Recently, high temperature corrosion and oxidation wastage on boilers and gas turbines has increased for improve on plants efficiency, development of new systems and using of low grade fuels, etc. This paper describes typical high temperature corrosion and oxidation of boilers and gas turbines also introduce various application examples of thermal spray coatings such as APS, HVOF, VPS, WPS, etc. for its protection.
A heat exchanger for vaporizing liquefied natural gas (LNG) is made of aluminum alloy tubes, and the way of protecting the tubes from corrosion in sea-water is to form a sacrificial anode layer by thermal-spray coating of Al-Zn alloy on aluminum alloy tubes. However, corrosion damage has been sometimes found in the lower part of heat exchangers exposed to near frozen sea-water. The purpose of this study is to investigate the corrosion behavior of Al-2mass%Zn alloy thermalspray coating in freezing sodium chloride solution and to examine a possibility of electrochemical impedance spectroscopy for monitoring of the corrosion process of heat exchanger. The Al-2mass%Zn thermal-spray coating corrodes at 253K in freezing sodium chloride solution. The characteristic of polarization change a mixed control at this temperature. The corrosion rate obtained by the measurement of electrochemical impedance spectroscopy didn't change with immersion time, corresponding with the corrosion rate obtained by the weight loss method. It was found that this technique is a convenient tool for monitoring of the corrosion processes of heat exchanger in near frozen sea-water.
The effect of rotation rate upon potential and etch rate of steel is examined utilizing a rotating steel disk electrode. Being based upon these experiments, etching mechanism is discussed. The principal ingredient of etchants is FeCl3. The concentrations of FeCl3, FeCl2 and free HCl are 49 to 54 mass%, 1.9±0.1 mass% and 0.09±0.02mol/kg (molality), respectively. The temperature of the etchants is maintained at 70°C and the rotation rate of the steel disk is set at 400-4900rpm. The etch rate is obtained from the weight loss. The etch rate and the square root of rotation rate are in proportionate relations at any concentrations of FeCl3. The mechanism that anodic and cathodic reactions are controlled by mass transfer process is proposed.
We propose a new hybrid method for evaluating the fracture strength of the hard films deposited on soft substrate metal. The method utilizes acoustic emission (AE), corrosion potential fluctuation (CPF) data and Finite Element Method (FEM) analysis. Both the AE and CPF were simultaneously detected during the Vickers indentation test into the hard film (TiN) in a buffer solution (pH: 4.0 phthalic acid) and used for the determination of both the number and distance of ripple cracks. AE system detected Lamb wave signals from the Mode-I ripple cracks due to the tensile stress induced by the bending of the soft substrate (SUS 304). Event counts of rapid drop (RD-) type CPF with a large shift rate and AE signals agreed well with the number of ripple cracks. Non-linear FEM analysis revealed the Mode-I crack generation in the film at the slight outside of the contact boundary of the indenter and can predict the fracture strength of the film.
Corrosion resistance of the newly developed stainless steel, type 310MoCu, was estimated under the various conditions for the coal fired power plant application with a single-loop desulfurization equipment, comparing with other stainless steels and Ni-base alloys. It was found that the corrosion resistance index, Isc (=-0.6(%Cr)+(%Ni)+1.5(%Mo)+7.5(%Cu)+122(%N)), of over 40 and addition of Cu were desirable against general corrosion under the sulfuric acid conditions. Corrosion resistance of clad plates of the new type steel was also investigated, taking processing factors into account. It was found that accelerated cooling after clad plate rolling, with cooling rate over 0.4K/s, greatly improved corrosion resistance, due to refinement of intermetallic phase along grain boundaries. Trial products of the new type stainless clad steels showed sufficient corrosion resistance including their welds.