- Email: [email protected]
Reply to the comments of F. Mansfeld
Ekcmchfmiea Acta, Vol. 39, No. IQ pp. 1453.1994 Copyri@ 8 1994Ehicr Science Ltd. Printed in Great Britain. All right8rcsemd 0013~4686/ws7.00 + 0.00
Pergamon
REPLY TO THE COMMENTS
OF F. MANSFELD
F. DEFLORIAN, L. FEDRIZZIand P. L. B~NORA MaterialEngineeringDepartment,Universityof Trento, Italy
In our opinion, herewith confirmed, the break-point method in the original version[l] is able to evaluate only the area of the defects through the organic coatings, and not the total delaminated area. Moreover it is very important to stress that our conviction arises from the experimental data shown in Fig. 3 and not from equation (ll), which concerns the limits of the break-point method. Concerning the new theoretical equations proposed by Mansfeld, we note that his work is not fully available since the second part of it (the more relevant) is still in press. However, in our work we intend to discuss the break-point method as proposed by Haruyama[ 11, and not the various successive elaborations and modifications of the theory[2,3,4]. The study of a phosphatized substrate involving a complex transmission line as a model is not the subject of our paper. It is true that the transmission line related to a phosphatized substrate is more complex than the proposed model[5], however even the mere presence of a second time constant is sufficient to affect the value off&. We are grateful for the careful revision of our paper which pointed out the presence of some typographical errors, such as the values of the coating thickness and the dielectric constant in page 1612 (actually 30pm and about 10, respectively). However, we would like to confirm that the most important part of our paper is the mathematical treatment, which is not under discussion. We have obtained experimental support of our assertions, some of which are now in press[6] using both different substrates and organic coatings with different chemical composition. A typical trend with time of the experimental product R,f,, is shown in Fig. 1, in which we also report a hypothetical evolution with time of the theoretical value of the product, considering also the deviations of the value of the dielectric constant. The essential fact is that the experimental value and the theoretical value approach each other only after some time of immer-
A
I
c
Days Fig. 1. Trend with time of the experimental data of R, fb5 and d/2me,.
sion, that is when the value of the double layer capacitance is much larger than the coating capacitance.
We agree with Mansfeld that the increase with time of the experimental value of R,f,, is surprising. This result pressed us toward a deeper investigation of the mathematical bases of the break-point method.
1.
2. 3. 4. 5. 6.
S. Haruyama, M. Asari and T. TSUN.in Corrosion Protection by Organic Coatings, (Edited by M. Kendig and H. Leidhaiser) Proc. Vol. 87-2, pp. 197-207. Electrochemical Society Pentington, NJ (1987). H. P. Hack and J. R. Scully, J. efectrochem. Sot. 138, 33 (1991). F. Mansfeld and C. H. Tsai, Corrosion 47,958 (1991). R. Hirayama and S. Haruyama, Corrosion 47, 952 (1991). F. Mansfeld, M. W. Kendig and S. Tsai, Corrosion 38, 478 (1982). F. Deflorian. L. Fedrbzi and P. L. Ronora, Corrosion SO, 113(1994).
Pergamon
REPLY TO THE COMMENTS
OF F. MANSFELD
F. DEFLORIAN, L. FEDRIZZIand P. L. B~NORA MaterialEngineeringDepartment,Universityof Trento, Italy
In our opinion, herewith confirmed, the break-point method in the original version[l] is able to evaluate only the area of the defects through the organic coatings, and not the total delaminated area. Moreover it is very important to stress that our conviction arises from the experimental data shown in Fig. 3 and not from equation (ll), which concerns the limits of the break-point method. Concerning the new theoretical equations proposed by Mansfeld, we note that his work is not fully available since the second part of it (the more relevant) is still in press. However, in our work we intend to discuss the break-point method as proposed by Haruyama[ 11, and not the various successive elaborations and modifications of the theory[2,3,4]. The study of a phosphatized substrate involving a complex transmission line as a model is not the subject of our paper. It is true that the transmission line related to a phosphatized substrate is more complex than the proposed model[5], however even the mere presence of a second time constant is sufficient to affect the value off&. We are grateful for the careful revision of our paper which pointed out the presence of some typographical errors, such as the values of the coating thickness and the dielectric constant in page 1612 (actually 30pm and about 10, respectively). However, we would like to confirm that the most important part of our paper is the mathematical treatment, which is not under discussion. We have obtained experimental support of our assertions, some of which are now in press[6] using both different substrates and organic coatings with different chemical composition. A typical trend with time of the experimental product R,f,, is shown in Fig. 1, in which we also report a hypothetical evolution with time of the theoretical value of the product, considering also the deviations of the value of the dielectric constant. The essential fact is that the experimental value and the theoretical value approach each other only after some time of immer-
A
I
c
Days Fig. 1. Trend with time of the experimental data of R, fb5 and d/2me,.
sion, that is when the value of the double layer capacitance is much larger than the coating capacitance.
We agree with Mansfeld that the increase with time of the experimental value of R,f,, is surprising. This result pressed us toward a deeper investigation of the mathematical bases of the break-point method.
1.
2. 3. 4. 5. 6.
S. Haruyama, M. Asari and T. TSUN.in Corrosion Protection by Organic Coatings, (Edited by M. Kendig and H. Leidhaiser) Proc. Vol. 87-2, pp. 197-207. Electrochemical Society Pentington, NJ (1987). H. P. Hack and J. R. Scully, J. efectrochem. Sot. 138, 33 (1991). F. Mansfeld and C. H. Tsai, Corrosion 47,958 (1991). R. Hirayama and S. Haruyama, Corrosion 47, 952 (1991). F. Mansfeld, M. W. Kendig and S. Tsai, Corrosion 38, 478 (1982). F. Deflorian. L. Fedrbzi and P. L. Ronora, Corrosion SO, 113(1994).