Gerhard Hans Knörnschild

The negative-difference effect during the localized corrosion of magnesium and of the AZ91HP alloy

Foram realizados testes galvanostaticos em solucoes de NaCl com magnesio puro e com a liga AZ91HP, tendo sido medido o volume de hidrogenio desprendido durante os testes. A fim de estudar o efeito da diferenca negativa (NDE), uma ideia foi desenvolvida para separar o desprendimento de hidrogenio (H.E) na superficie passiva do desprendimento dentro dos pites. A analise dos dados volumetricos mostrou que a corrente de reducao de hidrogenio dentro dos pites, sobre o Mg, e muito mais alta do que nos pites sobre a liga AZ91HP. Supondo que a razao entre a taxa de reducao de hidrogenio e a taxa de dissolucao de magnesio, expressa pelo fator k, nao muda com a polarizacao, foi encontrado um valor de aproximadamente 0,5 para o Mg puro. Para a liga AZ91HP foi alcancado somente um valor em torno de 0,3. Galvanostatic tests with pure magnesium and with the AZ91HP alloy were performed in NaCl solutions, and the volume of hydrogen evolved during the tests was measured. In order to examine the negative-difference effect, a concept was developed to separate the hydrogen evolution (H.E.) on the passive surface and the H.E. inside the pits. An analysis of the volumetric data showed that the current density of H.E. inside the pits is much higher for Mg than for the AZ91HP alloy. With the assumption, that the ratio between the hydrogen reduction rate and the rate of the anodic metal dissolution, expressed by the factor k, does not change with the polarization, a k value of ca. 0.5 for pure Mg was found, while for the AZ91HP alloy this value was only ca. 0.3.

Electron Microscopic Studies of Anodic Oxide Films on the AZ91HP Alloy

A Mg-9wt.Al-1wt.%Zn-alloy was anodized up to 90 V with constant current/constant voltage in an electrolyte which contained the compounds of the HAE-process (KOH, Al(OH) 3 , KF, Na 3 PO 4 and KMnO 4 ). Electron microscopic examinations revealed a highly porous and irregular film structure. The distribution of the elements in the film was measured with energy dispersive spectrometry on specimens prepared in cross section for the transmission electron microscope. The main characteristic found was a fluoride-enriched zone of about 100 nm thickness at the metal / film interface. Practically no manganese from the permanganate was detected in this fluoride-enriched zone.

Comparative studies of WC-Co and WC-Co-Ni composites obtained by conventional powder metallurgy

The present work reports a comparative study of cemented carbides of compositions WC-6Co, WC-10Co, WC-20Co, WC-6Co-6Ni and WC-12Ni-6Co. The purpose was to study the powder metallurgical production process of these compositions starting from a commercial WC-6Co powder, obtaining the desired compositions by mass balance with pure Co and pure Ni powders. During the process steps mixing, milling, compacting and sintering the powders were described by its apparent density, green density, shrinkage and sintered density. Lower densities were observed in composites with higher binder content. The process was monitored by scanning electron microscopy and EDS analysis to evaluate the homogeneity of the powders, to detect contaminations by the process and to characterize the microstructure of the sintered materials. A finer microstructure was found when the binder contained Ni. Potentiodynamic polarization tests in sulfuric acid revealed pseudo-passive behavior for all the tested hard metals.

Soil corrosion of the AISI1020 steel buried near electrical power transmission line towers

Soil corrosion of carbon steel samples buried up to hundred days close to a high voltage power transmission line tower was examined by weight loss vs. time. A higher weight loss was observed if the samples were electrically connected to the tower than if they were not. This was attributed to the influence of alternating current (AC) signals induced in the soil by the transmission line. This field study showed for the first time the influence of the AC power line on the buried structure of the tower, while other studies so far were focused only on AC corrosion of cathodically protected coated pipelines, running parallel to the transmission line. An improved method was used to measure weight loss by descaling in Clark solution. The new method substitutes discontinuous measurements, proposed in the ASTM-G1-90 standard, by in situ measurements of the weight loss during descaling, using a computer controlled microbalance.

A study of Al-Mo powder processing as a possible way to corrosion resistent aluminum-alloys

Elementary Al and Mo powder mixtures have been processed by high energy ball milling up to milling times of 100 hours. The shift of the pitting potential and the X ray analysis of green milled samples showed that part of the Mo has formed a supersaturated solid solution of Mo in Al. Elementary Mo powder, however, was still present after 100 hours of milling. Sintering led to the formation of the intermetallic Al12Mo phase.