Controlling the rate of degradation of Mg using magnesium fluoride and magnesium fluoride-magnesium phosphate duplex coatings

The thin and porous Fluoride Conversion Coating FCC with many cracks could not offer a significant improvement in corrosion resistance for Mg. Magnesium phosphate coating improves the corrosion resistance of Mg, good bioactivity, promotes cell viability and cyto-compatibility and exhibits antibacterial activity. However, rapid dissolution in Mg in acidic magnesium phosphate containing solutions leads to the development of an inhomogeneous coating. The present study attempts to prevent the excessive dissolution of Mg by forming a fluoride conversion coating as a pre-treatment in the first stage followed by deposition of magnesium phosphate coating in the second stage to develop magnesium fluoride-magnesium phosphate duplex coatings. The morphological features, structural characteristics, nature of functional groups, corrosion behavior in Hanks’ balanced salt solution and bioactivity in simulated body fluid are assessed to ascertain the suitability of the magnesium fluoride-magnesium phosphate duplex coating in controlling the rate of degradation of Mg and improving its bioactivity using uncoated Mg and fluoride conversion coated Mg as reference. The findings of the study reveal that the magnesium fluoride-magnesium phosphate duplex coating could offer an excellent corrosion resistance and improve the bioactivity of Mg.

Characterization of Failure Mechanisms of Duplex and Graded Thermal Barrier Coatings Exposed to Thermal Shock Test

The beginning of failure of a (ZrO2-7%Y2O3)/(Ni-22%Co-17%Cr-12.5%Al-0.6%Y) duplex andgraded coating systems on lnconel 617 and IN738LC in burner rig tests has been characterized.The test conditions are 40 s heating up to 75O℃ substrate temperature followed by 80 s aircooling. Failure is considered at the appearance of the first bright spot during heating period.Stresses due to thermal expansion mismatch strains on cooling are the probable cause of life-limiting in this conditions of testing.

CARBON FIBRE ELECTROPLATED BY Cu-Ni DUPLEX COATING AND ITS COMPOSITE

The carbon fibre was electroplated continuously by a duplex coating of Ni prior to Cu,and the composite made of this carbon fibre together with Cu matrix has been investigated,the compatibility between carbon fibre and duplex coating and flexural strength,linear thermal expansion coefficient and electric resistivity of the composite have also been examined.Owing to the transformation of face-centred cubic Cu-Ni solid solution in the duplex coating under high temperatures,both the nodulizing shrinkage of Cu coating alone and the graphitization of carbon fibre accelerated by single Ni coating with a consequent loss of strength were im- proved.Because of the dissolving of minute carbon fibre in the interfacial Cu-Ni solid solu- tion,the bonding strength and flexural strength of the composite were significantly increased, and the length of carbon fibre pulled out on the fracture surface was obviously reduced.The interface of the composite seems to be of the dissolution bonding.

Improvement in the corrosion protection and bactericidal properties of AZ91D magnesium alloy coated with a microstructured polypyrrole film

In this work hollow rectangular microtubes of polypyrrole(PPy)films were potentiostatically electrodeposited on magnesium alloy AZ91D in salicylate solution.The substrate was previously anodized under potentiostatic conditions in a molybdate solution in order to improve the adherence of polymer.Finally the duplex film was modified by the incorporation of silver species.The obtained coatings were characterized by scanning electron microscopy(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopies(XPS)and the antimicrobial activity against the bacteria Escherichia coli was evaluated.The corrosion protection properties of the coatings were examined in Ringer solution by monitoring the open circuit potential,polarization techniques and electrochemical spectroscopy(EIS).The duplex coating presents an improved anticorrosive performance with respect to the PPy film.The best results concerning corrosion protection and antibacterial activity were obtained for the silver-modified composite coating.

Synthesis and Characterization of Electroless Ni-P/Ni-Mo-P Duplex Coating with Different Thickness Combinations

In this work, a novel duplex Ni-P/Ni-Mo-P coating upon the aluminum（Al） substrate was synthesized via an electroless plating, i.e., the binary Ni-P coating as a transition layer and the ternary Ni-Mo-P coating on the top. It was found that the duplex coating was of a high hardness,large elastic modulus,low porosity and excellent corrosion resistance. In addition, experimental results revealed that for a total 20 μm coating thickness, the duplex coating with a 7 μm of the Ni-Mo-P coating exhibited the best corrosion resistance in 0.5 mol/L sulfuric acid solution, which was attributed to its compact structure and low porosity. This duplex Ni-P/Ni-Mo-P coating with a thin ternary Ni-Mo-P layer is expected to solve the problem of low deposition rate of ternary alloy coating and thereby may expand applications of Al and its alloys in the fields of machine manufacture and corrosion environment.