Corrosion sulfide layers

Many metal sulfides produce poorly adherent corrosion product layers. This leads to rapid spalling during thermal cycling or turbulent flow. In particular, nonadherent and easily spalled sulfides form on steel and cast irons. 

The corrosion behavior and dissolution mechanism of nickel in acid solutions with hydrogen sulfide (H2S) was studied. It was found that the dissolution of nickel is influenced by both the nickel sulfide layer formed on the electrode surface and the acceleration effect of H2S [56]. 

For determing the concentration of hydrogen sulfide in the product gas, a Gow-Mac thermal conductivity cell (Model 10-952) was used. The cell was equipped with four matched pairs of AuW filaments, especially used because of their resistance to corrosion from the hydrogen sulfide. Layers of styrofoam were used to insulate the cell from changes in ambient temperature. This detector was found to be very sensitive to changes in the flow-rate. 

It has been shown from corrosion studies (97, 98) that bulk sulfide formation involves metal cation diffusion through the sulfide layer to the surface with the formation of a new metal sulfide layer on the outer surface beyond the original metal. Apparently the formation of multilayer sulfides occurs slowly at room temperature and at Ph2s — 1 atm, cation diffusion through the sulfide layer controlling the rate (9, 96). Presumably this step would also be rate limiting at higher temperatures and at H2S concentrations as low as 10-100 ppm. However, in most catalytic processes H2S concentrations are below those needed for bulk sulfide formation. 

Dowling [53] proposed a mechanism for corrosion of structural steel exposed to wet soKd elemental sulfur (Fig. 8). Freestanding moisture and steel/sulfur contact are requisites for corrosion of structural steel by solid elemental sulfur. The principal form of attack in S/H2O media is not due to secondary acid generation resulting from hydrolysis of the sulfur. Instead, the author demonstrated that steel oxidation was coupled to sulfur reduction through an electron conductive iron sulfide layer. Evidence is also presented for the direct electrochemical reduction of solid elemental sulfur in the presence of FeS, supporting the role of this process as the partial cathodic step in the mechanism of the sulfur corrosion reaction. 

The reaction with CO2 leads to oxy-carbonates. The reaction with sulfur can lead to sulfide layers, e.g., on silver ware. A very prominent example of secondary corrosion products is also the green patina on copper sheets. Details can be found in the special literature on this subject. 

Since essentially all corrosion product layers have some ion exchange properties, and since these result in an osmotic pressure gradient across the interphase, one can expect a structural relaxation following a potential perturbation. It has been shown that iron sulfide scale, for instance, has a different permeability for corrosion reactions depending on solution pH [39]. This structural relaxation due to surface pH changes is slow (much slower than interfacial capacitance charging) and makes it practically impossible to measure a representative polarization curve. 

Corrosion problems and required control methods appear to be quite similar in the Flex-sorb HP and other amine activated hot potassium carbonate processes. During pilot plant tests the corrosion rate was monitored by the use of conoaon coupons and Conosometer probes. In many of the tests a small amount of H2S, ranging from 0.1 to 0.5 mole %, was present in the feed gas. This amount proved adequate to form a protective sulfide layer that inhibited corrosion of carbon steel equipment In tests without H2S or inhibitor, severe corrosion was observed (Say et al. 1984). 

Results of the study of corrosion control by inhibitors in producing oil wells in carbon dioxide flooded fields showed imidazolines are successful in protection in CO2 brines. The inhibitor was found to be incorporated in the carbonate corrosion product layer but was still more effective if the surface film contained sulfide. Also, better results were obtained with inhibitors, such as nitrogen-phosphorus compoimds or compounds with sulfur in the organic molecules. 

Aluminum itself cannot be used as a current collector or cell case material due to the formation of a high-resistivity aluminum sulfide layer. Therefore a surface protection has to be applied which necessitates a corrosion-resistant but electrically conductive coating. Good results have been obtained with aluminum coated with a layer of nichrome and covered by a secondary protective coating of carbon, which formed a tightly adherent, corrosion-resistant, outer layer. Post-mortem examination after four years of operation showed that such a current collector had been largely unaffected by the very aggressive environment. 

A low O2 condition is produced at a corrosion interface in the presence of protective scales, and complex corrosion reactions such as chlorination, sulfidation and oxidation occur below the corrosive deposit layer. Thick scales have pores and cracks due to temperature fluctuations and the vaporization of chlorides. As the thickness increases, the scales easily peel off from the surface. In particular, severe thermal cycles or increased gas velocities due to soot blowing accelerate the breakdown and spalling of the scale. Also, as a result of continuously repeated variations of gas conditions on the scales, the balance of chlorination, sulfidation and oxidation reactions at the corrosion interface and in the scales is forced to be changed by the penetration of O2. An increase of the partial pressure of O2 ( /qj ) temporarily halts the chlorination and sulfidation reactions. Therefore, a multi-layered scale stracture is produced. The presence of multi-layered oxides formed by corrosion resistant elements such as chromium, nickel, aluminum, silicon and molybdenum increases the protective effect of the scales against the... 

For the corrosion process to proceed, the corrosion cell must contain an anode, a cathode, an electrolyte and an electronic conductor. When a properly prepared and conditioned mud is used, it causes preferential oil wetting on the metal. As the metal is completely enveloped and wet by an oil environment that is electrically nonconductive, corrosion does not occur. This is because the electric circuit of the corrosion cell is interrupted by the absence of an electrolyte. Excess calcium hydroxide [Ca(OH)j] is added as it reacts with hydrogen sulfide and carbon dioxide if they are present. The protective layer of oil film on the metal is not readily removed by the oil-wet solids as the fluid circulates through the hole. 

The Alclad alloys have been developed to overcome this shortcoming. Alclad consists of a pure aluminum layer metallurgically bonded to a core alloy. The corrosion resistance of aluminum and its alloys tends to be very sensitive to trace contamination. Very small amounts of metallic mercury, heavy-metal ions, or chloride ions can frequently cause rapid failure under conditions which otherwise would be fully acceptable. When alloy steels do not give adequate corrosion protection—particularly from sulfidic attack—steel with an aluminized surface coating can be used. 

The corrosion of metal surfaces and the precipitation of a metal sulfide by an aqueous acid solution can be prevented by an aldol-amine adduct. Aldol (from acetaldehyde) CH3CH(OH)CH2CHO has been utilized as a H2S scavenger that prevents the precipitation of metal sulfides from aqueous acid solutions. However, when the aldol or an aqueous solution of the aldol is stored, the solution separates quickly into two layers, with all of the aldol concentrated in the bottom layer. The bottom layer is not redispersible in the top layer or in water or acid. In addition, the aldol in the bottom layer has very little activity as a sulfide scavenger. Thus the use of aldol as a H2S scavenger in aqueous acid solutions can result in unsatisfactory results [245,247]. However, the aldol can be reacted with an amine, such as monoethanoleamine (=aminoethanol), to form an aldol-amine adduct to overcome these difficulties. The amine utilized to prepare the aldol-amine adduct must be a primary amine. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the... 

Almost all new metallic surfaces exposed to the environment are sooner or later coated with a layer of corrosion products metal oxides, sulfides, and carbonates, for example, are common corrosion products formed when a metal or alloy interacts with contaminants in the environment. If the layer is continuous and stable, as in uniform corrosion, it may conceal the underlying metal from further exposure and protect it from additional corrosion if it is discontinuous, or chemically unstable, however, the metal surface below the initial layer of corrosion products remains in contact with the environment. Exposed to humidity and pollutants, the corrosion process continues, penetrating deeper into the metallic bulk and eventually resulting in its total destruction. 

Copper and the Copper Alloys. Copper and its alloys are relatively resistant to corrosion dry, unpolluted air rarely affects them at normal temperatures surfaces of the metal or its alloys exposed to polluted air, even under ordinary atmospheric conditions, however, are tarnished by pollutants such as hydrogen sulfide and/or carbon dioxide. Given sufficient time, the activity of the pollutants result in the formation of a usually green layer, known as patina, which coats and surrounds the bulk of the metal or alloy (see Fig. 40). If the patina is chemically stable, that is, if it is hard, is non-porous, and covers the entire surface of an object, it protects the underlying metal core from further corrosion. Such a patina consists mostly of basic... 

Hydrogen Sulfide. Hydrogen sulfide is a foul-smelling gas that is released into the atmosphere from volcanoes as well as in the course of decay of animal tissues. As an air pollutant, it reacts with almost all metals, with the exception of gold, forming a dark-colored corrosive layer of metal sulfide, commonly known as tarnish, which discolors the exposed surface of most metals. 

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