Corrosion mechanism, salts

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Sulfate ions have reactions similar to those of chloride. They are corrosion-causative agents (similar to oxygen and hydrogen) of the various types of concentration cell corrosion. In addition, they also are depassivation agents and may greatly accelerate the risk of stress corrosion mechanisms. Saline corrosion pits resulting from high concentrations of chloride and sulfate salts also may be associated with low pH corrosion because hydrochloric acid and sulfuric acid can form within the pit, under deposits. 

Electrochemical techniques have been utilized for many years to study metal corrosion. Two of these techniques, linear polarization (LP) and cyclic voltammetry (CV), complement each other, LP providing corrosion rates under conditions where the surface is minimally altered and CV furnishing information about the corrosion mechanism. With the advent of impedance spectroscopy (IS), both kinds of information can be gleaned simultaneously and more rapidly, while leaving the surface almost intact. In this paper, we discuss the application of IS to the study of rapid steel corrosion and describe a study we undertook to elucidate the roles played by adsorption and film formation in the inhibition mechanisms of the above-named compounds. For comparison, we also investigated two quaternary nitrogen salts, which appear to adsorb electrostatically and presumably do not form macroscopic films (8). 

Civilian applications are numerous, but most funding of SCWO technology has stemmed from the military s need to find a safe and effective alternative to incineration of their wastes, as well as the need to clean up mixed wastes (radioactive and hazardous organic materials) at DOE weapons facilities. For better utilization of SCWO for its application to a wide range of waste types, a better fundamental understanding of reaction media, including reaction rates, reaction mechanisms, and phase behavior of multicomponent systems is required. Such an understanding would help optimize the process conditions to minimize reactor corrosion and salt... 

Basic research efforts with respect to supercritical water and SCWO have been directed within a number of areas that are critical for design and optimization of a SCWO reactor and ancillary equipment. These areas include physical property measurement and correlations, kinetics and reaction mechanisms, salt equilibrium and transport behavior, and corrosion. 

Corrosion mechanisms in molten sulfates consist of a sequence of chemical reactions and transport processes including oxide dissolution, transport of dissolved species through the salt film, and subsequent precipitation of oxide within the salt film in contact with the gas atmosphere. 

In aircraft turbines, especially nickel-and cobalt-based alloys suffer from salt-induced corrosion. On these alloys, type II corrosion mechanisms were observed, depending on the surface temperature of the turbine blades. Studies were performed on Co-Cr, Co-Al, Ni-Cr, Ni-Cr-Al, and Co-Cr-Al-Y alloys [38, 39]. It was shown that on Co-Cr and Co-Cr-Al pits were formed on the alloy, filled with corrosion products. On Co-Cr and Co-Cr-Al, the morphology... 

Regarding molten chlorides, mainly KCl-ZnCl2 mixtures are present in the ashes and fast corrosion occurs at relatively low temperatures. Figure 21 presents results of a thermogravimetric measurement on 2.25Cr-lMo-steel beneath a molten 50 wt. % KCl-50 wt. % ZnCl2 mixture at different temperatures in He-5 vol.% O2 gas mixture. At 350 °C, significant corrosion occurs by the molten salt. The main corrosion mechanism is the dissolution of... 

Filiform corrosion of AZ91 magnesium alloy involves a corrosion mechanism different from the conventional mechanism. In this case, dissolved oxygen is not necessary, and the filiform corrosion propagation is fueled by hydrogen evolution at the filament head and is controlled by mass transfer because of the salt film on the tip of the filament (20, 21). 

Polyester primers are widely used in Western Europe. Although their corrosion resistance (salt spray test) is not as good as that of the other primer types, their mechanical properties are superior. 

Corrosion by Salt Melts (Hot Corrosion) Data and reviews of the mechanisms of the hot corrosion behavior of SiC have been presented in a number of papers by Jacobson and coworkers [65-68]. 

Corrosion by Salt Melts Hot corrosion of Si3N4 has been extensively studied and reviewed by researchers at NASA labs [67,68,77,125,126]. The basic attack is that on the protective silica layer, which makes the behavior similar to that of SiC and basic mechanisms can be taken from the chapter on SiC above. 

The most widely used cabinet test is the neutral salt spray (Fog) test (ASTM B 117), which consists of a fog of 5 % sodium chloride within the chamber at 35 C [46. Controversy exists over the validity of B 117 as a performance test because corrosion mechanisms are not always the same as those observed in automobile service. Also, not all materials can be successfully evaluated in the test. However, the value of the salt spray test as a quality assurance test is well documented [46]. Several modifications to the salt spray test have been developed including acetic acid salt spray (ASTM G 85, Annex 1), copper accelerated acetic acid salt spray (ASTM B 368), acidified synthetic seawater fog (ASTM G 43, Method of Acidified Synthetic Seawater (Fog) Testing), and modified salt spray (ASTM G 85). ASTM G 85 also includes cyclic tests. 

Formation of lead soaps appears to be the mechanism by which lead-based paints inhibit corrosion of clean steel. When formulated with linseed oil, lead reacts with components of the oil to form soaps in the cured film in the presence of water and oxygen, these soaps degrade to, among other things, salts of a variety of mono- and di-basic aliphatic acids. The lead salts of azelaic, suberic, and pelargonic acid act as corrosion inhibitors lead azelate is of particular importance in LBP. These acids may inhibit corrosion by bringing about the formation of insoluble ferric salts, which reinforce the metal s oxide film until it becomes impermeable to ferrous ions, thus suppressing the corrosion mechanism. 

Appleman and Campbell [55] have examined each of the accelerating stresses in the salt spray test and its effect on the corrosion mechanism compared to outdoor or real-life exposure. They found the following flaws in the salt spray test ... 

Lyon, Thompson, and Johnson [56] point out that the high sodium chloride content of the salt spray test can resnlt in corrosion morphologies and behaviors that are not representative of natnral conditions. Harrison has pointed out that the test is inappropriate for use on zinc—galvanized snbstrates or primers with zinc phosphate pigments, for example — becanse, in the constant wetness of the salt spray test, zinc undergoes a corrosion mechanism that it wonld not nndergo in real service [57]. This is a well-known and well-docnmented phenomenon and is discnssed in depth in chapter 7. 

Chromium is the least thermodynamically stable element among the materials of construction. This and other corrosion mechanisms resulted in the development of Hastelloy-N as a material of construction, which offers very good corrosion resistance in liquid salt systems. 

On the other hand, on the positive side, the opportunity may be taken to include corrosion inhibitors, such as strontium chromate, into the polymeric adhesive, or primer, layer. Such inhibitors will slowly leach out and are especially selected to retard the rate of the electrochemical-corrosion mechanism. They typically achieve this by increasing the polarisation of the anodic sites by reaction with the ions of the corroding metallic substrate to produce (a) thin passive films, or (b) salt layers of limited solubility which coat the anodic sites [34]. 

---