The New Mechanism Of Corrosion In Pore
THE NEW MECHANISM OF CORROSION IN PORE
OF CATHODE COATINGS ON STEEL
A.I. Kostrjitskiy, A.Yu. Kalinkov
Odessa State Academy of Food Industry,
Kanatnaya st. 112, 65039 Odessa, Ukraine, e-mail: profAIK@ipss.net
INTRODUCTION
Porosity is one of the main factors that defined protective properties of cathode coatings [1]. For the first approaching, intensivity of corrosion process in pore was determined by the efficiency of coating action as a cathode. This method has been used for protection corrosion control of different coatings [1,2]. It lets to compare the protective properties of cathode coatings on steel for 72-hours tests, if electrochemical mechanism of corrosion is assumed. It\’s often observed the through failure of substrates with thick coatings during prolonged (up to 1000 h) tests, while in thin coatings damage localized at film-substrate interface.
THEORY AND DISCUSSION
Developing principal position of Rosenfeld\’s galvanic element theory [3], we propose another approach to consideration of corrosion processes in pore of cathode coatings on steel. This approach is based on the following reasons:
– the identity of corrosion seats permits absolutely different character of depth corrosion processes;
– some cathode coatings, for example - vacuum evaporated coatings, – are characterized internal stresses, nature and initiation causes of which are well investigated in [2];
– the main reason – there are no systematic data about internal stress influence on corrosion mechanism in pore of cathode coatings on steel.
First trials have been made when studying chromium vacuum coatings corrosion behavior [2]. However, the primary attention has been spared on strained coating state, substrate was out of consideration, just so the conclusions were highly approximated.
The new corrosion mechanism of corrosion in pore of cathode coatings on steel has been based on the presence of internal stresses in some type of
cathode coatings [4]. Using the classical ideas about corrosion nature of strain deformed metal surfaces as well as results of L. Petrov [5], we can explain not only the final results of observed corrosion damage of steel in pores of cathode coatings, but also propose the new mechanism of corrosion failure seat development.
The initial cause of corrosion failure of steel in pore is the electrochemical processes, taking place in system "film – substrate". If the pore depth is nearly small (thin films), not only the walls of pore channel, but also adjacent parts of film surface may act as a cathode [6]. The corrosion current at the pore bottom for initial time-periods of immersion into solution may de written as
(1)
where DE – electromotive force of galvanic pair "pore bottom – coating"; Pa/Sa – specific polarizability of anode process.
Assumed, that anodic reaction polarizability Pa is time-indepedent or changes very slowly with time (by analogy with Petrov\’s theory of corrosion cracks [5]) and cathode process potential remains constant during observation period (Ek = const), the expression (1) may be rewritten as
(2)
The account of kinetics of electromotive force DE(t) and Sa(t) [1] as well as Est versus h [6] shows, that depth corrosion process development inhibits sharply with film thickness increasing. It is in good agreement with natural testing results, but doesn’t explain the appearance of deep and through corrosion failures in thick coatings as well as the cases of corrosion process under coatings.
Let us consider the role of mechanical factor in pore of thick cathode coatings. We imagine the corrosion process in such compressive strained coatings in the following way. At the first step pitting formation process takes place in consequence of galvanic pair action DE2 (potential differences coating potential Ec and bottom potential Eb). The approximate rate of pitting motion is determined by follows:
(3)
where F – electrochemical equivalent of iron, r - density of steel.
As pitting motion into strain deformed layers, its top becomes a concentrator of stresses and begins to work the second galvanic pair DE3 = s · a (s – stress level for given depth of pitting, a – deformation polarizability). The total rate of pitting motion is equal:
(4)
as far as both galvanic pair acting simultaneously. The conditions of such mechanism realization have been analysed in [1,4].
Considering the corrosion behavior of complicated electrode «porous coating – substrate), we assumed identity of physics-chemical processes in pore and in pitting. The estimation of veritable behavior of such complicated electrode and electrode process description may be produced only on the indirect indications. Recently the external evidence of corrosion process development in these system has been estimated by the following parameters [7]: corrosion elements current, anode current values, potential distribution along the sample surface, electrode potential dependence on film, measured from electrode potential kinetics [6], etc. The potential data of electrodes being exposed to local corrosion, that one can find in literature, are uncertain and depend on reciprocal arrangement of comparative electrode and corroded sample surface.
We also proposed the estimation technique of the interplay between veritable potential distribution along the surface of complicated electrode "porous coating – substrate" and literature data of stationary potential of such system [7]. Average potential shift for electrode, having N surface corrosion seats, may be determined as
. (5)
where DE – average potential shift for the sample, having total square Stowards negative region consequence of electrical field distortion near i-seat; Si – square of electrical fields distortion region near the i-seat; dE(Si) – the function, characterizing the field distortion near i-seat with respect to average electrode potential of coating material Ek; dS – electrode surface element.
Taking in to account (5) the stationary potential of steel sample with porous cathode coating may be expressed:
(6)
Obviously, if the coating is unporous, the function dE (Si) is equal to zero for any seat (electric field distortion is absent) and measured stationary potential value Est = Ek, so defined only by coating material properties.
SUMMARY
The proposed simplified model [7] for estimation of interplay between veritable potential distribution along the surface of complicated electrode "porous coating – substrate" and stationary potential values, measured by distant probe, assume the presence of the information about potential values Efor "coating – electrolyte" interface, that is in the pore outlet. This problem may be solved, using three-dimensional Laplace\’s equation for corrosion seat model as narrow capillary or narrow crack. For sufficiently narrow and long capillary, when axis system is realized, the total problem transfers to one-dimensional case. The example of such discussion have been analized in [8].
References
1. A.I. Kostrjitskiy, O.V. Lebedinskiy. Mnogocomponentnie vakuumnie pokytiya. – Moskva, Mashinostroenie, 1987. – 208 s.
2. I.L. Roych, L.N. Koltunova, S.N. Fedosov. Nanesenyie zhaschitnykh pokrytij v vakuume. – Moskva, Mashinostroenie, 1976. – 358 s.
3. I.L. Rosenfeld, L.V. Frolova. Zaschita metallov, v.4, 680, (1968).
4. A.I. Kostrjitskiy. Zaschita metallov, v.21, 64, (1985).
5. L.N. Petrov. Korrosiya pod napryazheniem. - Kiev, Vysshaya shkola, 1986. – 142 c.
6. A.I. Kostrjitskiy. Phusico-Chimichtskaya mtkhanica materialov, №3, 48, 1985.
7. A.I. Kostrjitsriy. Zaschita metallov, v.22, 960, 1986.
8. A.I. Kostrjitskiy. UkrNIINTI, Kiev, №961Uk-85Dep.,1985.
Related articles::