Ciência e Natura, Santa Maria, 15: 35-43, 1993. 35

ELECTROCHEMICAL EVIDENCE OF THE INFLUENCE OF LIGHT ON THECORROSION PROCESSES OF LOW-CARBON STEEL IN DILUTED SULFURIC ACID

Reinaldo Simões Gonçalves and Roni Fábio Dalla CostaDepartamento de Química - Centro de Ciências Naturais e ExatasUFSM - Santa Maria, RS

ABSTRACT

Two different electrochemical methods confirm the influence of light onthe anodic current values observed when low carbon steel electrodes werepolarized in deaerated 1.0 N H2S04 solutions in the anodic potential range.The h ighest current dens ity va lues were obta ined when the system is underillumination. The effect of natural illumination conditions on the weigh-lossexperiments in aerated 1.0 N H2S04, confirms that the corrosion rate is higherin an illuminated medium than in the dark.

GONÇALVES, R.S. e DALLA COSTA, R.F. 1994. Evidência eletroquímica dainfluência da luz nos processos de corrosão do aço carbono em ácido sulfúricodi luído.

Dois métodos eletroquímicos diferentes confirmaram a influência da luzsobre as dens idades de corrente anód icas regi stradas durante a po lari zaçãoanódica do aço carbono em H2S04 1,O N desarejado. Os ma isaltos va lores dedensidades de corrente foram observados quando o meio esteve sob iluminação.A luz natural também afetou os ensaios de perda de massa em H2S04 1,0 Narejado, confirmando que a velocidade de corrosão é maior no meio iluminado doque no escuro.

Photocorrosion, low carbon steel, diluted sulfuric acid

INTRODucnONIt is well-known that the action of light in changing the measured

photopotentials or photocurrents is dependent on: a) wavelength and intensity

of the light beam, b) nature of the electrolyte in solution, c) metal used for

36the electrode , d) presence or absence of oxygen, and e) prepolarizat ion orpo1arizat ion condit ions of the e1ectrode"'.

eorrosion processes of metals occur as a result of many competinginfluences so that the sequence of corros ion init iat ion and propagat ion canvary wide1y with different metals and corrosion environments. The influenceof radiat ion on the corros ion processes of copper(2·3), iron"', and irona110y"'" has been described in the 1iterature as a we 11-known facto

The work reported here was undertaken to make a systematic study of theeffect of light on the corrosion processes of 10w carbon steel in 1.0 N H2S04at 25°e in order to show that this variable must be taken under contro1 to geta more reproducib1e data. Two different e1ectrochemica1 methods were used: a)partia1 po1arization curves, and b) potentiodynamic profi1es. To obtaincomparative performance data, weight 10ss' tests were made in the sameaggressive solutions.

EXPERIMENTAL

E7ectrochemica7 methods

lhe e1ectrochemica 1 experiments were carried out in a three- e1ectrodece11 with a p1atinum counter-e1ectrode and a saturated ca1ome1 e1ectrode (SeE)as reference. A 11 potent ials are reported with respect to it. The work ingelectrode was a 10w carbon stee1 (e 0.049; Mn 0.227; S 0.0005; er 2.34 wt%).The samp1es were cut from the same sheet in a rectangu1ar form (2.0 x 2.5 x0.1 cm thickness). After mechanica1 po1ishing with emery papers, the specimenedges were covered with an epoxy glue. 10 ca1culate the current density, theoperating surface was measured with a sliding ca1iper.

lhe acid solutions were prepared from distil1ed water and Merck su1furicacid (p.a.). lhey were de-oxygenated by bubbling pure N2 through thesolutions.

lhe e1ectronic arrangement consisted of a PAR 173 potentiostat with a PAR376 loqar ithm current conversor, a PAR 175 universal programer, and a PARRE0089 X-V recorder.

B1ack plastic was used to cover the electrochemica1 ce11 to prevent the

37entrance of the. In the illuminated experiments a light source of 250 wattlamp was mounted in a slide projector fitted with condensing and collimatinglenses.

/IIeight 10ss experiments

The weight loss tests were made with disk samples of the metal. Thecoupons were cleaned with distilled water then degreased with carbontetrachloride, acetone, ethyl alcohol in this sequence, and dried. Thistreatment was carried out immediately before and after making the tests. Thecorros ion rates were a1so ca leulated on the bas is of the apparent surfacearea. The immersion time for the weight losses at 25·C in aerated conditionswas 30, 60, 90, 120, 150 and 180 mino

~ ANO OISCUSSION

Partial polarization curves

The current-potential curves under potentiostatic control were determinedafter the potential program shown in Fig I. This was necessary in order to

-340

wUV')

Ecorr

>E-w-550

0.0 30 60

Fig.tis

The potential program applied to the working electrodebefore the potentiostatic experiments.

obtain a better reproduction of the data. Anodic potential steps were appHedto oxidize the organic material initially adsorbed on the electrode surface.Cathodic potential steps were immediately applied to reduce the oxide layer

38formed. After this, the electrode potential was kept in the cathodic regionfor 30 s in order to start from a reproducible surface conditions. The anodicand/or cathodic polarization curves start from the corrosion potential.

The partial cathodic and anodic polarization curves of low carbon steelin deaerated 1.0 N H2S04 measured at 25 ·C are shown in Fig 2, under differentillumination conditions.

600~-----------------------------.

500

UJ __--IUV) 450 --.>E~, 400

350

300-2.0 -1.0 0.0 1.0 2.0

550

Fig.In (j/mA em")

Partial polarization curves of the low carbon stee 1 indeaerated 1.0 N H2S04 in: (6) illuminated; (O) unilluminated medium.

The current values shown that both anodic and cathodic polarizationcurves increases when the system is under illumination. This effect wasobserved in all potential range studied. This may mean that the corrosionprocesses are influenced by illumination of the system.

Cyc7ic voltammetric studies

These studies were carried out after a pre-activation of the electrode inthe aggressive solution. This pre-activation process consisted of a cyclicvariation of the potential at a sweep rate of 50 mV S-1 during a 5 min period,between -550 mV and -340 mV(SCE). Following this pre-activation process apotential program was applied to the electrode before each voltanrnogram, asdescribed already'?) .

39The potentiodynamic I(E) profiles recorded for the low carbon steel in

deaerated 1.0 N H2S04 under illuminated and unilluminated conditions arepresented in Fig. 3.

'l"Eo<E

o~--

a)

Jb)//

/// /

// I/ I

, I/ /

//

/

-550 -340EtmV(SCE)

Fig. 3 - Potentiodynamic curves of low carbon steel in deaerated 1.0 N H2S04at 2 mVs-' in: (a) illuminated; (b) unilluminated medium.

The main information taken from these curves are related to the current

corros ion potent ia 1, Ecorr.density variations during anodic sweep rate in the potential range near the

As it is possible to see, the anodic currents related to the anodicprocesses were higher in the illuminated medium than in the dark one. In theother side, the cathodic currents did not change as significantly as theanodic currents under the light influence.

This may mean that the light influences the anodic processes due probablyto the enhancement of the charge transfer rate. This effect is in goodagreement with that observed with partial polarization curves.

In considering that the potential sweep rate was very small (v =

2mVs-'), the current density values were plotted against the potential, in thesame way that applied to the partial polarization curves. The Fig. 4 show the

40

520r----------------------------,

420

--/------;~-.•.~--_::_--.•.....

500480

w~ 460

:e-w 440I

400 L __ ..L....__ -L __ -L- __ -:1::--_---7-3.0 -2.0 -1.0 0.0 1.0 2.0

Fig. 4 - Taffel curves taken from l(E) profiles of low cabon steelin deaerated 1.0 N H2S04 at 2 mVs-> in: (~) illuminated; (O) unilluminatedmedium.

plot E x ln(j) taken from l(E) curves, in deaerated 1.0 N H2S04 underdifferent illumination conditions at different potentials near the corrosionpotential. lt is confirmed that the light can influence the anodic processesthat occur during anodic sweep rate after the corrosion potential, Ecorr, inall potential range. This means that the illumination condition of the systemis a variable important to be taken under control in order to obtain morereproductive data in this potential region.

The extrapolation of anodic and/or cathodic Tafel lines gives thecorrosion current density, jcorr, at the corrosion potential, Eoor:O

). Thesevalues taken from both electrochemical methods are presented in table 1.

The corrosion potential values, Ecorr, are in good agreement with thosepublished before(7) and do not change significantly with the illuminationconditions. This effect may be related to the fact that the corrosionmechanisms are not affected by illumination. However, the corrosion densitycurrents, joorr, were higher in the illuminated medium than in the dark. Th fsmay mean that the corros ion k inet ics are affected by 1ight. Thisphotocorrosion phenomena may be related to the photo-electrochemical response

41of oxide layers on the metal"'.

Table I - Theoretical data taken from Partial Polarization Curves (PPC) andCyclic Voltammetry Curves (CVC) in both cathodic and anodic potential range,on low carbon steel in deaerated 1.0 N H2S04 under different illuminationconditions.

PPC CVC

- Eeo=/mV (SCE)jeorr/mA cm'"

Illum.445

0.419

Uni llum.444

0.377

Illum.469

0.380

Uni llum.466

0.345

Illum. = IlluminatedUnillum. = Unilluminated

Weight 70S5 experiment5

Weight loss tests were made in the same aggressive solutions in order tocompare the influence of the natural light on the corrosion processes thatoccur on the metallic surface. In these experiments the illumination controlwas made by keeping the coupons under different illuminat ions condit ions. Apart of them was kept in the dark and the other was placed in the laboratoryunder the influence of natural light.

Figure 5 shows the weight loss changes at different immersion times inaerated 1.0 N H2S04.

As it is possible to see even under natural illumination conditions, thecorrosion rate is significatively higher in the illuminated medium than in thedark. The difference of the weight loss variations increases with increasingimmersion time. This effect is in good agreement with that observed by othertechniques and the result confirms that the light is an important variable incorrosion studies.

CONCLUSIONS

42

4.0

~Eu 3.0-õ-:;( 2.0

Ê<J

1.0

0.00·0

F iq , 5

5.0r-------------------------------~

..-------L--... -'- .l.- ...J

50.0 100 200

Weight- loss changes of low carbon steel in aerated 1.0 N

150I/min

H2S04 at different immersion times in: (i~) illuminated; (O) unilluminatedconditions.

The electrochemica 1 data suggested that the light can influence thet;corrosion processes of low carbon steel in deaeretad aqueous sulfuric acid

~olutions. This effect is more pronounced in the anodic potential range where

in the dark ,the currents related to the anodic processes were higher than those observed

Weight loss experiments confirm this influence in aerated aqueous

illumination than in the dark.sulfuric acid solutions, giving higher corrosion rates values under natural

The effect of 1ight on the corros ion processes of low carbon stee 1 inaqueous sulfuric acid must be keep under control in order to obtain morereproducible data in corrosion experiments.

ACKNOWLEOGMENTS

The authors express their thanks to FAPERGS; CNPq; and, GTZ/Germany fortheir support to this work.

-------- ------------------------------

43

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