Short and long term velocity experiments were performed utilizing a rotating cylinder electrode. This was to determine the best way to model in the laboratory the flow conditions in service on a cathodically polarized surface or on anodically polarized zinc. This was studied by measuring the cathodic current required to protect HY-80 steel, nickel-aluminum-bronze and commercially pure titanium in flowing seawater. Potentiodynamic scans on a one inch diameter cylindrical specimen were run at rotation speeds of 50, 100, 200, 800 and 2000 rpm. The limiting current portion of the resulting polarization curves indicated that all three materials illustrated rotation speed dependent behavior. This behavior was consistent with a mechanism whereby the reaction kinetics are governed by oxygen diffusion through a fluid boundary layer. The currents measured for HY-80 were approximately an order of magnitude lower than those consistent with boundary layer diffusion-, due to the formation of a barrier layer of calcareous deposits or fouling. Increased rotation speeds, which result in increased shear stresses resulted In currents more consistent with the diffusion model, indicating that higher velocities result in stripping away surface deposits or change the morphology of the barrier film.

The effect of velocity on the anodic current output for zinc sacrificial anodes was also evaluated to determine the best way to model flow over a sacrificial anode. Some velocity effects were observed, but could not be related to a specific reaction control mechanism.

Long term potentiostatic tests were run to determine the effect of corrosion products and/or biological films on flow modelling of cathodic current demand. Rotation speeds of 40, 160, 800, 1600 and 2000 rpm were studied. The initial currents observed for bronze and steel correlated well with those values predicted for boundary layer diffusion. Shortly after polarizing these materials, the cathodic current decreased more than an order of magnitude, probably due to the formation of a film, such as a calcareous deposit. Titanium illustrated low currents initially, possibly due to the presence of a surface film. After a brief period of cathodic protection the current increased, indicating that the initial film was reduced. Upon reaching steady state the currents were still lower than for boundary layer diffusion, indicating the presence of another film.

Subject
Nernst layer, Electric current, Materials, Diffusion layers, Diffusion, Cathodic current, Bronze, Film formation, Steel, Oxygen, Titanium, Zinc, Corrosion potential
© 1987 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1987
Association for Materials Protection and Performance (AMPP)
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