N. Boshkov

Galvanic Zn-Mn alloys-electrodeposition, phase composition, corrosion behaviour and protective ability

Abstract The electrodeposition conditions and the peculiarities of the phase composition of Zn–Mn alloys with different contents (∼6 and ∼11 wt.%) of the alloying component are described and discussed. Their corrosion behaviour and protective ability are investigated in a model medium of free aerated solution of 5% NaCl in open circuit conditions as well as under external anodic polarization—potentiodynamic polarization curves, polarization resistance ( R p ) measurements, etc. The results obtained are compared to these of pure zinc galvanic coatings. For determination of the corrosion products of the layers X-ray diffraction as well as X-ray photoelectron spectroscopy are used. As a result the compound zinc hydroxide chloride monohydrate (ZHC)—Zn 5 (OH) 8 Cl 2 ·H 2 O—is registered on Zn–Mn alloys. ZHC supplement increases the corrosion resistance due to its low product of solubility and ensures better protective ability. Some suggestions about the protective action of the manganese are also given.

COMPOSITION OF THE CORROSION PRODUCTS OF GALVANIC ALLOYS ZN–CO AND THEIR INFLUENCE ON THE PROTECTIVE ABILITY

Abstract The composition of the corrosion products of non-chromated and chromated galvanic coatings of Zn and Zn–Co(1–5wt.%) alloys in free aerated solution of 5% NaCl is described and discussed. Corrosion treatment is realized at open circuit conditions for different periods of time. The corrosion products of monolayer galvanic coatings were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). As a result of the treatment the compound zinc hydroxide chloride monohydrat (ZHC)—Zn 5 (OH) 8 Cl 2 .H 2 O—was registered on Zn–Co alloys after the experimental period. ZHC supplementary increases the corrosion resistance due to its low solubility product and ensures better protection. In addition, the protective ability of mono- and bilayer galvanic systems (chromated and non-chromated) was determined using polarization resistance measurements.

Electrochemical Behavior, Microstructural Analysis, and Morphological Observations in Reinforced Mortar Subjected to Chloride Ingress

The behavior of steel reinforcement was studied using electrochemical impedance spectroscopy (EIS) and polarization resistance (PR) techniques in conditions of chloride-induced corrosion in ordinary Portland cement-mortar specimens immersed in 7% NaCl for a test period of 120 days and compared to specimens immersed in demineralized water for the same period as reference specimens. This study was an initial phase of ongoing research on electrochemical methods for corrosion protection in reinforced concrete structures and aimed at investigating the applicability of widely accepted techniques as EIS and PR and their possible correlation with structural observations of the bulk matrix, relevant to cement-based materials science and product-layers distribution, to corrosion and further protection. The results indicate that the concept of EIS modeling and the components used in the latter correspond well to alterations in structural properties of the bulk matrix, while the electrochemical behavior can be additionally supported by morphological observations of the steel/cement paste interface.

Electrochemical Performance of Low-carbon Steel in Alkaline Model Solutions Containing Hybrid Aggregates

This work reports on the electrochemical performance of low-carbon steel electrodes in model alkaline solutions in the presence of 4.9.10-4 g/l hybrid aggregates i.e. cement extract, containing PDADMAC (poly (diallyl, dimethyl ammonium chloride) / PAA (Poly (acrylic acid)/ PDADMAC over a CaO core. The main objective was to determine if the addition of hybrid aggregates will lead to increased corrosion resistance of the steel surface layers, generally formed in the hereby investigated environmental medium. Further, it was expected that when chlorides are involved, as corrosion accelerating factor, the presence of hybrid aggregates will delay corrosion initiation and therefore lead to increased corrosion resistance. This investigation forms part of a novel approach to control steel corrosion in reinforced concrete, using self-healing mechanisms. The results from this study denote for indeed superior corrosion performance of steel in chloride-free and chloride containing alkaline solution, when hybrid aggregates are involved. The mechanisms are related to increased barrier effects of the formed layer and CaO release from the core of the aggregates.

Electrochemical and Microstructural Studies in Reinforced Mortar, Modified with Core-Shell Micelles

This work reports on monitoring chloride-induced corrosion in reinforced mortar specimens, with and without addition of polymeric nano-aggregates in the mortar mixture. The investigation is a novel approach to control steel corrosion in reinforced concrete, hereby reporting the preliminary results, related to one of the main objectives: studying the influence of admixed polymer nano-aggregates (in the form of PEO113-b-PS218 core-shell micelles with a very low concentration of 0.006 wt.% per mortar weight) on the corrosion behavior of the steel reinforcement, compared to reference, micelles-free mixtures.

Corrosion properties of Zn-Ni-P alloys in neutral model medium

AbstractThe presented work reports on the peculiarities of the anodic behavior, corrosion resistance and protective ability of electrodeposited Zn-Ni-P alloys with a different composition in a model corrosion medium of 5% NaCl. Three characteristic coating types have been investigated using experimental methods such as potentiodynamic polarization (PD) technique and polarization resistance (Rp) measurements. In addition, X-ray diffraction (XRD) analysis as well as scanning electron microscopy (SEM) coupled with an Energy-dispersive X-ray (EDAX) device were applied to determine the differences in the chemical composition and surface morphology which appeared as a result of the corrosion treatment. The data obtained are compared to those of electrodeposited pure Zn coatings with identical experimental conditions demonstrating the enhanced protective characteristics of the ternary alloys during the test period in the model medium. The influence of the chemical and phase composition of the alloys on its corrosion resistance and protective ability is also commented and discussed.

Zinc Composite Layers, Incorporating Polymeric Nano-aggregates: Surface Analysis and Electrochemical Behavior

This study reports on a comparative investigation of the corrosion behavior of zinc (Zn) and nano-composite zinc (ZnC) galvanic layers in 5% NaCl solution. The metallic matrix of the ZnC layers incorporates nano-sized, stabilized polymeric aggregates, formed from the amphiphilic tri-block co-polymer: poly(2-hydroxyethyl methacrylate) - poly (propylene oxide - poly (2-hydroxyethyl methacrylate) (PHEMA15PPO34PHEMA15). The main objective was to evaluate the electrochemical properties and surface characteristics of both coatings, thus further to investigate if the nano-composite layers will have better corrosion resistance, compared to pure galvanic zinc. The electrochemical behavior, investigated by Impedance spectroscopy (EIS) and Scanning vibrating electrode technique (SVET), supported by surface analysis, using Atomic-force microscopy (AFM) and Scanning electron microscopy (SEM), reveals higher corrosion resistance and consequently better performance of the nano-composite layers, compared to pure galvanic zinc. The mechanism of incorporation of the polymeric nano-aggregates in the coating and their influence on the barrier properties of the composite layers are also briefly discussed.

Corrosion Investigations of Black Chromite Films on Zn and Zn-Co Coatings with Low Cobalt Content

The corrosion resistance and protective ability of black-colored chromite (Cr3+ based) and chromium-free conversion films (CFs) on electrodeposited zinc and Zn-Co alloy coatings having low cobalt content in a neutral model medium of 3% NaCl solution have been investigated and characterized. Test methods such as polarization resistance measurements, scanning electron microscopy, EDS, x-ray diffraction analysis, and x-ray photoelectron spectroscopy analyses have been applied in order to scrutinize the actual protective characteristics of these films as well as to determine the composition of the corrosion products appearing as a result of the treatment in the corrosion medium. The experimental results revealed better protective characteristics, consequently superior performance of the electrodeposits with chromite films compared to these with chromium free or without any additional CF. The processes occurring on the sample’s surface during the immersion in the chemical conversion solutions as well as the influence of the H3PO4 in the course of the treatment are also commented on and discussed.

Optical characterization of Zn coatings deposited on low carbon steel substrates

We studied the optical reflectance profile of zinc layers with thickness of 2-10 μm electrodeposited on low carbon steel in the spectral range of solar radiation (0.3 - 2.5μm) and in the thermal infrared (2.5 - 15μm). Spectral selectivity is an important parameter of the solar absorbers used in solar thermal collectors. It is defined by low reflectance in the range 0.3 - 2.5 μ, and high reflectance in the thermal infrared, 2.5 - 15 μm. The Zn coatings are thick and their surface is rough. If the surface features size is in the order of the visible wavelengths, then absorption of light at the surface is possible. The investigations revealed the formation of ZnO on the Zn coating surface. The Raman study showed a weak peak around 512 cm−1, which can be attributed to the ZnO oxide phase. The longer wavelengths penetrate the oxide surface and reach the bulk part of the Zn coating where they are reflected. As a result, a step-wise profile is expected in the reflectance spectrum within the 0.3 -15 μm range, with a change from a comparatively low reflectance in the visible to a high reflectance in the infrared.