Flow-accelerated corrosion has been studied by means of computational fluid dynamics (CFD) techniques and rotating cage (RC) autoclave tests according to ASTM G170. This study evaluated through CFD and laboratory tests the influence of RC geometry and baffles on the wall shear stress (τw) proposed by the aforementioned standard. The experimental were performed at 120 °C and 2 angular velocities (500 and 800 rpm) during 25 h in a solution with 600 mg/L of Cl- ions and 10-3 mol/L of sodium thiosulfate. Corrosion rates were obtained by ASTM G31 and the surfaces were evaluated by OM and XRD. Corrosion rate values and XRD analyses showed divergence between samples in different positions in the RC and between distinct regions in the same sample. The numerical simulations and the experimental tests showed a great influence of the baffles on τw. The conservatism in the values obtained with this technique is caused by excessive corrosion close to the samples edges and the non-uniformity of τw on the internal samples surfaces. Thus, the main conclusion of the study is related to the importance of better knowledge on experimental methodology and mathematical approximations for updating the criterion for the correct τw calculation on sample surfaces.

A resistência à corrosão de eletrodepósitos de Cr e Ni-Mo e a influencia do tratamento térmico na estrutura cristalográfica, na morfologia e nas propriedades de microdureza foram investigados. As caracterizações foram feitas usando as técnicas de microscopia eletrônica de varredura (SEM), difração de raios X (XRD) e energia dispersiva de raios X (EDX). Os ensaios de corrosão foram feitos à temperatura ambiente, em solução de NaCl 10-1 mol dm-3 e usando a técnica de polarização linear potenciodinâmica. O teor de Mo na camada e a eficiência de corrente aumenta com a concentração do íon molibdato no banho de eletrodeposição, enquanto que a morfologia superficial evolui de rugosa e homogênea para trincada com o aumento do teor de Mo na camada. Os ensaios eletroquímicos de corrosão mostram que os eletrodepósitos de Cr são mais resistentes à corrosão que os eletrodepósitos de Ni-Mo e que todas as camadas estudadas corroem em meio de cloreto. Dentre os eletrodepósitos de Ni-Mo estudados, o de Ni-13Mo foi o que apresentou o comportamento de resistência à corrosão mais nobre. A microdureza do eletrodepósito Ni-13Mo aumenta com o aumento da temperatura de tratamento térmico e está relacionado à formação de fases de Ni, Ni4Mo e NiMo, durante o tratamento térmico. Ni-13Mo é um potencial candidato a substituir o Cr em aplicações industriais quando a temperatura de operação for maior que 100 ºC e que boas propriedades de microdureza sejam requeridas.

The corrosion resistance of electrodeposited Cr and Ni-Mo coatings and the influence of heat treatment on the crystallographic structure, morphology and microhardness properties were investigated here. The characterisations were carried out using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were performed at room temperature in 10-1 mol dm-3 NaCl solutions and by potentiodynamic linear polarization technique.TheMo content in the layer and current efficiency increased with the molybdate ion concentration in the plating solution, while the surface morphology evolved from rough and homogeneous to cracked surface with the increase of the amount of Mo in the layer. The electrochemical corrosion tests showed that the Cr coatings have better corrosion resistance than the Ni-Mo coatings in chloride medium and that all the studied coatings corrode in chloride medium. Ni-13Mo coating has the nobler corrosion behavior among the studied Ni-Mo coatings. The microhardness of the Ni-13Mo coatings increased as the annealing temperature increased which is related with the precipitation of Ni, Ni4Mo and NiMo phases during the heat treatment of this coating. Ni-13Mo coating is a potential substitute for chromium coating in industrial applications when operating at temperatures higher than 100 ºC and good microhardness properties are required.