Handbook of Modern Coating Technologies

Application of the scanning vibrating electrode technique to the characterization of modern coatings

A.C. Bastos, M.G.S. Ferreira

DEMAC—DEPARTMENT OF MATERIALS AND CERAMIC ENGINEERING, CICECO—AVEIRO INSTITUTE OF MATERIALS, UNIVERSITY OF AVEIRO, AVEIRO, PORTUGAL

• Introduction

The scanning vibrating electrode technique (SVET) uses a vibrating microelectrode to sense the electric field in an electrolyte solution associated with the ionic currents flowing therein. When this is done close to a surface it is possible to identify the points where oxidation reac­tions occur (anodic regions) and where reductions take place (cathodic regions). The SVET is a noninvasive and nondestructive technique and is well suited for corrosion studies because it makes available in a single map the global picture of the corrosion process occur­ring at the surface, displaying the spatial distribution of the regions where oxidation and reductions take place, and their local current density magnitudes. The corrosion evolution of the sample can be described by a succession of maps acquired over time. In the context of coatings, which is the focus of this chapter, SVET can detect porosity and defects on a pro­tective film, identify self-healing properties of responsive layers, or characterize the activity of an electrocatalytic surface.

In spite of being around for a few decades, SVET use is still scarce and remains an exotic and unexplored technique. Taking this into account, this chapter covers a list of matters that should be of interest to those who never had a contact with the technique and should also benefit all others that already work with it. The chapter starts with an introduction to metals and corrosion, followed by a short description of coatings for corrosion protection, their modes of action, and forms of degradation. This provides the background and justification for the application of SVET. Then, the principles of the technique, experimental details, and examples of use with the different types of coatings are presented. It continues with a litera­ture assessment of works using SVET to characterize coatings applied on metals, followed by a discussion about the capabilities and limitations. The chapter ends with a comparative description of alternative localized techniques that can be used to analyze modern coatings.

Handbook of Modern Coating Technologies. DOI: https://doi.org/10.1016/B978-0-444-63239-5.00001-9

© 2021 Elsevier B.V. All rights reserved.

• Metals and corrosion

Modern civilization is based on metals as the main building blocks in all areas of human activ­ity, including tools, instruments and infrastructures, in industry, construction, transportation, and communications [1]. The great importance of metals comes from their availability on Earth's crust and the unparalleled set of properties, such as castability, machinability, recycla­bility, electrical, and thermal conductivities, together with the outstanding range of mechanical properties, encompassing strength, hardness, ductility, toughness, and resilience [2].

The main drawback is their tendency to corrode, that is, the natural predisposition to react with the environment and change the chemical state from the metallic (reduced) form to their original (oxidized) state in nature. Through corrosion, metals lose their properties, putting in danger the function of the structures and equipment they are part. In the crust of the Earth most metals exist in the oxidized state, as ores, and energy is necessary to bring them to the reduced form. As soon as metals are produced, they are ready to spontaneously return to the native state and this readiness is proportional to the energy spent in transform­ing them.

The corrosion process is an electrochemical phenomenon that can be split in the metal oxidation and in the reduction of chemical species in the adjacent environment. The metal (M) oxidation is usually written as

M(s) ! Mⁿ¹(aq) 1 ne                                                                      (1.i)

In acidic media the main reduction is

2H⁺(aq) 1 2e² ! H2(g)                                                                  (1.ii)

In neutral or basic media the dominant reduction is

O2(g) 1 2H2O(l) 1 4e ! 4OH (aq)                                                         (1.iii)

The electrochemical half-reactions take place at the same rate but separated in space. Fig. 1 — 1 shows an idealization of a corrosion cell together with a sketch of the corresponding

current lines and electric field in solution, which will be important for understanding the measuring principle of SVET.

The oxidation and reduction currents must be balanced. In fact, the current is the same, just named differently and having opposite sign when associated with anodes or cathodes. Often the rate of corrosion is controlled by the cathodic process. This is because the metal is there ready to oxidize but the actual velocity and extension of the reaction are dependent on the availability of chemical species able to accept the electrons, the most common being O₂ and H₂O. In normal conditions, the oxygen dissolved in water (~2.5 X 10^_4 M or 8 mg/L [3]) determines the corrosion rate but in acidic conditions the concentration of H⁺(aq) is much larger and corrosion increases exponentially with the decrease in pH.

After the initial electrochemical half-reactions, the process continues with the formation of solid corrosion products by chemical reaction involving the products of (1.i)—(1.iii),

M"⁺(aq) 1 nOH(aq) ! M(OH)_n(s)                                                        (1.iv)

xM"⁺(aq) 1 yH₂O(l) ! M_xO_y(s) 1 2yH⁺(aq)                                                 (1.v)

Many other reactions with chemical species existing in the environment are possible. Common corrosion products include oxides, hydroxides, chlorides, sulfates, and carbonates.

An impressive figure to retain is the cost of corrosion, estimated to lie between 3% and 5% of the gross domestic product [4,5]. Given the enormous economic impact of metallic corrosion, a great effort has been undertaken in the past century to understand this phenom­enon and control it.

• Corrosion protection and coatings
• Strategies of corrosion control

Corrosion is a thermodynamic inevitability and the question is not whether the metal corrodes but how long will it take until it does. Fortunately it is possible to significantly slow down the process. The procedures for control corrosion (1) act over the metal, (2) or act over the envi­ronment, or (3) separate the metal from the environment [6].

Acting over the metal can be done during the product design by selecting more corro­sion resistant alloys or even nonmetallic materials. In service, the action on the metal takes place by changing its electric potential. A potential shift in the positive direction might form a passive layer on the metal surface protecting it from corrosion (anodic protection). A shift in the negative direction turns the metal into a cathode (cathodic protection) and the potential can be placed at values where the metal oxidation is thermodynamically impossible. It is then said to be immune to corrosion. Acting over the environment includes controlling room temperature and humidity, elimination of aggressive species (e.g., Cl^_ and SO4²), purging dissolved oxygen from aqueous solution, and the use of cor­rosion inhibitors. Separating the metal from the environment can be achieved by applying coatings to the surfaces. The coating hides the surface for the redox reactions and blocks the ionic paths between anodes and cathodes, interrupting the electric circuit of the corro­sion cell.

Most objects are coated and corrosion protection is only one of the functionalities. Other functions are decoration, reflection, adhesion, wettability, wear resistance, chemical resis­tance, thermal shielding, electrical conductivity, catalysis, etc. Such functionalities are obtained with coatings chemically very distinct, applied in many different ways, with very diverse forms of film formation, resulting in films morphologically and structurally very dif­ferent. These coatings are usually classified as metallic, inorganic, and organic.

• Metallic coatings

Metallic layers are applied to the surfaces for various reasons including corrosion resistance, appearance, brightness, smoothness, hardness, wear resistance, thermal resistance, and electri­cal contact. For corrosion protection, the thicknesses range from 400 nm (tin in tinplate) to

1. 5 mm or even more (zinc or aluminum coats on large steel structures). The coatings can be applied in various ways being electrodeposition, immersion in molten metal bath, thermal spraying, and cladding, the most common ones. The primary mechanism of protection is the shielding (or barrier) effect, which isolates the substrate from the aggressive environment. This mechanism is common to all coatings. The applied metallic films are more resistant to corro­sion than the underlying metal. Corrosion still occurs but the durability of the coated material is significantly extended, principally if the coating is intact and covers completely the base material. When the two metals become exposed to the environment, at defects, pores, or cut- edges, a galvanic corrosion process starts with the metal of more negative potential being oxi­dized, protecting the other metal from corrosion. If the coating is more negative, it will provide cathodic protection to the substrate preventing its corrosion (a zinc layer on steel is a typical example). Conversely if the potential of the base metal is more negative than that of the coat­ing (e.g., chromium-plated steel), it will corrode at the points open to the environment with an increased rate, owing to the large cathodic area of the nobler coating.

• Inorganic coatings

Inorganic coatings for corrosion protection comprise vitreous (porcelain) enamels for deco­ration and protection, ceramic thermal barrier coatings for high-temperature applications, phosphate layers as base for paints, chromate conversion coatings for metal finishing or paint pretreatment, and anodized layers for finishing and base for paint. These coatings pro­vide a barrier to the environment and higher protection may be attained by impregnating them with corrosion inhibitive compounds. Inorganic coatings are inert and their degrada­tion usually occurs by mechanical damage or extreme acid—base conditions.

• Organic coatings

Organic coatings (or paints) are the most common, universal, and economic method for corro­sion control. They are applied with many different objectives, from protection and decoration to sanitation and visibility [7,8]. A paint formulation contains constituents from five families: binder, pigments, fillers, solvents, and additives. The binder (nowadays predominantly syn­thetic resins) is responsible for the adhesion to the substrate, is the medium that keeps together all other constituents, and determines most of the coating properties; pigments are added for color and opacity; fillers (extenders) are included to reduce price by partly replacing the more expensive binder and pigments; solvents are necessary for adjusting the viscosity during manufacture and application;and additives are important to aid pigment dispersion and for special purposes (dispersants, wetting agents, defoamers, thickeners, rheol­ogy modifiers, driers, flash-rust inhibitors, and antiskin agents) [9—11]. A typical paint scheme is composed by at least one layer of primer, to insure good adhesion and to level surface roughness, and by a topcoat for color, gloss, and weather resistance. The protection against corrosion is obtained by (1) a barrier to oxygen, water, and ions, (2) incorporation of anticor­rosive pigments, and (3) adhesion to the substrate [12—14]. A polymeric film with high ionic resistance and good adhesion to the substrate represents an extremely difficult path for the flow of electric current between anodes and cathodes needed to close the corrosion cir­cuit. The durability of paint schemes is limited and even the best systems rarely last more than 15 years without repainting [15]. The degradation is originated by abrasion, mechanical impact, ultraviolet (UV) radiation, cracking, hydrolysis, and oxidation reactions [16]. The cor­rosion of the metal underneath the paint appears in different forms: blistering, cathodic delamination, anodic undermining, filiform corrosion, and flash-rusting [8,12,13].

• Techniques for assessing coating degradation

Taking into account the plethora of coatings, functionalities, fields of application, and envir­onments of service, it is clear that the mechanisms of degradation and the techniques and parameters for estimating the deterioration and predict service life can be very different. The most common testing conditions are natural weathering and accelerated tests in UV light, humidity, or corrosion chambers. The degradation state can be assessed simply by visual inspection (sometimes comparing with reference images) or by spectroscopic and surface analysis methods, such as gloss and color measurements, infrared spectroscopy, and scan­ning electron microscopy. Often, the tests follow international standards so that academia, industry, and end users have a common ground of understanding. By following normalized tests, anyone will know how they were performed and understand the results, independently on when or where they were produced. Corrosion testing is frequently done using electro­chemical methods, such as open circuit potential monitoring, linear sweep voltammetry (polarization curves), and electrochemical impedance spectroscopy (EIS) [17]. For noncon­ducting films such as organic coatings, due to the high resistance, and consequent high ohmic drop, DC techniques should not be used [18,19] and EIS is the technique of choice [20—23]. These methods give the average response of the sample but lack spatial resolution which is important to fully describe the degradation process. For this, localized techniques are needed. Among the several techniques available (see Section 1.8), the SVET stands out because it has the advantage of offering the global picture of the corrosion process,
identifying the places where oxidation and reduction take place, their magnitudes, and evo­lution in time [24,25]. The technique was first used in biology [26—28] and its application to the corrosion field was pioneered by Hugh Isaacs in the 1980s [29,30].