Breakdown Of Passive Layer Research Articles

Gaseous corrosion resistance of Fe–Al based alloys containing Cr additions: Part II. Scale morphology

Iron–aluminum based weld overlay claddings are currently being considered as corrosion resistant coatings for corrosion protection at 500 °C in aggressive sulfidizing and oxidizing atmospheres. These alloys rely on the formation of a thin passive Al 2O 3 layer for their corrosion protection. Once the alloy can no longer maintain the protective oxide scale, passive layer breakdown can occur, where non-protective nodules form at random locations along the surface of the sample. These nodules can continue to grow together and eventually overgrow the protective oxide rendering the sample no longer corrosion resistant. In this current study, 10 iron–aluminum based alloys containing chromium and titanium additions were exposed to three high-temperature corrosive environments at 500 °C for 100 h. The passive layer breakdown phenomenon was observed in all three environments on several non-protective alloys. The extent of nodule growth was measured and used to identify corrosion resistant alloy compositions. It was found that aluminum and chromium additions help to form and maintain a protective oxide layer and critical alloying contents were required to prevent passive layer breakdown during 100 h of exposure. Critical alloying values necessary to prevent nodule formation were compared to critical alloying levels required to prevent rapid corrosion kinetics. While corrosion kinetics are helpful in determining protective alloy compositions, the corrosion scale morphology must be characterized to ensure that the alloy is completely protective during the exposure time.

The Effect of Water Vapor on Passive-Layer Stability and Corrosion Behavior of Fe–Al–Cr Base Alloys

Although it has been reported that additions of water vapor to high-temperature corrosion environments accelerate corrosion, the role of water vapor on increasing the corrosion rate of Fe–Al–Cr alloys and coatings is not very well understood. In order to better understand these effects, multiple Fe–Al–Cr base alloys were tested at 500°C for 100 hr in three different multi-component corrosive gases with and without water vapor. The three gases were a sulfidizing gas, a mixed oxidizing/sulfidizing gas, and an oxidizing gas. Thermogravimetric testing showed that the corrosion kinetics increased when water vapor up to 6% was added to the atmospheres. The surfaces of the exposed samples were considered carefully to determine if the addition of water vapor changed the morphology of the corrosion products and, more importantly, affected the passive layer. It has previously been shown that the formation of nodules can be caused by the inability of the passive layer to re-heal itself after the presence of a defect allowing fast growing non-protective corrosion products to externally grow from the surface. In this study, the amount of external nodules that formed on the surface of the alloys was shown to increase with the addition of water vapor. An increase in the amount of external nodules present due to additions of water vapor gives an indication that water vapor accelerates the passive-layer breakdown. It was observed that the scale morphology correlated well with the corrosion kinetics for the exposed alloys, which showed that water vapor increased the corrosion rates of the alloys.

Passive Layer Breakdown on Titanium in Doped and Undoped Artificial Saliva

AbstractThe aim of this work is to investigate the long term (3000 exposure hours) tendency of passive layers on titanium to breakdown in Tani-Zucchi and Carter-Brugirard artificial saliva undoped and doped with chloride and fluoride ions. The non-uniformity of the saliva pH (from 7.88 to 5.73 and extreme case 2.15) was simulated. The following techniques were used: potentiostatic, potentiodynamic and linear polarization, the monitoring of the open circuit potentials vs. exposure time and atomic force microscopy. The breakdown (pitting) potential at about +2.9 V and the re-passivation potential at +2.4 V in Tani-Zucchi artificial saliva were determined. So, the pitting protection potential is very noble and can not be reached in human body.

Studies on Galvanized Carbon Steel in Ca(OH)2 Solutions

The behavior of galvanized carbon steel samples was studied in Ca(OH)2 solutions, simulating the alkaline environment in reinforced concrete. Under shorter periods, it was seen that a passive layer was formed on the surface of the zinc coating. The film, however, was not stable for large periods of time. This was revealed by the long duration tests where the passive layer was disrupted and the carbon substrate was protected sacrificially by zinc dissolution. The presence of chlorides accelerated the passive layer breakdown. The role of calcium nitrite in inhibiting the corrosion process of zinc was also studied. It was found that zinc was not protected by nitrite in the presence of chloride ions. The inhibitor, however, significantly reduced the corrosion rate of the underlying steel.

Breakdown of the Iron Passive Layer by Use of the Scanning Electrochemical Microscope

The scanning electrochemical microscope (SECM) is used to generate localized corrosion at passivating iron surfaces by using the tip to generate ions. Use of the SECM allows the rapid establishment of a locally aggressive chemical environment at a preselected site on the iron surface. The susceptibility for passive layer breakdown and corrosion initiation was examined as a function of the time between the start of the passive layer growth and the formation of ions. The breakdown of the passive layer was found to depend strongly on the passivation potential and the site of formation on the iron surface. In addition to generating ions, the SECM tip was simultaneously used to detect large iron ion concentration fluctuations as corrosion began. Current fluctuations at the tip were observed and ascribed to precursors to the passive layer breakdown.