Volume 29, Issue 5-6 - Special Issue: Corrosion in Italy, Ausgabe 5-6 (Nov 2011), S. 177–296

The use of electrochemical techniques in the repair of reinforced concrete suffering carbonation-induced corrosion is increasing thanks to advantages connected with the possibility of preserving carbonated concrete and minimising the alteration of the surface, which make these techniques attractive for facades or monuments that have a high cultural, historical or architectural value. Electrochemical techniques allow repassivation of the steel through the application of a temporary or permanent cathodic current. These techniques have been used for several decades, although some aspects connected with their application have not been clarified yet. This paper presents a state-of-the-art on the techniques of electrochemical realkalisation and cathodic protection used in carbonated concrete, and describes the recent developments in this field, with particular reference to experimental investigations and on-site applications carried out in Italy in recent years.

Interest on nanometric conformal coatings is currently growing across a wide range of applications, from electronic components to corrosion protection, chemical barriers or even wear decrease. Currently, atomic layer deposition (ALD) is one of the most promising nanometric deposition technologies: it offers the possibility to obtain conformal coatings even on very complex tridimensional substrates, with a strict thickness tolerance. During an ALD cycle, only one molecular layer is deposited on the substrate surface, enabling the theoretical possibility to tailor the composition of the deposit up to molecular resolution. In the first part of this review, a brief history of ALD is presented to better understand the evolution of the technique during the past five decades; in the second part, a short description of the main ALD techniques is given, considering their main advantages and drawbacks. In the third part, a short review about ALD applications is presented, and in the fourth part the most useful and used instruments for the analysis and characterization of ALD are listed; some results are discussed and particular emphasis on the corrosion protection of different metallic alloys of common industrial interest is given.

Austenitic stainless steels are employed in many industrial fields due to their very good resistance to general corrosion in several environments. However, their use is limited due to the fact that they suffer localised corrosion in specific environments, particularly in chloride-ion rich solutions. Low temperature nitriding treatments can improve both corrosion resistance in chloride-ion containing media and surface hardness, due to the formation of a metastable phase known as expanded austenite or S phase; this phase can be outlined as a supersaturated interstitial solid solution of nitrogen in the expanded and distorted γ-Fe f.c.c. lattice. We review the main experimental results obtained in our research on low temperature glow-discharge nitriding of austenitic stainless steels. By means of proper treatment parameters, low temperature glow-discharge nitriding is able to markedly improve the corrosion resistance of austenitic stainless steels, such as AISI 316L and AISI 202 in chloride-ion rich solutions in comparison with the untreated alloys.

Silicon alkoxides sol-gel pre-treatments act as coupling agents between metal substrates and organic coatings. This kind of metal finishing is widely studied pre-treatment, but relatively little study is done on the effect of this sol-gel pre-treatment on the durability and on the corrosion protection properties of the complete protection system which consist of a pre-treated and, subsequently, painted metals. In this paper the performance of a sol-gel layer produced from a mixture of silicon alkoxides precursors was applied on different metals (galvanized steel, copper, aluminum) as a pre-treatment prior the application of an organic coating. The effect of the sol-gel pre-treatment on the durability of the painted metals was investigated evaluating the dry and wet adhesion of the organic coating to the substrate as well as the corrosion resistance in the salt spray chamber and the resistance to cathodic delamination. The laboratory testing proved that the experimental sol-gel layer provides the three different painted metals with an improved corrosion resistance, thus leading to a reduced extent of delamination of the coating from the substrate after the accelerated tests.

The present study deals with focused ion beam (FIB) coupled with transmission electron microscopy (TEM) analysis of corrosion mechanisms in cathodic arc evaporation (CAE)-PVD CrN/NbN multilayer coatings on aluminum alloy substrate. Samples were exposed to a 500-h ASTM B-117 salt spray test. Surface morphology of tested specimens was preliminary analyzed by SEM-EDS techniques. The corrosion rate in CAE-PVD coated components (ASTM B-117 standard) was significantly lower than uncoated substrates. Crevice corrosion in relation to surface defects (microdroplets) was observed after FIB and TEM cross-section analysis. Nevertheless, results of TEM analysis also showed a more complex crevice/galvanic corrosion mechanism in relation to the bottom droplet/coating voids, close to the coating/substrate interface, which could be concurrent with usually proposed corrosion mechanisms. Moreover, modification of coating microstructure and composition is observed close to the peripheral area of droplets. These results give further and deeper insight into the corrosion mechanisms for PVD thin coatings, and could only be obtained by FIB sectioning and TEM analysis. Obtained results also allow the correct selection of the most suitable coating material and process parameters as a function of the nature of the substrate and the specific environmental conditions.

This research illustrates a spatially distributed cellular automaton aimed at simulating the pitting corrosion process in stainless steels. The cellular automaton model is based on the metal surface discretization into square cells; in any instant, each cell is found to be in one of the following states: (i) passivity (the passive film on the cell surface is stable and protective), (ii) metastability (the passive film is unstable and can either break, causing pitting initiation, or repassivate), and (iii) pitting (the cell has turned into a stable attack area, with no repassivation chance). At the very beginning, all cells are in passivity state. Subsequently, a stochastic process drives the changes in the state of cells, following rules drawn from the kinetics of pitting localized corrosion. In particular, the transition probabilities of each single cell from one state to another are influenced by the adjacent cells. The comparison between simulations run with the proposed model and laboratory experimental results demonstrates that the model is suitable for describing pitting corrosion processes.

In the past years, a growing interest in the use of electrochemical noise (EN) for corrosion studies has been seen in the main literature. As a result, several methods for analyzing noise data have been developed. In this paper, a review of the investigative possibilities offered by EN in the field of corrosion is given. All the experimental data reported are from research projects developed in our laboratory where, for more than 15 years, EN has been applied to the solution of different corrosion problems and some dedicated software have been developed for EN data analysis (fast Fourier transform-maximum entropy method, power spectral density slope detection and discriminant analysis). Much attention will be given to the use of statistical parameters and their significance in terms of corrosion behavior of the electrode. Moreover, the use of experimental care to have meaningful data will be highlighted.

The interest in the electrocodeposition has been revived due the development of new nanotechnology methods for the production of nano-particles. The codeposition of nanoparticles might enhance the mechanical properties of composite deposits without penalizing the corrosion resistance and avoid the formation of an abrasive third body during the wear process. On the other hand, the volume fraction of codeposited nano-particles is generally low and usually inversely proportional to their size. Many efforts have been done in order to improve the codeposition rate of nanoparticles. Most of the research on nickel composites is focused on the electrodeposition process or on the mechanical and wear properties rather than on the corrosion resistance. In this work, the protective properties of nickel matrix nanocomposite coatings have been studied by means of potentiodynamic curves and electrochemical impedance spectroscopy. Two different nanopowders, i.e., silicon carbide and alumina, were added to a Watts type galvanic bath in order to deposit the nanocomposites coatings on steel substrate. Ultrasonic vibrations have been considered as new process parameter to improve the dispersion of the powder into both the plating bath and metal matrix coating and to substitute pitting control agents for the production of defect-free coatings. Unique, functional properties of composite coatings are derived not only from the presence of the particles dispersed in the bulk of the metallic matrix but also on the matrix microstructural changes induced by the interaction between particles and electrocrystallization process. It has been demonstrated that the codeposition of the silicon carbide particles induces an important microstructural refinement. On the contrary, the aluminum oxide powder is strongly agglomerated and only under ultrasonic vibrations can be dispersed. Ultrasounds have a positive effect not only in hindering the porosity but also in dispersing the ceramic powder and increasing the codeposition rate leading thus to the production of protective and very refined coatings. Moreover, the better dispersed powder induces an improvement also in the corrosion protection leading to the formation of a more stable and resistant passive nickel oxide.

This paper deals with hydrogen embrittlement (HE) under cathodic protection of steel used or recently proposed for buried pipelines of the oil and gas industry. High strength low alloy steel with different chemical composition, microstructure and ultimate tensile strength ranging between 530 and 860 MPa were considered to compare new steel with traditional grades. Their behavior was assessed through constant load tests, fracture mechanics tests on modified wedge-opening load specimens, slow strain rate tensile tests and interrupted slow strain rate tensile tests in solutions simulating soil moisture and sea water, at room temperature and cathodic potentials between -2 and -0.8 V vs. saturated calomel electrode reference. All the steel was found immune under static load and HE phenomena were observed only during slow strain rate tests at potentials more negative than a critical value, depending on steel susceptibility and strain rate. Quenched and tempered steel with high tempered martensite and very fine precipitation distribution showed better behavior than the rolled steels with banded microstructure, exhibiting the highest HE resistance. The resistance of steel characterized by banded microstructures increased with ultimate tensile strength until ∼700 MPa and then decreased. The increase of ultimate tensile strength occurred by changing the steel microstructure, from hot rolling with coarse ferrite/pearlite to fine ferrite/pearlite microstructure, and very fine acicular ferrite, yields the resistance of steel to HE. A further increase of ultimate tensile strength related to untempered martensite inside the acicular ferrite based microstructure produces a worsening of resistance.

One of the most common strategy to protect metallic substrates from corrosion is the use of organic coatings. The accurate prediction of long-term performances of this protective system is a very important issue for the coating industry. Outdoor exposure of organic coatings can produce the loss of their aesthetic and anticorrosion properties because of weathering conditions: sunlight, temperature, water, oxygen and pollutants. Coating service life is generally evaluated by using laboratory tests simulating in accelerated way the natural conditions (temperature, UV radiation, moisture, condensation, pollution). It is well known that it is difficult to find a clear correlation between natural exposure and artificial weathering. The time of exposure in a natural site is not sufficient to define the weathering conditions; it is important to consider different environmental parameters affecting the organic coating properties (energy from UV radiation, wet time, temperature). In this work the change of aesthetic and protective properties of organic coatings was investigated. Different systems were studied: epoxy-polyester and polyester coatings crosslinked with different hardeners. The coatings have distinct surface finishes and colours with particular aesthetic effects. The samples were exposed in Trento area (alpine region, north of Italy), monitoring all meteorological parameters. The same samples were artificially weathered by a cyclic test consisting of 72 h of UV exposure (ASTM G154 UVA, 340 nm) and 72 h of salt spray chamber exposure (ASTM B117, 5% NaCl, 35°C, 100% relative humi­dity). The aesthetic degradation was evaluated by gloss and colour variation measurements; the surface chemical changes were studied by FTIR analysis. Environmental scanning electron microscopy was used to obtain some information about morphological characteristics of the exposed surfaces. The degradation of protective properties was studied by electrochemical impedance spectroscopy. Different results have been obtained for aesthetic and protective properties. Concerning gloss and colour variations, a good relationship was found between artificial and natural tests. When considering the protective properties, laboratory test results were more aggressive due to the salt spray environment in which the samples have been exposed.

Aluminium-based alloys are widely used in seawater as galvanic anodes. Conversely, their use in soil is not recommended as they tend to passivate when chloride content is low. However, there are chloride contaminated soils where the use of aluminium anodes may be of interest. The aim of this research is to evaluate the effectiveness of aluminium galvanic anodes in low chloride solutions, in presence of sulphates. The research followed two phases: electrochemical characterisation of anodes through potentiodynamic polarisation tests in solution and galvanic coupling tests in soil saturated with chloride and sulphate solutions. The potentiodynamic tests showed that pitting potential decreased as chloride concentration increased; sulphate ions exhibited a passivating effect or increased nobility. The results have also been analysed by statistical methods. In galvanic coupling tests, anodes showed an acceptable working potential also at the lowest tested chloride concentration.