- Email: [email protected]
Ti coating elaborated by cathodic arc PVD process onto mild steel substrate
ELSEVIER
Thin Solid Films 346 (1999) 150-154
Corrosion behaviour of AllTi coating elaborated by cathodic arc PVD process onto mild steel substrate J. Creusa, H. Idrissia, H. Mazille”‘*, “Lubmmire
de Physicochimie Indmtrielle, ‘Soci6rle Tnnovatique (HIT),
F. Sanchetteb, P. Jacquotb
INSA. bcif 401. F69621 Villeurbmne B.P. 143, F-69680 Chassieu, France
cede.r. Frunce
Received 30 December 1997: received in revised form 20 July 199s; accepted 22 November
199s
Abstract The useof cadmiumcoatingsin industryis stronglyrestrictedby anEuropeandirective.cadmiumbeingdangerous for humanhealthand Aluminium-based coatings are potential candidates for replacing cadmium coatings. In this paper, we compare>he corrosionbehaviourof Al and AVTi coatedsteel.We demonstrate that the galvaniceffect throughthe openporusity of the AI coating leadsto a rapidlossof sacrificialprotectionduringlongimmersion time.It is shownthatthedouble-layercoatingAlRiisteel presents a b_etter the environment.
corrosion resistance than monolayer Al coating in saline solution. The intermediate titanium layer permits the reduction of the open porosity
andalsoallowsthe reductionof the galvaniceffect betweensteelandaluminium.In orderto enhancetheelectrochemical propertiesof the double-layercoatingwe alsoinvestigatedthe effect of PVD post-oxidationtreatments.0 1555ElsevierScienceS.A. All rights reserved. Ke.w~o&:
Aluminium:
Titanium: Physical vapour deposition coating: Multilayer
1. Introduction Depositedaluminium, in the form of a dense,uniform and adhesive coating, has been used for many years and may often replace cadmium plating for aeronautical or automotive applications [2]. Aluminium coatings are widely used becauseof their good corrosion resistance[3]. The protection properties of the aluminium basedcoatings depend on the formation of an oxide or hydroxide film on the surface. In NaCl solution, the pitting potential of AI alloys is very close to the open circuit corrosion potential [4-61. Foley [5] and Garrigues [6] found a potential difference of only 20 mV. Consequently, in chloride solution, aluminium corrosion is mainly due to pitting. Furthermore, the galvanic corrosion effect between Al alloys and mild steel, for a unit surface ratio between them. leads to a mixed potential very close to the pitting potential, with a very important galvanic current density. Such a galvanic effect considerably reduces the lifetime of Al coatings in NaCl solution [ 1,7,8]. To enhancethe durability of the coating, it is necessary to reduce the presence of defects such as pinholes. pores, cracks. It is also possible to increase the coating thickness[ 1,9]. However. the industrial requirementsoften needan accurate thickness,for instancelessthan 15 pm for screwing applications. A further possibility is to fabricate * Corresponding E-vtuil addrev;
author. Fax: + 33 472-33-87-15;. [email protected] 1H. Mazille)
0010~6090/99/S - see front matter PII: s0040-6090(98)01732-8
0 1999 Elsevier
coatin,. 0’ Corrosion;
Electrochemicai
behaviour
multilayer coatings,with an intermediatecorrosion r&stant metal such as titanium. Previous work 19,lo] reported the corrosion behaviour of multilayer TilAl coatingsmadeup of six alternated layers of titanium and aluminium; the combination, with an initial internal titanium layer and an external aluminium one, provides the best corrosion resistance. In this paper. we comparethe corrosionresistanceof steel protected by dual Al/Ti layers with bare steel or steel protected by only onelayer of Al. Then, the electrochemical behaviour of multilayer coatings ALTiOJTi/steel And AlzOj/Al/Ti/steel is also investigated.
2. Materials and experimental procedure 2.1. Materials and test sollrtioil The composition of AISI 113.5steel substrateis reported in Table 1. Pure solid sources(Ti or Al) are evaporated by meansof PVD cathodic multi-arc technology in a low pressureatmosphereof pure argon. The main depositionparameterswere a negative bias ranging between 100 and 150 V, a pressure ranging between 0.5 and 4 Pa and a growth rate ranging between 4 and 6 pm h-‘. The multilayer AlEi coatings are fabricated by a successiveevaporation from Ti and Al cathodes.The duration of coating deposition is adapted-to reach a total coating thickness of 15 pm. The oxidation
Science S.A. All rights reserved.
J. Crerrs et al. /Thin
Solid Films 346 (19991 150-154
1.51
Table 1 AISI 4135 steel composition Element
C
Si
Mn
s
P
Ni
Cr
MO
Fe
4c By weight
0.337
0.36
0.69
<0.003
0.01
0.23
1.11
0.23
Balance
treatments are realized by the addition of a reactive oxygen flow in the reactor during the 5-min following the PVD deposition of the previous metal. The morphology and microstructure of the deposited coatings are investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) in secondary electron (SE) and backscattering electron (BSE) modes. EDX analysis are used for measuring the coating surface composition and for following the concentration profile of Ti and Al elements in a sectioned sample. The test solution for electrochemical investigations is an aerated 3% NaCl solution, stirred at 500 rpm with a rotating working electrode; generally, the samples, mounted as the rotating working electrode, are cylinders of 10 mm diameter and have a total exposed area of 3 cm’.
2.2.2. Poterztiodynamic polarization
cun’es: i(E)
After an initial potential stabilization of 1 h, the curves i(E) are plotted from -50 mV vs. open circuit potential
(OCP) in the cathodic side up to + 100 mV in the anodic side, using a sweep rate of 10 mV/min. 2.2.3. Electrochemical
impedance spectroscopy
(EIS)
Impedance determinations are made with a Solar&on 1254 frequency response analyser (FRA) for frequencies ranging from 4 mHz up to 64 kHz. The amplitude of sinusoidal signals is set at 10 mV around the free corrosion potential. Results are interpreted in terms of Nyquist plots. 3. Results and discussion
2.2. Esperirilerltal procedure
3.1. Corrosion
The electrochemical experiments are controlled by an EGG 273A potentiostat, using the conventional three-electrode technique. The potential is referred to a saturated calomel electrode (SCE) and the counter electrode is a platinum sheet. The electrochemical techniques used to characterize the behaviour of coated steel samples are: 2.2.1. Open circuit potential recordings drrring protracted irnrnersion tests: E(t) md R,,(tj
After being cleaned and dried, samples are immersed from 48 to 75 h in the saline solution. The open circuit potential is recorded versus time with simultaneous periodic measurements of polarization resistance performed between 220 mV around the free corrosion potential. For the polarization resistance determination. a sweep rate of 10 mVlmin was employed.
behaviour
qf Al and AUTi coated steel
3.1.1. Morplzology and wicrostructure
The morphology of Al coatings have been discussed in a previous paper [I], and we just mention here their cellular morphology, typical of PVD coatings. We find the same characteristic for Al/Ti coatings, with the presence of some defects such as droplets, preferential growths, micropores, inducing local heterogeneities in coating thickness (Fig. 1). SEM observations, on cross-section views. show distinctly the two layers, an internal compact one (5 p,rn thick) corresponding to the titanium layer and a external layer of aluminium (10 p,rn thick). The Ti and Al composition profiles. obtained by EDX analysis, do not suggest the existence of a thick interdiffusion region. However, due to a -460 T
. Al/steel
“-e e
Fig. 1. SEM observation
(X1250)
of a cross-section
of AliTi coated ste&
Fig. 2. Potential evolution of Al and AliTi immersion time in a 3% NaCl solution.
coated
steel during
a long
J. Crew
et al. /Thin
Solid Films 346 (1999)
Fig. 3. SEM observation 1x100) in backscattering electron mode iBSE) of ALTi coated steel, after a long immersion time in a 3% NaCl solution.
relatively large electron probe size (= 1 km3), it is not possible to conclude on a real lack of interdiffusion.
3.1.2. PotentiaI nnd polarization resistanceevollttions during n long immersionperiod During the first hours of immersion, the potential of the AUTi coatings onto steelstabilizesat around -730 mV/ECS (Fig. 2). The potential-time behaviour of steel coated by only one layer of aluminium is markedly different. with the open porosity of the Al coating enabling a potential shift towards the pitting potential, and a consequent,drastic increasein the corrosion current density after a few hoursof immersion. A suddenpotential shift occurs. associatedwith the corrosion of the substratewhich is no longer protected by the sacrificial coating. The fast, but short rise up to -450 mV is attributed to the dissolution of the substrate: the potential variation is similar to that observed on bare steel. For Al/l? coatings. the intermediate titanium layer decreasesthe open porosity, thus increasing the duration of the sacrificial behaviour of the outer aluminium coating. SEM observations, in backscattering electron mode @SE) (Fig. 3) of AIRi coated steel show the presenceof different areas:dark needle-like are essentiallycomposedof Al, with dispersedwhite areasbeing composedof 95% of titanium and a generalgreyish matrix-rich in titanium (67 at.%). Thesedifferent zones correspondto the heterogeneity Table 2 Polarization resistance (R,) evolution during a 75-h immersion test in a 3% NaCl solution Immersion duration ih)
0.5 4.0 15.0 35.0 75.0
R, (fi cm’) Al/Ti/steel
Al/steel
Bare Al
45000 20000 1cQoo 120000 145000
760 185 95 475 390
100000 90000 50000 19000 -
1X?-I54
in the coating thickness,exacerbatedby a corrosion immersion test of 80 h. SEM observations of cross-sectionviews confirm the decreaseof Al thickness after immersion tests of 80 h. EDX analyses(at low energy 10 KeV) alsoreveal an important amount of oxygen on the surface. This suggests&e formation of an oxide or hydrated oxide film during the corrosion test, asconfirmedalso by electronoptical observations of a samplepreviously immersedduring 75 h in the saline solution. R, determinations on multilayer Al/Ti coatings are reported in Table 2. They are compared with the values for bare aluminium and Al coated steel. The Al coated steel presentsthe lowest I?, values, essentially due to a high galvanic effect between the substrateand Al through the coating porosity. The openporosity increases with time of immersion, increasing the galvanic effect, which is very deleteriousfor the aluminium. The corrosion behaviour of the bilayer coating is improved over the monolayer one, and is very similar to the bare aluminium behaviour. The intermediate titanium layer considerably reduces the galvanic effect between steel and aluminium, essentially due to a decreasein the open porosity betweenaluminium and steel substrate. We should also underline that titanium coating is more noble than steel, which could suffer from localized corrosion through the porosity. However, the external layer of aluminium prevents this phenomenon,due to its sacrificial behaviour. Furthermore, a thorough investigation of the galvanic corrosion effect between bare metals (Ti, Al and steel) allowed us to conclude that Ti-Al galvanic corrosion is less harmful than Al-steel couple, although the open circuit potential difference is larger in the former case[7]. We notice a decreaseof R, values during the immersion time for all samples,which is associatedwith the formation and spreadingof pits. However, for coated samples.we observe different behaviour during long immersion times, for which an increaseof R, values occurs. This subsequent increasecorrespondseither to a repassivationof the initiated pits or more probably to a plugging of the porosity by corrosion products. 3.1.3. Polari:atioi? clinw The polarization curves of the different materials are presented in Fig. 4. The electrochemical characteristics determined from the polarization curves are reported in Table 3. The corrosion potential of a coated steel sample is a mixed potential between Al and steel. As previously stated. that potential is close to the pitting potential of aluminium which induces high corrosion current density. The bilayer coating presents a more negative potential and a much reduced current density compared with the monolayer one. The former leads to better corrosjon resistance, 55th the intermediate titanium layer providing the beneficial effect.
J. Creus
IE-3
ef al. /Thin
Solid
Films
346
(1999)
150-154
153 I 0
T
lE-9 -800
0 A
0,Ol Hz
: Aliriisteel 5 -750
Ih Zbh48h
cp
-700
-650
-600
Re(Z) n.ml
E (mV/ECS) Fig. 4. Pohxization curves of-Ah% and Al coated steel compared to bare aluminium after 1 h of immersion in a 3% NaCl solution.
3.1.4. Impedance spectroscop> The impedance diagrams, plotted in Nyquist or Bode mode at the free corrosion potential after I, 26 and 48 h of immersion, are representedin Fig. 5. According to the diagram shape, it is assumedthat several reactions may occur, without separationon the diagrams.Thus, the overall evolution of the loop during immersion is consideredhere. This evolution, during the first hours of immersion,confirms the deterioration of the outer aluminium layer. According to the impedancediagrams,we observe that no interconnection of porosity exists between Al and Ti layers. Furthermore, the galvanic effect is negligible between theseelements[7]. Consequently, the behaviour of the bilayer coated steel is similar to the bare aluminium one during the first hours of immersion. After 20 h of immersion, we notice an increase of charge transfer resistancewhich is associatedwith plugging of the porosity by corrosion products. The oxide film growth is confirmed by an XRD pattern and by EDS analysis. Impedance diagrams of monolayer Al coatings (Fig. 6) differ from the previous. During the first hour of immersion, the Nyquist diagram presentstwo clearly separatedcapacitive loops which correspond,at the highest frequencies, to the aluminium corrosion reaction strongly enhancedby the galvanic corrosion effect through the open porosity, and at the lowest frequencies, to the presence of Al corrosion products. The latter capacitive loop disappearsrapidly for longer immersion durations and the diameter of the first capacitive loop is reduced. The galvanic effect between steel and aluminium is important and leadsto a rapid dissolution of Al coating with an increaseof pore sizes together
Fig. 5. Nyquist impedance diagrams of AUWsteel, after immersion times of 1. 26 and 4S h in a 3% NaCI solution.
with creation of new opened porosity. This explains the disappearanceof the low frequencies capacitive loop and the decreaseof the transfer resistanceof the high frequencies capacitive loop. The sacrificial protection is no more effective for long immersion time. In fact, we observe a change in the impedance diagram shape which becomes similar to the bare steel impedancediagram. 3.2. Improvernent of corrosion resistanceof the bilayer coating by PVD oxidation treatment As previously stated, open porosity of the coating is an important parameter in the corrosion resistanceof a coated steel, whatever the nature of the consideredcoating. Essential for cathodic coatings, it also induces a shortening of sacrificial protection duration for anodic coatings. In order to improve the corrosion behaviour of Al/T? coated steel by reducing the open porosity, we investigated two types of oxidation treatments.Firstly, the oxidation of the intermediate titanium before the deposition of the outer Al layer (Al/ TiO$lYsteel), and then the oxidation of the final layer of aluminium (Al,O,/Al/Ti/steel). The changesof potential with time of both multilayer 1 A
A
lh
Table 3 Electrochemical characteristics of coated steel after 1 h of immersion in a 3% NaCl solution
I&,, (mV/ECS) iLoi,(@/cm’)
Al
Al/steel
-750 0.25
-710 13
-722 0.25
Fig. 6. Nyquist impedance diagrams of Al/steel, after immersion times of 1, 27 and 44 h in a 3% NaCI solution.
154
J. Cress et al. /Thin
Solid Films 346 (1999)
150-1.54
Table 5 Electrochemical characteristics 3% NaCl solution
E,,,, (mVECS) L, Mkm?
Fig. 7. Polarization curves of .41/Ti01/Ti and A&O~/Al/Ti coated compared to ALTi coated steel, and bare Al, after 1 h of immersion 3% NaCl solution.
steel in a
coatings are similar to the bare aluminium potential behaviour. The variation of polarization resistance with time and the electrochemical characteristics determined from the polarization curves after one hour of immersion in saline solution (Fig. 7), are summarized, respectively, in Table 4 and Table 5. The variation of R, with time are similar during the first hours of immersion. essentially due to the effect of galvanic corrosion between Al and Ti through the open porosity of the outer Al layer. After a few hours of immersion, an increase in R, is observed for Al/T? coated steel due to the plugging of the porosity of the aluminium layer by corrosion products. For samples with a final oxidation treatment of the aluminium layer, the corrosion potentia1 is shifted towards more positive values, and the presence of defects in this oxide layer leads to an increase in localized corrosion through the porosity and thus a much higher corrosion current density. Nonetheless, the impedance data allow us to conclude that this multilayer coating presents a very good corrosion behaviour during the first hours of immersion, but rapidly suffers from a strong corrosion due to the defects of the oxide film. The AliTiOjTi/steel also has a similar corrosion beha-
Immersion
0.5 2.0 3.0 15.0 35.0 75.0
duration
(R,) evolution
(h)
during
a 75-h immersion
test in a 3%
R, (0 cm’) AL’Ti/steel
AyTiOZ/Ti/steel
Al~O~/ArlTisteel
45000 30000 20000 10000 120000 145000
40000 3000 2700 3800 2000
40000 2200 3000 3000 17000
Al/Ti/steel
ALTiOJTilsteel
-722
-718 -
025
~_
ina
-695 -71
viour to the A1203/AUTi/steel one, but the mechanism of degradation involved differs from the previous. A delamination of the aluminium layer rapidly occurs after the first hours of immersion. This d&dminatio?-~risesfrfrom a lack of adhesion between the Ti02 layer and the final layer of aluminium. Finally, contrary to our expectation, the oxidation treatments do not lead to an additional improvement of Al/E coated steel corrosion behaviour, and it may be worse~in some cases, for instance for long immersion duration (Table 4).
4 Conclusions Porosity is an important criterion for the corrosion resistance of Al coated steel and could lead to a shortening of the sacrificial properties of an aluminium coating for long immersion times. The beneficial effect induced by an intermediate layer of titanium is underlined. It considerably reduces the pomsity and also reduces the galvanic effect between steel and aluminium. The PVD post-oxidation treatments unfortunately do not offer additional beneficial effects for corrosion resistance. The delamination of alumnium layer on one hand and the porous oxide layer on the other hand lead to high corrosion current densities. Perhaps. dense Al203 films, like those elaborated by~anodisation, would strongly enhance the corrosion resistance of the duplex coating. A further possibility is the use of polymer impregnation which may promote the same effect and therefore offer additional means to improve the corrosion resistance of duplex coatings.
References Ul
Table 4 Polarization resistance NaCl solution
of coated steel after 1 h of immersion
J. Creus, H. Idrissi, H. Mazille, C.Charrier, P. Jacquot, First online corrosion conference 11996) Intercorr’96, Texas, Session 3. -~ Van Wesling, G.M. FeEti, PI M.P.W. Vreijling, P.R. Willensen,E.P.M. J.H.W. De Wit, Eurocorr. Nice 1996 (1996) XI, OR 33, pp. l-5. Cijutk n(19927 7W; ~~ t31 H. Kagech?&, H. f(ibe;&o&n 141 H.S. Isaacs, Corrosion 43 (1987) 594. ~ 151 R.T. Foley, Corrosion 42 (19861 277. 161L. Garrigues, N. Pebere. F. Dabosi, Elecuochim. Acta 41 (1996) 1209. 171 J. Creus, H. Idrissi, H. Mazille, Surf. Eng. 13 (1997) 41.5. ~~~ ~ [sl F. Mansfeld. D.H. Hengstenberg, J.V. Ker&el, Corrosion 30 (l-974) 343. PI A. Merati, Ph.D. INSA Lyon (France), 199d pp. 130 . -_. --. IlOl C. Charrier, P. Jacquot, E. De&se, J.P. Millet, H. Mazilie, Surf,~t. Technol. 90 (1997) 29.
Thin Solid Films 346 (1999) 150-154
Corrosion behaviour of AllTi coating elaborated by cathodic arc PVD process onto mild steel substrate J. Creusa, H. Idrissia, H. Mazille”‘*, “Lubmmire
de Physicochimie Indmtrielle, ‘Soci6rle Tnnovatique (HIT),
F. Sanchetteb, P. Jacquotb
INSA. bcif 401. F69621 Villeurbmne B.P. 143, F-69680 Chassieu, France
cede.r. Frunce
Received 30 December 1997: received in revised form 20 July 199s; accepted 22 November
199s
Abstract The useof cadmiumcoatingsin industryis stronglyrestrictedby anEuropeandirective.cadmiumbeingdangerous for humanhealthand Aluminium-based coatings are potential candidates for replacing cadmium coatings. In this paper, we compare>he corrosionbehaviourof Al and AVTi coatedsteel.We demonstrate that the galvaniceffect throughthe openporusity of the AI coating leadsto a rapidlossof sacrificialprotectionduringlongimmersion time.It is shownthatthedouble-layercoatingAlRiisteel presents a b_etter the environment.
corrosion resistance than monolayer Al coating in saline solution. The intermediate titanium layer permits the reduction of the open porosity
andalsoallowsthe reductionof the galvaniceffect betweensteelandaluminium.In orderto enhancetheelectrochemical propertiesof the double-layercoatingwe alsoinvestigatedthe effect of PVD post-oxidationtreatments.0 1555ElsevierScienceS.A. All rights reserved. Ke.w~o&:
Aluminium:
Titanium: Physical vapour deposition coating: Multilayer
1. Introduction Depositedaluminium, in the form of a dense,uniform and adhesive coating, has been used for many years and may often replace cadmium plating for aeronautical or automotive applications [2]. Aluminium coatings are widely used becauseof their good corrosion resistance[3]. The protection properties of the aluminium basedcoatings depend on the formation of an oxide or hydroxide film on the surface. In NaCl solution, the pitting potential of AI alloys is very close to the open circuit corrosion potential [4-61. Foley [5] and Garrigues [6] found a potential difference of only 20 mV. Consequently, in chloride solution, aluminium corrosion is mainly due to pitting. Furthermore, the galvanic corrosion effect between Al alloys and mild steel, for a unit surface ratio between them. leads to a mixed potential very close to the pitting potential, with a very important galvanic current density. Such a galvanic effect considerably reduces the lifetime of Al coatings in NaCl solution [ 1,7,8]. To enhancethe durability of the coating, it is necessary to reduce the presence of defects such as pinholes. pores, cracks. It is also possible to increase the coating thickness[ 1,9]. However. the industrial requirementsoften needan accurate thickness,for instancelessthan 15 pm for screwing applications. A further possibility is to fabricate * Corresponding E-vtuil addrev;
author. Fax: + 33 472-33-87-15;. [email protected] 1H. Mazille)
0010~6090/99/S - see front matter PII: s0040-6090(98)01732-8
0 1999 Elsevier
coatin,. 0’ Corrosion;
Electrochemicai
behaviour
multilayer coatings,with an intermediatecorrosion r&stant metal such as titanium. Previous work 19,lo] reported the corrosion behaviour of multilayer TilAl coatingsmadeup of six alternated layers of titanium and aluminium; the combination, with an initial internal titanium layer and an external aluminium one, provides the best corrosion resistance. In this paper. we comparethe corrosionresistanceof steel protected by dual Al/Ti layers with bare steel or steel protected by only onelayer of Al. Then, the electrochemical behaviour of multilayer coatings ALTiOJTi/steel And AlzOj/Al/Ti/steel is also investigated.
2. Materials and experimental procedure 2.1. Materials and test sollrtioil The composition of AISI 113.5steel substrateis reported in Table 1. Pure solid sources(Ti or Al) are evaporated by meansof PVD cathodic multi-arc technology in a low pressureatmosphereof pure argon. The main depositionparameterswere a negative bias ranging between 100 and 150 V, a pressure ranging between 0.5 and 4 Pa and a growth rate ranging between 4 and 6 pm h-‘. The multilayer AlEi coatings are fabricated by a successiveevaporation from Ti and Al cathodes.The duration of coating deposition is adapted-to reach a total coating thickness of 15 pm. The oxidation
Science S.A. All rights reserved.
J. Crerrs et al. /Thin
Solid Films 346 (19991 150-154
1.51
Table 1 AISI 4135 steel composition Element
C
Si
Mn
s
P
Ni
Cr
MO
Fe
4c By weight
0.337
0.36
0.69
<0.003
0.01
0.23
1.11
0.23
Balance
treatments are realized by the addition of a reactive oxygen flow in the reactor during the 5-min following the PVD deposition of the previous metal. The morphology and microstructure of the deposited coatings are investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) in secondary electron (SE) and backscattering electron (BSE) modes. EDX analysis are used for measuring the coating surface composition and for following the concentration profile of Ti and Al elements in a sectioned sample. The test solution for electrochemical investigations is an aerated 3% NaCl solution, stirred at 500 rpm with a rotating working electrode; generally, the samples, mounted as the rotating working electrode, are cylinders of 10 mm diameter and have a total exposed area of 3 cm’.
2.2.2. Poterztiodynamic polarization
cun’es: i(E)
After an initial potential stabilization of 1 h, the curves i(E) are plotted from -50 mV vs. open circuit potential
(OCP) in the cathodic side up to + 100 mV in the anodic side, using a sweep rate of 10 mV/min. 2.2.3. Electrochemical
impedance spectroscopy
(EIS)
Impedance determinations are made with a Solar&on 1254 frequency response analyser (FRA) for frequencies ranging from 4 mHz up to 64 kHz. The amplitude of sinusoidal signals is set at 10 mV around the free corrosion potential. Results are interpreted in terms of Nyquist plots. 3. Results and discussion
2.2. Esperirilerltal procedure
3.1. Corrosion
The electrochemical experiments are controlled by an EGG 273A potentiostat, using the conventional three-electrode technique. The potential is referred to a saturated calomel electrode (SCE) and the counter electrode is a platinum sheet. The electrochemical techniques used to characterize the behaviour of coated steel samples are: 2.2.1. Open circuit potential recordings drrring protracted irnrnersion tests: E(t) md R,,(tj
After being cleaned and dried, samples are immersed from 48 to 75 h in the saline solution. The open circuit potential is recorded versus time with simultaneous periodic measurements of polarization resistance performed between 220 mV around the free corrosion potential. For the polarization resistance determination. a sweep rate of 10 mVlmin was employed.
behaviour
qf Al and AUTi coated steel
3.1.1. Morplzology and wicrostructure
The morphology of Al coatings have been discussed in a previous paper [I], and we just mention here their cellular morphology, typical of PVD coatings. We find the same characteristic for Al/Ti coatings, with the presence of some defects such as droplets, preferential growths, micropores, inducing local heterogeneities in coating thickness (Fig. 1). SEM observations, on cross-section views. show distinctly the two layers, an internal compact one (5 p,rn thick) corresponding to the titanium layer and a external layer of aluminium (10 p,rn thick). The Ti and Al composition profiles. obtained by EDX analysis, do not suggest the existence of a thick interdiffusion region. However, due to a -460 T
. Al/steel
“-e e
Fig. 1. SEM observation
(X1250)
of a cross-section
of AliTi coated ste&
Fig. 2. Potential evolution of Al and AliTi immersion time in a 3% NaCl solution.
coated
steel during
a long
J. Crew
et al. /Thin
Solid Films 346 (1999)
Fig. 3. SEM observation 1x100) in backscattering electron mode iBSE) of ALTi coated steel, after a long immersion time in a 3% NaCl solution.
relatively large electron probe size (= 1 km3), it is not possible to conclude on a real lack of interdiffusion.
3.1.2. PotentiaI nnd polarization resistanceevollttions during n long immersionperiod During the first hours of immersion, the potential of the AUTi coatings onto steelstabilizesat around -730 mV/ECS (Fig. 2). The potential-time behaviour of steel coated by only one layer of aluminium is markedly different. with the open porosity of the Al coating enabling a potential shift towards the pitting potential, and a consequent,drastic increasein the corrosion current density after a few hoursof immersion. A suddenpotential shift occurs. associatedwith the corrosion of the substratewhich is no longer protected by the sacrificial coating. The fast, but short rise up to -450 mV is attributed to the dissolution of the substrate: the potential variation is similar to that observed on bare steel. For Al/l? coatings. the intermediate titanium layer decreasesthe open porosity, thus increasing the duration of the sacrificial behaviour of the outer aluminium coating. SEM observations, in backscattering electron mode @SE) (Fig. 3) of AIRi coated steel show the presenceof different areas:dark needle-like are essentiallycomposedof Al, with dispersedwhite areasbeing composedof 95% of titanium and a generalgreyish matrix-rich in titanium (67 at.%). Thesedifferent zones correspondto the heterogeneity Table 2 Polarization resistance (R,) evolution during a 75-h immersion test in a 3% NaCl solution Immersion duration ih)
0.5 4.0 15.0 35.0 75.0
R, (fi cm’) Al/Ti/steel
Al/steel
Bare Al
45000 20000 1cQoo 120000 145000
760 185 95 475 390
100000 90000 50000 19000 -
1X?-I54
in the coating thickness,exacerbatedby a corrosion immersion test of 80 h. SEM observations of cross-sectionviews confirm the decreaseof Al thickness after immersion tests of 80 h. EDX analyses(at low energy 10 KeV) alsoreveal an important amount of oxygen on the surface. This suggests&e formation of an oxide or hydrated oxide film during the corrosion test, asconfirmedalso by electronoptical observations of a samplepreviously immersedduring 75 h in the saline solution. R, determinations on multilayer Al/Ti coatings are reported in Table 2. They are compared with the values for bare aluminium and Al coated steel. The Al coated steel presentsthe lowest I?, values, essentially due to a high galvanic effect between the substrateand Al through the coating porosity. The openporosity increases with time of immersion, increasing the galvanic effect, which is very deleteriousfor the aluminium. The corrosion behaviour of the bilayer coating is improved over the monolayer one, and is very similar to the bare aluminium behaviour. The intermediate titanium layer considerably reduces the galvanic effect between steel and aluminium, essentially due to a decreasein the open porosity betweenaluminium and steel substrate. We should also underline that titanium coating is more noble than steel, which could suffer from localized corrosion through the porosity. However, the external layer of aluminium prevents this phenomenon,due to its sacrificial behaviour. Furthermore, a thorough investigation of the galvanic corrosion effect between bare metals (Ti, Al and steel) allowed us to conclude that Ti-Al galvanic corrosion is less harmful than Al-steel couple, although the open circuit potential difference is larger in the former case[7]. We notice a decreaseof R, values during the immersion time for all samples,which is associatedwith the formation and spreadingof pits. However, for coated samples.we observe different behaviour during long immersion times, for which an increaseof R, values occurs. This subsequent increasecorrespondseither to a repassivationof the initiated pits or more probably to a plugging of the porosity by corrosion products. 3.1.3. Polari:atioi? clinw The polarization curves of the different materials are presented in Fig. 4. The electrochemical characteristics determined from the polarization curves are reported in Table 3. The corrosion potential of a coated steel sample is a mixed potential between Al and steel. As previously stated. that potential is close to the pitting potential of aluminium which induces high corrosion current density. The bilayer coating presents a more negative potential and a much reduced current density compared with the monolayer one. The former leads to better corrosjon resistance, 55th the intermediate titanium layer providing the beneficial effect.
J. Creus
IE-3
ef al. /Thin
Solid
Films
346
(1999)
150-154
153 I 0
T
lE-9 -800
0 A
0,Ol Hz
: Aliriisteel 5 -750
Ih Zbh48h
cp
-700
-650
-600
Re(Z) n.ml
E (mV/ECS) Fig. 4. Pohxization curves of-Ah% and Al coated steel compared to bare aluminium after 1 h of immersion in a 3% NaCl solution.
3.1.4. Impedance spectroscop> The impedance diagrams, plotted in Nyquist or Bode mode at the free corrosion potential after I, 26 and 48 h of immersion, are representedin Fig. 5. According to the diagram shape, it is assumedthat several reactions may occur, without separationon the diagrams.Thus, the overall evolution of the loop during immersion is consideredhere. This evolution, during the first hours of immersion,confirms the deterioration of the outer aluminium layer. According to the impedancediagrams,we observe that no interconnection of porosity exists between Al and Ti layers. Furthermore, the galvanic effect is negligible between theseelements[7]. Consequently, the behaviour of the bilayer coated steel is similar to the bare aluminium one during the first hours of immersion. After 20 h of immersion, we notice an increase of charge transfer resistancewhich is associatedwith plugging of the porosity by corrosion products. The oxide film growth is confirmed by an XRD pattern and by EDS analysis. Impedance diagrams of monolayer Al coatings (Fig. 6) differ from the previous. During the first hour of immersion, the Nyquist diagram presentstwo clearly separatedcapacitive loops which correspond,at the highest frequencies, to the aluminium corrosion reaction strongly enhancedby the galvanic corrosion effect through the open porosity, and at the lowest frequencies, to the presence of Al corrosion products. The latter capacitive loop disappearsrapidly for longer immersion durations and the diameter of the first capacitive loop is reduced. The galvanic effect between steel and aluminium is important and leadsto a rapid dissolution of Al coating with an increaseof pore sizes together
Fig. 5. Nyquist impedance diagrams of AUWsteel, after immersion times of 1. 26 and 4S h in a 3% NaCI solution.
with creation of new opened porosity. This explains the disappearanceof the low frequencies capacitive loop and the decreaseof the transfer resistanceof the high frequencies capacitive loop. The sacrificial protection is no more effective for long immersion time. In fact, we observe a change in the impedance diagram shape which becomes similar to the bare steel impedancediagram. 3.2. Improvernent of corrosion resistanceof the bilayer coating by PVD oxidation treatment As previously stated, open porosity of the coating is an important parameter in the corrosion resistanceof a coated steel, whatever the nature of the consideredcoating. Essential for cathodic coatings, it also induces a shortening of sacrificial protection duration for anodic coatings. In order to improve the corrosion behaviour of Al/T? coated steel by reducing the open porosity, we investigated two types of oxidation treatments.Firstly, the oxidation of the intermediate titanium before the deposition of the outer Al layer (Al/ TiO$lYsteel), and then the oxidation of the final layer of aluminium (Al,O,/Al/Ti/steel). The changesof potential with time of both multilayer 1 A
A
lh
Table 3 Electrochemical characteristics of coated steel after 1 h of immersion in a 3% NaCl solution
I&,, (mV/ECS) iLoi,(@/cm’)
Al
Al/steel
-750 0.25
-710 13
-722 0.25
Fig. 6. Nyquist impedance diagrams of Al/steel, after immersion times of 1, 27 and 44 h in a 3% NaCI solution.
154
J. Cress et al. /Thin
Solid Films 346 (1999)
150-1.54
Table 5 Electrochemical characteristics 3% NaCl solution
E,,,, (mVECS) L, Mkm?
Fig. 7. Polarization curves of .41/Ti01/Ti and A&O~/Al/Ti coated compared to ALTi coated steel, and bare Al, after 1 h of immersion 3% NaCl solution.
steel in a
coatings are similar to the bare aluminium potential behaviour. The variation of polarization resistance with time and the electrochemical characteristics determined from the polarization curves after one hour of immersion in saline solution (Fig. 7), are summarized, respectively, in Table 4 and Table 5. The variation of R, with time are similar during the first hours of immersion. essentially due to the effect of galvanic corrosion between Al and Ti through the open porosity of the outer Al layer. After a few hours of immersion, an increase in R, is observed for Al/T? coated steel due to the plugging of the porosity of the aluminium layer by corrosion products. For samples with a final oxidation treatment of the aluminium layer, the corrosion potentia1 is shifted towards more positive values, and the presence of defects in this oxide layer leads to an increase in localized corrosion through the porosity and thus a much higher corrosion current density. Nonetheless, the impedance data allow us to conclude that this multilayer coating presents a very good corrosion behaviour during the first hours of immersion, but rapidly suffers from a strong corrosion due to the defects of the oxide film. The AliTiOjTi/steel also has a similar corrosion beha-
Immersion
0.5 2.0 3.0 15.0 35.0 75.0
duration
(R,) evolution
(h)
during
a 75-h immersion
test in a 3%
R, (0 cm’) AL’Ti/steel
AyTiOZ/Ti/steel
Al~O~/ArlTisteel
45000 30000 20000 10000 120000 145000
40000 3000 2700 3800 2000
40000 2200 3000 3000 17000
Al/Ti/steel
ALTiOJTilsteel
-722
-718 -
025
~_
ina
-695 -71
viour to the A1203/AUTi/steel one, but the mechanism of degradation involved differs from the previous. A delamination of the aluminium layer rapidly occurs after the first hours of immersion. This d&dminatio?-~risesfrfrom a lack of adhesion between the Ti02 layer and the final layer of aluminium. Finally, contrary to our expectation, the oxidation treatments do not lead to an additional improvement of Al/E coated steel corrosion behaviour, and it may be worse~in some cases, for instance for long immersion duration (Table 4).
4 Conclusions Porosity is an important criterion for the corrosion resistance of Al coated steel and could lead to a shortening of the sacrificial properties of an aluminium coating for long immersion times. The beneficial effect induced by an intermediate layer of titanium is underlined. It considerably reduces the pomsity and also reduces the galvanic effect between steel and aluminium. The PVD post-oxidation treatments unfortunately do not offer additional beneficial effects for corrosion resistance. The delamination of alumnium layer on one hand and the porous oxide layer on the other hand lead to high corrosion current densities. Perhaps. dense Al203 films, like those elaborated by~anodisation, would strongly enhance the corrosion resistance of the duplex coating. A further possibility is the use of polymer impregnation which may promote the same effect and therefore offer additional means to improve the corrosion resistance of duplex coatings.
References Ul
Table 4 Polarization resistance NaCl solution
of coated steel after 1 h of immersion
J. Creus, H. Idrissi, H. Mazille, C.Charrier, P. Jacquot, First online corrosion conference 11996) Intercorr’96, Texas, Session 3. -~ Van Wesling, G.M. FeEti, PI M.P.W. Vreijling, P.R. Willensen,E.P.M. J.H.W. De Wit, Eurocorr. Nice 1996 (1996) XI, OR 33, pp. l-5. Cijutk n(19927 7W; ~~ t31 H. Kagech?&, H. f(ibe;&o&n 141 H.S. Isaacs, Corrosion 43 (1987) 594. ~ 151 R.T. Foley, Corrosion 42 (19861 277. 161L. Garrigues, N. Pebere. F. Dabosi, Elecuochim. Acta 41 (1996) 1209. 171 J. Creus, H. Idrissi, H. Mazille, Surf. Eng. 13 (1997) 41.5. ~~~ ~ [sl F. Mansfeld. D.H. Hengstenberg, J.V. Ker&el, Corrosion 30 (l-974) 343. PI A. Merati, Ph.D. INSA Lyon (France), 199d pp. 130 . -_. --. IlOl C. Charrier, P. Jacquot, E. De&se, J.P. Millet, H. Mazilie, Surf,~t. Technol. 90 (1997) 29.