2019

Correspondence Luca Pezzato, Department of Industrial Engineering, University of Padova, Via Marzolo 9, 35131 Padova PD, Italy. Email: luca.pezzato@unipd.it; lucapezzato@virgilio.it Abstract Plasma electrolytic oxidation (PEO) coatings were produced on AZ80 magnesium alloy in a solution containing silicates and phosphates and working at high current densities with short treatment times. The effect of a sealing treatment in boiling water on corrosion and mechanical properties of the coatings were investigated. Moreover, the corrosion mechanism of the samples with and without the sealing treatment was evaluated. The microstructure of the coatings was characterized with scanning electron microscope observation and X‐ray diffraction analysis. The mechanical properties were evaluated with nanoindentation tests and the corrosion resistance was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, and scanning vibrating electrode technique. The results showed tha...

Zastita materijala, 2016

In this paper, phenomena of chemical corrosion of metals and alloys in electrolyte solutions are analyzed. It is shown that iron, chromium and other metals and alloys dissolve much faster than is the corrosion rate determined by electrochemical methods. This means that the chemical dissolution takes place simultaneously with the electrochemical dissolution. The chemical dissolution does not depend on electrode potential. Under some conditions, chemical dissolution of metals is the dominant process of dissolution. Several mechanisms of chemical corrosion are described. Also, the consequences of chemical dissolution of various metals and alloys are discussed, as well as the hydrogen evolution during the chemical dissolution. The process of hydrogen evolution during the chemical corrosion is not subjected to the laws of electrochemical kinetics.

Metallurgical Factors Affecting Corrosion in Petroleum and Chemical Industries Abstract: Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. With a few exceptions, metals are unstable in ordinary aqueous environments. Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels. Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. The production of oil and gas, its transportation and refining, and its subsequent use as fuel and raw materials for chemicals constitute a complex and demanding process. Various problems are encountered in this process, and corrosion is the major one. Since metals are the principal material suffering corrosive deterioration, it is important to develop a background in the principles of metallurgy to fully understand corrosion. The control of corrosion through the use of coatings, metallurgy, nonmetallic materials for constructions cathodic protection and other methods has evolved into a science in its own right and has created industries devoted solely to corrosion control. Metallurgical factors that affect corrosion are chemical composition, material structure, grain boundaries, alloying elements, mechanical properties, heat treatment, surface coating, welding and manufacturing conditions. Understanding these factors are of great importance to decrease and control corrosion problem in many industrial applications.

Two methods of combating corrosion which are widely used in New Zealand are cathodic protection and chemical inhibitors. Both methods depend on controlling the charge on the metal surface, and this can be monitored by measuring the potential of the metal. The conditions needed to stop corrosion can then be predicted from an electrochemical phase diagram. Cathodic protection is effected by forcing the potential to a negative region where the metal is completely stable. This can be done by using a sacrificial anode made from a more reactive metal, or using an external power supply to change the amount of charge on the metal surface. Cathodic protection is well suited to steel structures in marine or underground environments. There is a class of chemical inhibitors which work by removing electrons from the metal, thereby pushing the potential into a positive region where an oxide film spontaneously forms. This results in a stable, passive surface with a very low corrosion rate. Industries apply this technology in processes where the inhibitor can be conveniently added without causing environmental or health problems.

Handbook of Sol-Gel Science and Technology, 2018

Deposition of sol-gel coatings is a promising method for corrosion protection of metal substrates. As an example, sol-gel coatings represent an alternative to the use of chromate conversion coatings for protection of aluminum alloys in the aerospace industry. In order to expand the use of sol-gel coatings, it is important to assess their corrosion behavior. The sol-gel coating should provide an efficient barrier against aggressive species limiting corrosion of the metal substrate. Moreover, the coating should offer the possibility to incorporate corrosion inhibitors in order to provide active behavior to the coating. The sol-gel coating should be well adherent to the base metal and should also promote the adhesion of organic primer and topcoat when the sol-gel coating is employed as a pretreatment of the substrate. This chapter focuses on the use of electrochemical methods for the characterization of corrosion properties of thin sol-gel coatings. Barrier properties, effect of corrosion inhibitors incorporated in the sol-gel coatings, and adhesion promotion are targeted in this chapter providing selected examples taken from our scientific work.

The investigations into the influence of important parameters on the corrosion behaviour of preselected container materials (carbon steel, Cr-Ni steels, Hastelloy in salt, granite and clay environments have been continued.