Two types of thermal sprayed coatings for corrosion prevention in natural environments are discussed in this review. One is a sacrificial anode type coating of zinc and aluminum, which has a long history dating back to 1940's and will become more important as the expected service life of infrastructures become longer and the existing structures require good maintenance technology. The other is a dense barrier type coating of materials such as stainless steels and Ni-base alloys, which becomes more applicable due to the recent development of spray processes such as HVOF. Basic corrosion properties and field performances as well as problems to be solved of HVOF sprayed alloy coatings are discussed.
Chlorides induced stress corrosion cracking (ESCC) behavior was examined for SUS304L, SUS316L and SUS304 stainless steels (SS) after depositing synthetic sea water simulating sea salt particles at a temperature range from 333 K to 353 K with relative humidity of 35%. A stress-ESCC failure time relationship was formulated for SUS304L SS as σ=-Alog(tf)+B, where σ : applied stress (MPa), tf : time to failure (h), A=23.7T-7020, B=43.7T-11600, T : absolute temperature (K). Even an incipient micro-crack was not observed on the specimen surface of the SS at the applied stress level of 0.25σy (σy : 0.2% proof stress), although SCC initiated at the applied stress level of 0.5σy. Thus the threshold stress should be between 0.5σy and 0.25σy for these SS, and the threshold stress would rise beyond these values if the stress concentration be considered at the bottom and circumference of a pit. A statistical analysis suggested that the ESCC crack length and crack depth conformed to the double exponential distribution.
The oxide films formed on metallic surfaces at high temperature corrosive environments often act as diffusive barriers of oxygen to protect the metal from corrosion. However, damage to metallic materials is possibly accelerated by mechanical fractures and spalling of the oxide films, caused by such as collisions of fly ash and thermal stresses of the oxide films. Investigations on mechanical properties and adhesion of oxide films formed on metal surfaces at high temperature corrosive environments lead to estimating the life time of materials and to searching high temperature corrosion prevention techniques. Both high temperature corrosion and round particle impact tests were performed on pure titanium coupon specimens. Both a particle impact method and an exfoliation/fracture model of oxide films were very useful to examine elastic modulus, fracture stresses and strains, and adhesive stresses of oxide films formed on pure titanium at high temperature environments. The spalling of the titanium oxide films was experimentally reproduced by the temperature decrement calculated by the elastic modulus and the adhesive stress obtained in this study, and by the thermal expansion coefficients of titanium and titanium oxide. These mechanical properties obtained in this study were, therefore, proved to be valid.