Estimation of Corrosion Rate of AISI 1016 Steel by the Analysis of Polarization Curves and Using the Method of Measuring Ohmic Resistance

By the example of the study of AISI 1016 steel corrosion in the HCl solution, the potentialities of various methods for determining the corrosion currents are studied. The corrosion rate of steel without polarization is determined using the method of measuring ohmic resistance of test specimen. It is shown that the iterative method used for the analysis of potentiodynamic curves in the vicinity of corrosion potential avoids the complications that arise when the Tafel extrapolation method is used in the absence of pronounced Tafel sections in these curves.

Morphological, Chemical, and Biological Investigation of Ionic Substituted, Pulse Current Deposited Calcium Phosphate Coatings

Ionic substituted calcium phosphate coatings (iCP) have been prepared by the electrochemical pulse current deposition technique with an alternate pulse on and off time of 5 ms onto a titanium alloy substrate. The elemental distribution and morphology of the deposited layers have been extensively studied by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM). The crystallinity and phase structure of iCPs have been investigated by X-ray diffraction (XRD). The corrosion characteristics and biodegradability of coatings have been determined by electrochemical measurements, recording potentiodynamic curves in a physiological solution over a long-term immersion period. The cell viability tests confirmed that the iCP coating was biocompatible, while the corrosion tests proved its biodegradable characteristic. In our paper, we compare the morphological, chemical, and biological characteristics of silver and zinc substituted calcium phosphate layers deposited by the electrochemical method.

Chlorides Entrapment Capability of Various In-Situ Grown NiAl-LDHs: Structural and Corrosion Resistance Properties

In this work, various NiAl-LDH thin films, exhibiting specific surface morphologies, were developed directly on aluminum AA 6082 substrate to understand the two main characteristics of layered double hydroxide (LDH), i.e., ion-exchange behavior and barrier properties, which are found to have a significant influence on the LDH corrosion resistance properties. The as-prepared NiAl-LDH films were analyzed through the scanning electronic microscope (SEM), X-ray diffraction (XRD), while the corrosion behavior of the synthesized films was investigated by the electrochemical impedance spectroscopy (EIS) and potentiodynamic curves. The results indicated that NiAl-LDH microcrystals grow in various fashions, from porous relatively flat domains to well-developed platelet structure, with the variation of nickel nitrate to ammonium nitrate salts molar ratios. The LDH structure is observed in all cases and is found to cover the aluminum surface uniformly in the lamellar order. All the developed NiAl-LDHs are found to enhance the corrosion resistance of the aluminum substrate, specifically, a well-developed platelet structure is found to be more effective in chloride adsorptive and entrapment capabilities, which caused higher corrosion resistance compared to other developed NiAl-LDHs. The comparison of the synthesized NiAl-LDH morphologies on their ion-exchange capabilities, barrier effect and their combined effect on corrosion resistance properties is reported.

SELECTION OF MODE OF ANODIC TREATMENT OF GRAPHITE IN A SPENT NITRIC ACID ETCHING SOLUTION FOR PRODUCING THERMALLY EXPANDING GRAPHITE COMPOUNDS

The possibility of using galvanic waste for the synthesis of graphite thermo expanded compounds has been shown. An electrolyte on the basis of a spent nitrate solution of etching copper parts containing cations of various metals in its composition was taken as an example, thereby solving an environmental issue related to the problem of utilization of solutions containing nitric or sulfuric anions in its composition. The processes occurring in the synthesis of thermally expanding graphite compounds on platinum and graphite electrodes in the electrolyte under investigation were studied by the method of potentiodynamic curves. The currents on the potentiodynamic curves before reaching the oxygen evolution potential were due to the oxidation of the surface-functional groups followed by intercalation of the graphite matrix. It was revealed that the potential for oxygen evolution in the electrolyte based on the spent nitric acid solution of etching of copper parts is slightly lower than in 58% HNO3 due to the presence of metal cations in the composition. Free metal cations attract a part of water molecules to form hydrate shells, while copper cations form complexes where H2O molecules are ligands, which makes it difficult to release oxygen. In the electrochemical synthesis of thermally expanding graphite compounds, potentiostatic and galvanostatic modes were applied. A range of potentials of 1.8 - 2.2 V was chosen for performing the synthesis in the potentiostatic mode. Recommendations are given for the application of the galvanostatic mode with the current of anodic treatment of 60 - 140 mA (per 1 g of graphite) for the synthesis of graphite thermo expanding compounds in industrial volumes. The thermo expanded graphite obtained in this manner is characterized by a bulk density of less than 2 g/dm3.

Effect of Fluoride Addition in Sulfuric Acid-Based Desmutting Solution for Aluminium Alloy AA8006

Aluminium alloys are covered spontaneously by a natural oxide layer, well adherent to the surface. For improving the material’s anticorrosion performance surface pre-treatments are needed in order to remove the superficial oxide and to prepare the alloy surface to subsequent treatments. The pre-treatment process involves several steps, including desmutting. The last requires the immersion of the part in an appropriate solution, typically composed of several chemicals, able to remove the oxide layer. In this work, the effect of the fluorides addition into a sulfuric acid-based solution, when used in a desmutting industrial process, was evaluated. Potentiodynamic curves, glow discharge optical emission spectrometry and contact angle techniques were used to characterize the surface of the aluminium alloy AA8006 after desmutting.

Corrosion resistance of AZ31 magnesium alloy treated by plasma electrolytic oxidation

The coating prepared by plasma electrolytic oxidation (PEO) was created on AZ31 magnesium alloy surface with the aim to evaluate its effect on corrosion resistance. The DC current was applied on the sample in solution consisted of 10 g/l Na3PO4·12H2O and 1 g/l KOH. Additional samples were prepared with 2 and 4 minutes of preparation to observe evolution of the PEO coating. Morphology of the coatings was evaluated by scanning electron microscopy and chemical composition was examined by EDX analysis. Electrochemical characteristic were measured by potentiodynamic polarization tests and electrochemical impedance spectroscopy in 0.1 M NaCl at the laboratory temperature. Obtained data were presented in form of potentiodynamic curves and Nyquist diagrams. Results of analysis showed that plasma electrolytic oxidation coating positively influence corrosion resistance of AZ31 magnesium alloy in chosen corrosive environment.

Corrosion Protective Coatings: Fabrication of Sputtered CeO2-La2O3 and La2O3-CeO2 Bilayers

This entry provides a comparative study on the corrosion protection efficiency of Ce, La films as well as Ce/La- and La/Ce-bilayered coatings deposited onto AA7075 and AA6061 substrates by the radio frequency magnetron sputtering technique. The coating thickness ranged from ~12 to 835 nm, which changed with the deposition parameters and substrate composition. The relationship between microstructure, roughness, and electrochemical performance is examined. The reactivity and crystallinity of rare earth (RE) films can be tailored by adjusting the sputtering parameters. Sputtered La films with a thickness of ~390 nm and an average roughness of 66 nm showed the best corrosion protection properties in chloride medium as determined by potentiodynamic curves and electrochemical impedance spectroscopy. The method to obtain RE-bilayered coatings, i.e., La/Ce or Ce/La as well as the substrate composition and applied power, conditioned their inhibition properties. The RE-bilayered coatings displayed better barrier properties than Ce films, which were worser than those featured by La films.

Effect of Thiosulfate on Pitting Corrosion of Ni-Cr-Fe Alloys in Chloride Solutions

Pitting corrosion of Alloys 600, 690, and 800 (UNS N06600, N06690, and N08800) was studied in 1 M NaCl solution with different concentrations of thiosulfate (). Alloys exhibited vastly different electrochemical behavior, depending on the concentration and chromium content of the alloy. Alloy 600 exhibited a breakdown and repassivation potential that decreased with decreasing concentration, in the range from 1 M to 10−4 M. Breakdown and repassivation potentials decreased about 300 mV and 600 mV, respectively, when 10−4 M was added to a 1 M NaCl solution. For Alloys 690 and 800, additions of in the range of 1 M to 0.01 M caused a decrease in the breakdown and repassivation potentials. Dilute solutions were more aggressive, and a 0.01 M addition of to a 1 M NaCl solution caused a decrease in breakdown and repassivation potentials of about 300 mV. In a solution containing 0.001 M Na2S2O3 + 1 M NaCl, Alloys 690 and 800 showed two different submodes of pitting, each one of them existing at a different range of potential. Chloride pitting corrosion was observed at high potentials and was characterized by pits with a lacy cover. The stable pit initiation potential associated with this process in both alloys was near 300 mVAg/AgCl and was preceded by frequent metastable events. Chloride plus pitting was observed at low potentials (near −225 mVAg/AgCl) and was characterized by hemispherical pits. In potentiodynamic curves, this submode of pitting showed a characteristic anodic peak of approximately 120 mV width and a maximum current density of 10 μA/cm2 for both alloys. Potentiostatic tests at potentials within this anodic peak led to stable pit growth. Those pits could be repassivated by scanning the potential either in noble or active directions. Low-potential and high-potential pitting submodes were separated by a stable passivity range, as determined by potentiodynamic curves. Upon a further decrease in concentration down to 10−4 M, only high-potential pitting corrosion was observed, with pitting and repassivation potentials similar than those in 1 M NaCl solution.

Corrosion behavior of wrought magnesium alloys AZ31 and AZ61 in Hank’s solution

Corrosion behavior of wrought magnesium alloys AZ31 and AZ61 was studied in Hank’s solution. Potentiodynamic curves measured after short-term of exposure showed higher corrosion resistance of AZ31 magnesium alloy in comparison with AZ61 magnesium alloy. On the contrary, long-term tests measured by electrochemical impedance spectroscopy showed higher corrosion resistance of AZ61 magnesium alloy in comparison with AZ31 magnesium alloy.

Evaluation of the Synergistic Effect of Erosion-Corrosion on AISI 4330 Steel in Saline-Sand Multiphase Flow by Electrochemical and Gravimetric Techniques

The synergistic effects of fluid flow, sand particles, and solution pH on erosion-corrosion of AISI 4330 steel alloy in saline-sand medium were studied through a rotating cylinder electrode (RCE) system by weight-loss and electrochemical measurements. The worn surface was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results show that, under all the test conditions assessed, the passivity of the steel alloy could not be maintained; as a result, an activation mechanism dominates the corrosion process of steel alloy. Furthermore, the potentiodynamic curves show that, with the increasing of the electrode flow rate and particle size, the anodic current density increased, which is due to deterioration of the electrode by the impacting slurry. Although the increase of particle size affects the anodic current density, the effect of particle size does not cause a significant change in the polarization behavior of the steel electrode. The electrochemical impedance and potentiodynamic curves suggest that erosion-corrosion phenomenon of the ASISI 4330 steel is under mixed control of mass transport and charge transfer. The inductive loops formed in the impedance plots are representative of an increase in roughness of the electrode caused by the particles impacting at the surface. The change in the passivity of the steel alloy as the pH is altered plays an important role in the corrosion rate.