistant to breakdown for y100 min, compared to y10 min for the uncoated steel. The average current of the 60 layers coated PAH/PSS was 1 mA cm22 compared to 2 mA cm22 for the uncoated substrate which is a 50% reduction. Figure 9 shows SEM images of a coated surface after polarisation at 700 mV v. Ag/AgCl. Some small pitting sites are observed. This is mainly because of the discontinuity in coating, which is locally scattered. Moreover, no specific pitting pattern is observed on the surface. However, channelling in the coating due to locally scattered uncoated surfaces and breakdown of the coating results in electrolytic penetration beneath the coating. This, in turn, results in corrosion product formation in this region. When comparing coated and uncoated surfaces after the electrochemical tests (Fig. 10), it is observed that large pitting sites occur on the uncoated surface whereas small and locally scattered pit sites are seen on the coated surface. This shows that coating prevents large scale
a as received before potential step; b as received after potential step; c coated after potential step 10 Scanning electron microscopy images for coated sample after applying 700 mV for 12 h in 0?1M NaCl solution (low magnification)
pitting. However, locally scattered uncoated regions are responsible for pitting of the coated surface.
Concluding remarks Electrochemical corrosion experiments carried on the PAH/PAA, PAH/PSS-co-MA and PAH/PSS nanofilms with increasing anionic acidity generally showed an enhancement in corrosion protection as indicated by a decrease in corrosion rate, which was the lowest for the 60 layers coated PAH/PSS nanofilm, and an increase in corrosion potential, which was the largest for the PAH/ PAA nanofilm. However, an increasing acidity was accompanied with a significant spreadout of transient currents. In contrast, the PAH/PSS showing the most intense transient pitting currents exhibited the smallest reverse area, indicating an enhanced ability to pit recovering. The SEM analysis showed surface pitting, which correlates with the observed current transients. It
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also shows the formation of channels and locally scattered corrosion byproducts. Thus, polyelectrolyte nanofilms show promise in minimising corrosion; however, further improvement of the films should be carried out to improve adherence, continuity and corrosion resistance.
Acknowledgement The authors acknowledge the support of KFUPM and KACST funding through project AC-80-25.
References 1. A. B. Cristobal, M. A. Arenas, A. Conde and J. de Damborenea: Electrochem. Acta, 2006, 52, 546–551
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2. S. A. Bradford: ‘Corrosion control’, 2nd edn; 2001, CASTI Publishing Inc. 3. R. A. Dickie and F. L. Floyd: ‘Polymeric materials for corrosion control’; 1986, Washington, DC, ACS. 4. A. D. Jones: ‘Principles and prevention of corrosion’, 2nd edn; 1996, Prentice Hall. 5. J. Li: ‘Study on corrosion protection of organic coatings using electrochemical techniques’, PhD thesis, NDSU, Fargo, ND, USA, 2001. 6. H. H. Uhlig and R. W. Revie: ‘Corrosion and corrosion control’; 1985, New York, John Wiley and Sons. 7. C. B. Bucur, H. H. Rmaile and J. B. Schlenoff: Polym. Mater. Sci. Eng., 2004, 90, 522. 8. T. R. Farhat and J. B. Schlenoff: Electrochem. Solid State Lett., 2002, 5, (4), B13. 9. M. Rinaudo: Corros. Eng. Sci. Technol., 2007, 42, (4), 324– 334.
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