The evaluation methods for the degree of rusting on stainless steels in architectural applications are reviewed. Conventionally, the atmospheric corrosion resistance of stainless steels has been ranked based on so-called rating number methods, which is either the evaluation of the percentage of area rusted, or the comparison of the specimen surface with the standard panels having different levels of dirtiness due to rusting. However, it is impossible to extract information about the corrosion mechanism that is contained in the rust form by using these methods. On the other hand, by showing relationship between the percentage of area rusted and the number of the rust spots on the xy-coordinate, the change in rust morphology can be characterized from the inclination of the locus of each datum point. Also, it can be known the quantitative dirtiness of specimen from the relationship between the logarithm of area percentage and the logarithm of the number of rust. Furthermore, it is mentioned about the application of digital image processing to measure the rust morphology.
We have measured the Kelvin potential, Vkp, of a low-alloy steel covered with adsorption water and a thin solution layer using the Kelvin probe apparatus. It was found that the Kelvin probe can detect the water adsorption both on the surface and in the surface oxide layer. Under a thin solution layer, corrosion potential, Ecorr, is dependent upon the thickness of the solution layer due to change in oxygen reduction rate and in surface characteristics. Long-time Ecorr measurement shows that Ecorr increases with time as a result of rust-layer formation on the steel surface; magnitude of which depends on characteristics of the rust-layer. The scanning Kelvin probe reveals the Vkp distribution for the steel surface with depositions of solution droplet and iron oxide.
For the measurement of electrode potential of metals covered with thin solution film, the Kelvin technique was applied, in which a silver wire of 0.5mm diameter with a AgCl coating was used as the Kelvin probe for potential detection. The probe placed in air was oscillated sinusoidally with respect to metal surface. It is thus possible to measure relative electrode potential without touching the electrolyte. A galvanic couple model comprising of iron and zinc was used for demonstrating potential transition at the interface. Experiments were carried out under pure water to 0.5%NaCl solution layer with 100 and 400μm thick. Under the solution films, potential on zinc was about -1.2V while that on iron was about -0.7V. Under the pure water film of 100μm thick, the zone of potential transition was 3-4mm wide from the boundary into Fe surface. The width of potential transition was influenced by both solution film thickness and salt concentration. The width of potential transition zone under the film of the NaCl solution was larger than that under pure water. This indicates that galvanic effect reaches further into Fe surface in salt solutions. The effective distance of corrosion protection by zinc is inversely proportional to a square root of ρ/δ, in which ρ is a resistivity of a solution and δ is a thickness of a solution film. Potential distribution at Zn/Fe boundary was explained in terms of“transmission line model”.
Surface potential distribution of Zn and Zn-Al alloy coated steels with a large scratch was measured under a thin electrolyte layer using the Kelvin sensor in order to study the sacrificial corrosion protection effect due to the Zn and Zn-Al alloy coatings. The measurement was conducted under the thin electrolyte layer containing NaCl and MgCl2 at 25°C and at 80%RH or 98%RH. The scratched part, where the steel substrate was directly exposed to the electrode layer, showed less noble potential as the electrolyte layer becomes thicker and the concentration of chloride ion becomes higher, because the couple current flowing between the scratched part and the coating through the electrolyte layer becomes larger. When the electrolyte layer was very thin, the surface potential of steel coupled with Zn coating shifted to noble direction with the distance from the Zn anode, indicating that the sacrificial corrosion protection effect is decreased. It was found that the Kelvin sensor is a very useful tool for studying the sacrificial corrosion protection effect under thin electrolyte layer.
In this paper the filiform corrosion of painted steel substrates was analyzed with a scanning Kelvin probe. The substrates were painted with clear acryl paint, and the dry film thickness was 5μm. In order to induce filiform corrosion, knife scribed specimens were set in a corrosive environment. (5%NaCl solution spray and 40°C 80%RH) By using the scanning Kelvin probe, a potential on the head of filiform corrosion was detected as less noble which showed anodic site. It is possible, with this technique, to measure the under-film corrosion.
Kelvin probe measurement was applied to measure corrosion potential of low-alloyed steels covered with thin electrolyte layer (0.1mM NaCl) and the thickness of electrolyte layer. A good correlation between the volta potential difference between gold Kelvin probe and working electrode (low-alloyed steel) and its electrochemical corrosion potential was observed. By Kelvin probe measurement, variations of corrosion potential and the electrolyte thickness was successfully monitored. In case of the working electrode covered with corrosion products, diminution rate of electrolyte layer was decreased at about 50μm, which are considered to be the effects of corrosion products “on time of wetness of specimen”. Below about 200μm of the electrolyte layer thickness, corrosion potential shifts cathodically according to the diminution of the thickness of electrolyte layer up to a few μm, which suggests the break down of passive film is occurred by chloride concentration.