Plasma electrolytic oxidation (PEO) is a method to obtain protective coatings on metallic light-weight construction materials. Here, the workpiece receives a strong anodic polarisation in a suitable aqueous electrolyte, which leads to the formation of a passive layer and a gaseous shell. Afterwards, plasma electrolytic discharges appear on the substrate surface and convert it into a ceramic layer. The properties of the passive layer are influenced by the selected substrate/electrolyte combination and are essential for the PEO process-initiation and characteristics. In this work, a new method for the systematic investigation of the substrate/electrolyte interactions during the pre-discharge stage is presented. The procedure is carried out by a polarisation experiment and allows for a quantitative characterisation of the passivation behavior, based on a small electrolyte volume. The method is used to investigate a literature-known electrical conduction mechanism on passive films formed on magnesium, by cross-comparison between different Mg and Al materials. In addition, the influence of phosphate, glycerol, and fluoride on the passivation behaviour of the Mg alloy AZ31 in an alkaline environment is considered and quantified. The results provide an explanatory approach for the positive influence of toxic fluorides within the electrolyte on the morphology of PEO layers on magnesium.

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is an innovative method in producing oxide-ceramic coatings on metals, such as aluminum, titanium, magnesium, zirconium, etc. The process is characterized by discharges, which develop in a strong electric field, in a system consisting of the substrate, the oxide layer, a gas envelope, and the electrolyte. The electric breakdown in this system establishes a plasma state, in which, under anodic polarization, the substrate material is locally converted to a compound consisting of the substrate material itself (including alloying elements) and oxygen in addition to the electrolyte components. The review presents the process kinetics according to the existing models of the discharge phenomena, as well as the influence of the process parameters on the process, and thus, on the resulting coating properties, e.g., morphology and composition. 

Volatile corrosion inhibitors (VCI) are a means of temporary corrosion protection based on the release of inhibitors from films, papers, diffusers or oil to the gaseous phase. By adsorption on a metal surface, the inhibitors prevent degradation of the primary oxide layer and impede corrosion. Thus, VCI provide corrosion protection for high-quality goods and semi-finished parts made from metals during transportation and storage. The application of oil on the metal surface for corrosion protection and its removal can be avoided. Due to rapid volatilization of the VCI after unpacking, no costly cleaning of parts prior to further use is required. In a new approach for evaluation of the protective effect of VCI, a metallic sample is exposed in a small chamber filled with air alongside a VCI-containing product. The air is humidified and the sample is cooled down under dewpoint, which leads to condensation of humidity and represents corrosive stress. The state of the metallic surface under the condensed water film is characterized by electrochemical methods. The present work demonstrates the evaluation of the anticorrosive effect of various VCI-films on mild steel. In summary, the application-oriented test allows for quick and quantified evaluation of VCI-containing products with a tabletop device.

An appropriate corrosion protection is required for transport and storage of high-quality goods and semi-finished parts made from metals. The application of volatile corrosion inhibitors (VCI) as a means of temporary corrosion protection is based on the release of corrosion inhibitors from films, papers, diffusers or oils to the gaseous phase. By adsorption on the metal surface, the inhibitors prevent the degradation of the primary oxide layer and hence impede corrosion. In principle, the anticorrosive effect is independent of the part geometry, because the protected surfaces do not need direct contact with the VCI material. Also, the VCI method does not require costly cleaning of parts prior to further use after transport and storage. The EXCOR jar test is a common method to examine the efficiency of VCI. A metallic sample and a VCI material are exposed in a jar. After conditioning in dry and humid atmosphere, corrosive stress is applied to the sample. To clarify the VCI's mechanism of action, the test steps, their duration and the type of inhibitors were varied. Thus, the operating principle of VCI during the test was identified. The acquired knowledge enables to understand and tailor the corrosion protection by VCI in service.

Volatile corrosion inhibitors (VCI), which are incorporated in packaging materials like films, papers, plastic racks etc., are a suitable means for temporary corrosion protection during transport and storage of metallic products. For the evaluation of the corrosion protection effect of VCI-containing products, numerous tests are established. In most cases, principally, a metallic sample is exposed to a climatic stress in presence of a VCI-containing product and the protection effect of the VCI is evaluated afterwards by visually assessing the occurrence/absence of corrosion products at the sample surface. In a newly proposed test method and a dedicated device, the evaluation of the VCI-containing product is based on electrochemical measurements at the interface between the metallic sample and a film of condensed water. The water film is precipitated during the test by actively cooling the metallic sample below the dew point, which imposes a corrosive stress on the sample surface. With a simple direct current potentiostat for polarization resistance measurement, the state of the metallic surface (corrosion-active, passive) is registered objectively during the test and thus the corrosion protection effect of the VCI is evaluated. In the present paper, the test method and the dedicated device are disclosed in detail. On the basis of a statistical analysis of a sample of independently repeated tests, limit values are established for the quality assessment of VCI products. Different commercially available VCI films are evaluated and the results are compared to the state-of-the-art jar test procedure based on German TL 8135. In conclusion, the proposed test reproducibly provides objective and quantitative information on the corrosion protection effect of VCI-containing products. 

The anodic oxidation process is an established means for the improvement of the wear and corrosion resistance of high-strength aluminum alloys. For high-strength aluminum-copper alloys of the 2000 series, both the current efficiency of the anodic oxidation process and the hardness of the oxide coatings are significantly reduced in comparison to unalloyed substrates. With regard to this challenge, recent investigations have indicated a beneficial effect of nitric acid addition to the commonly used sulphuric acid electrolytes both in terms of coating properties and process efficiency. The present work investigates the anodic oxidation of the AlCu4Mg1 alloy in a sulphuric acid electrolyte with additions of nitric acid as well as oxalic acid as a reference in a full-factorial design of experiments (DOE). The effect of the electrolyte composition on process efficiency, coating thickness and hardness is established by using response functions. A mechanism for the participation of the nitric acid additive during the oxide formation is proposed. The statistical significance of the results is assessed by an analysis of variance (ANOVA). Eventually, scratch testing is applied in order to evaluate the failure mechanisms and the abrasion resistance of the obtained conversion coatings. 

An appropriate corrosion protection is required for transport and storage of high-quality goods and semi-finished parts made from metals. The application of volatile corrosion inhibitors (VCI) as a means of temporary corrosion protection is based on the release of corrosion inhibitors from films, papers, diffusers or oils to the gaseous phase. By adsorption on the metal surface, the inhibitors prevent the degradation of the primary oxide layer and hence impede corrosion. In principle, the anticorrosive effect is independent of the part geometry, because the protected surfaces are not in direct contact with the VCI material. Further, the VCI method does not require costly cleaning of parts prior to further use after transport and storage. A new approach for the evaluation of VCI is presented. A metallic sample is exposed in a chamber filled with air along with a VCI-containing product. In the course of the test, the air is humidified and the sample is then cooled down below the dew point, which leads to condensation of humidity. Electrochemical DC and AC methods, namely polarization resistance measurements and electrochemical impedance spectroscopy, are applied to characterize the state of the metallic surface under the condensed water film. The time-resolved analysis of the measurements provides a basis for the quantitative evaluation of the VCI's anticorrosive effect or a possible corrosion attack. The present work focusses on establishing the interrelations between the electrochemical response of the system and the test regime including initial exposition time of the dry sample along with the VCI in the test volume, the amount of VCI and the dwelling time of the sample under condensed humidity. The validity of the measurements is verified and the range of deviation is estimated. Exemplarily, the anticorrosive effect of various VCI-containing products on mild steel is evaluated. In summary, the presented application-oriented test is proven as a suitable method for the quantified assessment of VCI-containing products.

With the progress in materials science and production technology and the establishment of light-weight design in many fields of the industry, the application of light metals no longer requires only mechanical strength, but also a significant protection of the material against wear and corrosion. Hard and wear-resistant oxide coatings on aluminium are produced by plasma electrolytic oxidation (PEO). During PEO, a conversion of the aluminium substrate to a ceramic oxide takes place. While the role of strength-giving alloying elements like Cu, Mg/Si, Zn, and Zn/Cu on the PEO process has selectively been subject of investigation in the past, the significance of the alloy composition for the service properties of the coatings is still unknown. Therefore, the performance of PEO coatings produced on the widely used commercial high-strength alloys AlCu4Mg1 (EN AW-2024), AlMgSi1 (EN AW-6082), and AlZn5.5MgCu (EN AW-7075) is examined with regard to their behaviour in the rubber-wheel test according to ASTM G65 and the current density-potential behaviour of the substrates with undamaged and worn coatings in dilute NaCl solution. To give a reference to the unalloyed material the testings were carried out also on Al 99.5 (EN AW-1050) which was treated in an adjusted PEO process. Although differences in the conversion of intermetallic phases during PEO and the phase composition of the coatings on the various substrates are determined, the service properties are hardly depending on the alloying elements of the investigated aluminium materials. The wear rates in the rubber-wheel test are low for all the alloyed samples. The current density-potential curves show a decrease of the corrosion current density by approximately one order of magnitude compared to the bare substrate. Eventually, previous wear of the coatings does not deteriorate the corrosion behaviour. PEO layers on technically pure aluminum can resist the testing regimes if they are prepared in an electrolyte with an elevated silicate content and without additional hydroxide ions, during a longer process time.

Magnesium and its alloys are increasingly in use as lightweight construction materials. However, their inappropriate corrosion and wear resistance often prevent their direct practical use. The plasma electrolytic oxidation (PEO) is a promising, environmentally friendly method to improve the surface characteristics of magnesium materials by the formation of oxide coatings. These PEO layers contain components of the applied electrolyte and can be shifted in their composition by increasing the concentration of the electrolyte constituents. Therefore, in contrast to the use of conventional low concentrated electrolytes, the process results in more stable protective coatings, in which electrolyte species are the dominating constitutes. In the present work, the influence of the composition of highly concentrated alkaline silicate electrolytes with additives of phosphate and glycerol on the quality of PEO layers on the magnesium alloy AZ31 was examined. The effect of ultrasound coupled into the electrolyte bath was also considered. The process was monitored by recording the electrical process variables with a transient recorder and by observation of the discharge phenomena on the sample surface with a camera. The study was conducted on the basis of a design of experiments. The effects of the process parameter variation are considered with regard to the coatings thickness, hardness and corrosion resistance. Information about the statistical significance of the effects of the parameters on the considered properties is obtained by an analysis of variance (ANOVA). 

The increasing multi-functionality of parts and assemblies in several fields of engineering demands, amongst others, highly functionalised surfaces. For the different applications, on the one hand, there is a need to scale up surface modification processes originating in the nano- and micro-scale. On the other hand, conventional macro-scale surface refinement methods offer a huge potential for application in the said nano- and micro-scale. The anodic oxidation process, which is established especially for aluminium and its alloys, allows the formation of oxide ceramic layers on the surface. The build-up of an oxide ceramic coating comes along with altered chemical, tribological and electrical surface properties. As a basis for further investigations regarding the use of the anodic oxidation process for micro-scale-manufacturing, the scale effects of oxalic acid anodising on commercially pure aluminium as well as on the AlZn5.5MgCu alloy are addressed in the present work. The focus is on the amount of oxide formed during a potentiostatic process in relation to the exchanged amount of charge. Further, the hardness of the coating as an integral measure to assess the porous oxide structure is approached by nano-indentation technique.