Organic and Inorganic Coatings for Corrosion Prevention

Research and Experience, Papers from EUROCORR '96

L. Fedrizzi

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9781000724899
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Organic and Inorganic Coatings for Corrosion Prevention - Research and Experiences is a collection of Papers from EUROCORR '96 and published for the European Federation of Corrosion by The Institute of Materials. In the session on Coatings the following topics were discussed: • Life-time prediction of organic coatings; • Environmentally friendly coatings; • Testing; and • Surface preparation techniques. This book contains a selection of the scientific work presented in the Conference with the aim of focusing on the research developments in the frame of corrosion protection coatings for industrial use. The book is in four sections describing, respectively, organic coatings, zinc coatings, other metallic coatings and ceramic coatings.

Part 1

Organic Coatings

1

Laboratory Corrosion Monitoring of Zinc-Rich Paint Coatings on Naval Steel in Seawater by Electrochemical Methods

E. C. BUCHARSKY^* and J. R. VILCHE

Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Sucursal 4-C.C.16, (1900) La Plata, Argentina

* Departamento de Ciencia y Tecnología, Universidad Nacional de Quilmes, Roque Saenz Peña 180, (1876) Bernal, Argentina

ABSTRACT

Laboratory assessment of the performance of zinc-rich paint (ZRP) coatings on naval steel in seawater has been carried out using electrochemical impedance spectroscopy (EIS) combined with cathodic protection potential measurements. Different formulations of painted naval steel samples were investigated during exposure times up to 50 days. ZRP coating formulations included those with the zinc pigment content covering a wide w/w-concentration range as well as those with different organic/inorganic binders based on chlorinated-rubber, epoxy-polyamide, ethyl-silicate and alkyd-resins. Experimental impedance diagrams have been quantitatively explained in terms of a transfer function analysis using identification procedures and non-linear fit routines. Information concerning the influence of both binder type and zinc pigment concentration on the corrosion protection behaviour of these ZRP coatings was obtained from the exposure time dependence of the system parameters, which were derived from the fitting procedure.

Characteristics and properties of the different naval steel/ZRP coating/seawater systems have been determined according to the application of an impedance transfer function model which describes the reactions taking place at the metal/film and film/solution interfaces as well as the diffusion processes through the active ZRP film. The kinetic parameters correlate well with gradual system deterioration and reflect the way in which the galvanic effect and barrier-action properties of the ZRP employed as primers progressively diminished. The whole set of results can be interpreted in terms of corrosion processes going on extensively through pores and cracks of these coatings.

1. Introduction

In modern technology, the use of coatings with organic and inorganic compounds provides a widespread method for the corrosion protection that is necessary to improve the durability of engineering metallic structures. In these industrial applications the corrosion protection properties are determined by a complex mechanism which includes the effects of factors such as coating formulation, chemical composition and pre-treatment of the metallic surface, environmental prevailing conditions, as well as electrochemical reactions occurring during the corrosion process either inside the coating itself and/or at the metal/coating interface during and after water uptake and oxygen permeation, respectively. Therefore, the protective properties of the coating can be attributed to an electrochemical action and/or a barrier-type effect. Due to the great diversity of products necessary to best serve the large number of specific purposes for which coatings are applied, the technology of protective organic/inorganic coatings has expanded tremendously in the course of the last decades. It is worth noting that high-performance coatings were developed as need for them arose and as materials became available which allowed their application.

The in situ deterioration of protective coatings during exposure to corrosive media can be explained by considering the typical service situations. Localised corrosion processes caused by inhomogeneities within the protective film applied on active metals are the most frequent reasons for damage of metallic surfaces which are covered by relatively thin organic/inorganic coatings. Even in the case of a high solid content in the coating, its inner structure will exhibit an inhomogeneous nature which becomes more so with penetration of aggressive electrolyte and with accumulation of corrosion products. Thus, the corrosion of polymer coated metals involves a gradual formation of defects followed by the penetration of corrosive species which promote progressive adhesion loss and subsequent attack of the underlying base metal [1]. Both electrical and electrochemical methods have been in general use for a long time to detect and evaluate the early degradation of coatings on steel [2].

The application of fast electrochemical transient methods for an appropriate evaluation of the relative ability of a painting scheme to provide anticorrosive protection to metallic substrates has generally not been available until recently. The situation appears to be changing, particularly with the advent of modern a.c. electrochemical techniques. There are now many examples in the literature describing the use of electrochemical impedance spectroscopy for investigations concerning protective coatings [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. In the field of polymer coated metals, this technique has mainly been employed to study the reaction kinetics and mass transport processes at electrodes with restricted diffusion to the polymer film structure [3,4], and to characterise the barrier properties of unpigmented organic polymer coatings [5].

In recent years, primers heavily pigmented with zinc dust have been extensively used as protective coatings of naval steels exposed to seawater environments. Zinc rich paints (ZRP) including binders based on epoxy, alkaline metal silicates or alkyd silicates gained initially similar acceptance. In the particular case of iron-based metallic subtrates exposed to either industrial or saline atmospheres, paints containing a high content of pure zinc particles homogeneously dispersed in both organic and inorganic binders are currently employed in corrosion protective ZRP coatings. In the case of inorganic binders, the use of ethyl-silicate appears to be predominant. It is interesting to note that the formulation of commercial ZRP coatings which are currently applied, includes v...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Introduction
• Part 1 – Organic Coatings
• Part 2 – Zinc Coatings
• Part 3 – Other Metallic Coatings
• Part 4 – Ceramic Coatings
• List of Abbreviations
• Index