Corrosion diagnostics with gel electrolytes - Part 1 - Gel electrolyte or liquid electrolytes?

In surface technology, there is often a need to reliably and quickly test the corrosion-relevant properties of metal products. However, this is either not possible with standard test methods or it is simply far too time-consuming, tedious and cannot be integrated into the manufacturing or service life cycle in a meaningful way.

1 Introduction

Corrosion diagnostics using electrochemical methods can now offer effective solutions for many issues in surface technology. The use of gel-like electrolytes based on agar is a sensible alternative to liquid electrolytes, as this makes new investigation concepts possible and easier to implement. Gel electrolytes form a thin electrolyte film on their surface and the remaining test electrolyte remains immobilized and stored in the gel. This makes handling easier and enables minimally invasive electrochemical tests, from which characteristic values can be derived that enable quality control or even a service life estimate. This article provides an overview of research work on gel electrolytes for corrosion diagnostics and presents some practical applications.

Corrosion protection is and remains an important measure in almost all areas of industry to ensure the function and service life of products. In order to test the effectiveness of corrosion protection measures in advance, tests are usually carried out under increased corrosion stress. It is hoped that this will provide the quickest possible indication of the effectiveness and correct implementation of the protective measures. Often, no comparable image of the real corrosion behavior is obtained, although this is not absolutely necessary. It is more decisive if the corrosion mechanisms change due to a tightening of the test conditions and the resulting time lapse. This can either result in an apparent safety (product passes the test but then fails in practice), or certain protective measures are oversized or even unnecessary. Both are bad!

Despite this initial situation, regular corrosion tests must be carried out to ensure the quality of the manufactured products and because rules and agreements require it. The tests, which are usually standardized or agreed between companies, are often very time-consuming and resource-intensive. The question therefore often arises as to whether new test approaches and methods exist that can be used to make correct statements about the quality of products and the effectiveness of corrosion protection measures more quickly and with less effort. The use of electrochemical methods can help here. With their help, corrosion systems can be characterized very quickly, a corrosion load can be intensified precisely or corrosion tests can be monitored. However, colleagues from industry are often skeptical about electrochemical methods. One hears, for example: What does the box with the cables have to do with corrosion anyway? - Why don't you do a salt spray test too? This reluctance is partly understandable. After all, a potentiostat has nothing to do with corrosion per se; it only becomes a valuable instrument with the user's expertise. It is therefore a constant challenge to explain the electrochemical methodology to potential users and to simplify it methodically without making false statements.

In corrosion diagnostics, an attempt is made to pursue a different strategy to conventional corrosion testing and is often based on metrological concepts of electrochemical analysis. Using electrochemical correlations and methods adapted to them, attempts are made to identify potential corrosion problems quickly and reliably or to make a rapid qualitative assessment of the current condition of a corrosion protection system. A major advantage here is that corrosion-relevant information can be recorded in a short time and often almost non-destructively, which can be used for development, improvement and, in particular, for prompt quality monitoring. This also makes it possible to take the step towards more digitalization in these areas of application, as relevant data is immediately available for further use in the company.

A promising part of this strategy is the use of gel electrolytes, which serve as a carrier for a special test solution and for connecting electrodes to a test surface. Agarose polymer, which is used in gel electrophoresis, as a culture medium in biology or as a thickening agent for foodstuffs, has proved particularly successful as a gel former. By immobilizing the liquid electrolyte, the measurement setup can be simplified and thus electrochemical measurements, whether in the laboratory or in an industrial environment, can also be significantly simplified. In addition, agar gels have other unique properties and applications, but also some special features. This article reports on the current state of developments in this field and presents some examples of applications.

2 State of the art

Agar is the basic structure for a gel electrolyte and is generally described as a gel-forming, macromolecular polysaccharide, which is obtained from the cell walls of various red algae by extraction with hot water, freeze-thaw processes or spray drying. It consists of approx. 70 % agarose (gelling) and approx. 30 % agaropectin (sulphated, non-gelling). Separation to pure agarose is carried out for use in gel electrophoresis. Agar is miscible in water, but insoluble at room temperature. It melts at 80 to 100 °C and cross-links physically on cooling. Even relatively low concentrations of 0.5-1 % agar in water lead to stable gels. The structure of the solidified macromolecules is described as double helices, which crosslink spatially and, depending on the concentration of agarose, form cavities in the range of a few hundred nanometers, which are filled with the solvent.

The targeted research and use of agar gels for corrosion studies is still comparatively limited. As early as 1955, Laque et al. used gel electrolytes mixed with indicators to visualize partial reactions of corrosion processes for teaching materials [1]. Similarly, many years later, Isaacs et al. added chloride and pH indicators to agar gel pads and used them to study aluminum and aluminum alloys [2]. They were able to visualize the location and sequence of anodic and cathodic partial reactions during local corrosion on aluminium using indicators in the gel. The visualization of corrosion using an indicator for iron ions is also the basic principle of the electrographic ferroxyl test. This test is still used today to determine the porosity of metallic coatings [3] or for testing the porosity of phosphate coatings on steel, whereby the use of gelatine instead of filter paper as an electrolyte carrier is often recommended. As early as 1959, the ferroxyl test with gels was used as a test method for implant materials made of stainless steels [4]. A few years ago, the method was further developed by the Federal Institute for Materials Research and Testing (BAM) as a "requirement for the detection of localized corrosion in stainless steels" [5], patented [6] and is available for purchase as the "Korropad" test equipment. It is based on a circular gel pad with an agar content of 3 % in water and contents of 0.1 mol/L NaCl and 0.001 mol/L potassium hexacyanoferrate(III). The latter leads to the formation of a stable redox potential which, as a test potential, lies in a range in which critical potentials also lie if the material is sensitive to corrosion. The chloride ions activate weak points in the passive layer and local metal dissolution takes place. The iron ions that dissolve are complexed by the potassium hexacyanoferrate(III) to form Berlin blue, a poorly soluble color pigment that indicates the areas of local corrosion in color and makes them assessable. In recent years, the corropad has been increasingly used in industry as a test medium and has been applied to questions relating to stainless steels in industry and research [7], e.g. to optimize their heat treatment [8], to show the influence of surface treatments [9] or for the detection of sensitization [10].

An application of gel-like electrolytes to simulate special corrosive conditions was demonstrated, for example, by Newton and Sykes [11]. They used agar gel mixed with NaOH and NaCl to examine steel in mortar-like electrolytes and were able to document clear differences to measurements in bulk electrolyte solutions in their investigations. In their studies, Spark et al [12] describe how agar gels can be used to simulate corrosion processes in clayey soils with the complex physico-chemical conditions prevailing there. The investigations were carried out on an unalloyed steel and free corrosion potentials and current densities were determined from polarization curves. In further publications, the same authors describe investigations with agar gels to which peptides were added [13] in order to simulate micro-biologically induced corrosion of pipelines in soils [14]. Vanbrabant et al. also used agar-based gel electrolytes as a corrosion medium to simulate special media conditions, with simultaneous electrochemical instrumentation to perform electrochemical measurements [15]. They see the use of gels as having the advantage of being closer to practically relevant media conditions, especially where the transport of corrosion products is inhibited. Another advantage they see is that the individual phases of corrosion can be better visualized in the course of the experiments. Applications were presented that better simulate the conditions of steel fibres in concrete, galvanized steel (zinc and zinc-aluminium alloys) in agricultural atmospheres and in animal husbandry.

Agar/agarose gels are also used in the field of corrosion research for tailor-made, often miniaturized reference electrodes in which the bridging electrolytes are immobilized by the gel [16]. In recent years, numerous authors have reported on the use of gels as electrolytes for electrode coupling. Monrabal et al., for example, added high proportions of glycerol (20-70 %) to agar gels in order to realize more flexible top-mounted measuring cells [17] and to couple them better to shaped components. For example, they try to identify weak points in weld seams of stainless steels [18] and carried out numerous other electrochemical investigations [19, 20]. Cano et al. also developed a measuring cell based on agar gel as a coupling electrolyte, which makes it possible to carry out electrochemical impedance measurements on monuments and sculptures without significantly damaging them [21]. Di Turo et al. pursued the same goal with the use of gel-based electrolytes for the electrochemical characterization of bronze patina on historical coins [22]. The first own electrochemical investigations with gel electrolytes were already started in 2014 and led, among other things, to a methodical approach with which electrochemical parameters can be determined on galvanized steels under atmospheric exposure in order to determine the current condition of the protective top layers [23]. Based on this, it is possible to quickly determine electrochemical parameters [24], which could be used to estimate the service life in the future. The use of gel electrolytes for corrosion tests can be summarized as follows:

• Triggering and visualization of corrosion processes using indicators in the gel electrolyte
• Electrochemical instrumentation of gel electrolytes to simplify the characterization of corrosion protection systems and to determine characteristic values
• Simulation of specific corrosion conditions (building materials, soils, food, biological tissue) with solid/liquid phases and inhibited mass transfer.

This article will mainly deal with the use of gel electrolytes for visualizing corrosion processes and electrochemical instrumentation for determining characteristic values. Corrosion diagnostics opens up new areas of application in these fields and can contribute to additional acceptance among users, as the practical implementation of the measurements is significantly simplified. Most of the investigations and results presented here were carried out as part of a research project funded by the German Research Foundation (DFG) on the subject of "Agar-based gel electrolytes for corrosion diagnostics".

3 Corrosion-relevant properties of agarose gels

When using gels as electrolytes for corrosion tests, it is important to determine the corrosion-relevant properties of the gels in advance. A certain percentage of the gel former, in this case agar or pure agarose, is added to the aqueous electrolyte. The chemical composition of the starting products must therefore be taken into account and the extent to which the corrosiveness of the electrolyte is additionally changed as a result. The agarose melts when the aqueous mixture is heated and the gel network is formed when it cools. Cross-linking turns the electrolyte solution into a gel with altered mechanical and rheological properties. When using the gel electrolyte in a sensor, care must be taken to ensure that the gel structure is not destroyed when it is placed on the test surface and the electrodes are connected. Therefore, rheological properties are also of interest, e.g. to maintain the permissible contact pressure with a sensor so that the agarose gel is not irreversibly damaged. Another important factor is the electrolyte film that forms on the gel or between the gel and the test surface due to the so-called syneresis effect. The electrolyte film thickness on the gel, or generally the supply of liquid electrolyte on the surface, is also an important property that influences corrosion reactions and electrochemical measurements and can depend on the agar concentration and electrolyte composition. In addition, the agar concentration determines the pore sizes and thus the mass transport in the gel. This can influence the uptake and removal of ions and corrosion products to varying degrees.

The gel electrolytes are prepared by mixing a certain amount of agar or agarose, which can usually be purchased as a powder in various degrees of purity in the laboratory. For the majority of our own investigations, gels in the concentration range of 1.5 to 6 % were examined, whereby gel electrolytes with 3 % polymer content have proven themselves for most corrosion diagnostic applications. The agar powder mixed with the aqueous electrolyte is slowly heated to 88 °C (agarose) or 95 °C with stirring until the polymer has melted. For temperature-sensitive electrolyte components, addition at approx. 65 °C and rapid cooling is advantageous. The mixture can then be poured, which reaches its gel point between 60 and 40 °C, at which point the polymer cross-links and the solution solidifies. The gels were poured into square acrylic Petri dishes with thicknesses between 2 and 6 mm. Any shape can be cut out of the resulting gel plates for further use. Ideally, the gels are stored in the dishes in the absence of air and in a refrigerator at approx. 5 °C in order to suppress any change in the organic material due to mold growth or heavy liquid leakage during storage over several weeks. Selected test results on the corrosion-relevant properties of agar/agarose gels are summarized below.

3.1 Chemical composition of the gel electrolytes

The chemical composition of the gel electrolyte is specifically adjusted for the respective application in order to achieve a certain corrosiveness. Even if the proportion of gel formers is usually very low (2 to a maximum of 6 % is reasonable), there may still be an undesirable entry of foreign ions. Chlorides, sulphates, phosphates and nitrates in particular must be taken into consideration, as even low concentrations of these anions can influence the corrosion reactions that take place. The purest possible starting materials are desirable, as the agar gel should only serve as a carrier for the test electrolyte. The ions mentioned also have an influence on the conductivity. The pH value of the mixtures before gel formation and the redox potentials (on platinum) before, immediately after and 3-72 h after gel formation were also examined, as heating to up to 95 °C during production is expected to influence the oxygen content. Table 1 summarizes the results of these investigations. Two nominally identical agar variants (Extra Pure) from Merck and Agarose Basic from AppliChem, representing one agarose product, were examined.

- to be continued -