Dusting, metal

Another ultimate effect of carburization can be metal dusting. This phenomenon may occur in process ojjerations in which oxidizing and reducing conditions are cycled. When the environment is on the reducing side (CO predominant), carburization of the metal to a shallow depth can occur at breaks in the protective oxide film. When the exposure then changes to oxidizing, the high-carbon area of the metal is burned out and the metal reacted to the oxide. A depression is left in the metal surface where the carburized area existed (Fig. 15.28), and the metal oxide is swept downstream in the process as metal dust. 

Metal dusting is usually associated with gas streams rich in carbon monoxide and hydrogen. Prediction and modeling of metal dusting are difficult, and little relevant quantitative data is available for engineering alloys to assist designers. It appears that most stainless 

6 Effect of sulfur on carburisation mass gain of Alloy 800 in CH4/H2/H2S atmospheres at 900, 1000 and 1100 °C plotted versus ratio H2S/H2, and decrease of mass gain by carburisation with increasing H2S/H2 ratio up to an optimum value, above which sulfidation occurs (35). 

As noted before, metal dusting is to be expected if metallic materials are carburised at carbon activities ac 1, i.e. under a strong driving force for graphite formation. The carbon from the gas molecules should react to graphite (and, in fact, that is the overall reaction which occurs in metal dusting) and destroy the materials. As yet, two different reaction paths have been observed. For iron and Fe-based alloys, the reaction sequence is as follows (see Fig. 1.7)  

7 Mechanism of metal dusting on iron and low-alloy steels [9-15]. 

Adsorbed sulfur effectively hinders step (iii), the nucleation of graphite, and in this way interferes with the mechanism. The adsorbed sulfur stabilizes the cementite and one can grow thick layers of cementite on iron and steels [14], The continuous presence of only a few ppm H2S in the atmosphere is an effective remedy against metal dusting of steels [14] (see Fig. 1.8). The data on the effect of sulfur on metal dusting can be readily combined with the data on its effect on carburisation [35-37]. 

Metal dusting occurs in strongly carburising atmospheres at carbon activities ac 1, which condition certainly was present in the furnace feed consisting of hydrocarbons and a few percent of hydrogen. The sulfur content of the 

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The ACGIH adopted TLV/TWA for 1992—1993 for fluorides as F is TWA 2.5 mg/m, and for cobalt as Co metal dust TWA 0.05 mg/m. Dust masks should be used while handling both the cobalt fluorides and all other cobalt compounds. CoF is shipped as an oxidizer and a corrosive material. 

Exposure to tantalum metal dust may cause eye injury and mucous-membrane irritation. The threshold limit value (TLV) in air is 5 mg/m, LD q is <400 mg/kg and the Occupational Safety and Health Administration (OSHA) time weighted average (TWA) exposure limit is 5 mg/m (47). The immediate dangerous to life or health (IDLH) concentration is 2500 mg/m (48). Whereas some skin injuries from tantalum have been reported, systemic industrial poisoning is apparently unknown (47). 

The adopted values for TWAs for airborne vanadium, including oxide and metal dusts of vanadium, is 0.5 mg/m the values for fumes of vanadium compounds is 0.05 mg/m. These limits are for normal 8-h workday and 40-h work-week exposures. The short-term exposure limit (STEL) is 1.5 mg/m for dusts (25). A description of health ha2ards, including symptoms, first aid, and organ involvement, personal protection, and respirator use has beenpubhshed (26). 

Products which decompose spontaneously, metal dust hazard... 

These facts would suggest that die electrolysis of molten alkali metal salts could lead to the inuoduction of mobile elecU ons which can diffuse rapidly through a melt, and any chemical reduction process resulting from a high chemical potential of the alkali metal could occur in the body of the melt, rather than being conhned to the cathode volume. This probably explains the failure of attempts to produce tire refractoty elements, such as titanium, by elecU olysis of a molten sodium chloride-titanium chloride melt, in which a metal dust is formed in the bulk of the elecU olyte. 

A number of attempts to produce tire refractory metals, such as titanium and zirconium, by molten chloride electrolysis have not met widr success with two exceptions. The electrolysis of caesium salts such as Cs2ZrCl6 and CsTaCle, and of the fluorides Na2ZrF6 and NaTaFg have produced satisfactoty products on the laboratory scale (Flengas and Pint, 1969) but other systems have produced merely metallic dusts aird dendritic deposits. These observations suggest tlrat, as in tire case of metal deposition from aqueous electrolytes, e.g. Ag from Ag(CN)/ instead of from AgNOj, tire formation of stable metal complexes in tire liquid electrolyte is the key to success. 

Metal dusting is a form of metal deterioration that occurs in carbonaceous gas streams containing carbon monoxide and/or hydrocarbons at elevated temperatures. 

The term, metal dusting, was first used about this time to describe the phenomenon associated with hydrocarbon processing. Butane dehydrogenation plant personnel noted how iron oxide and coke radiated outward through catalyst particles from a metal contaminant which acted as a nucleating point. The metal had deteriorated and appeared to have turned to dust. The phenomenon has been called catastrophic carburization and metal deterioration in a high temperature carbonaceous environment, but the term most commonly used today is metal dusting. 

Figure 2. Section of tube at right shows uniform thinning by metal dusting.

Figure 3. Metal dusting as a combination of pitting and thinning.

Among the measures which have successfully prevented metal dusting are the use of additives (steam, and compounds of S, As, Sb, and P) in the feed, reduction of pressure, reduction of temperature, and material change. The most common additives are sulfur compounds and steam. Susceptibility can be reduced by using a material in which the total percent of Cr plus two times the percent of Si is in excess of 22 percent. In some environments, a. small amount of a sulfur compound will stop the dusting. When sulfur compounds cannot be tolerated in the process stream, a combination of steam and an alloy with a Cr equivalent of over 22 percent may be most desirable. 

Because of the inexactly defined conditions causing metal dusting, mitigation methods will not be the same for each occurrence. Each problem must be carefully studied to determine the most effective and economic measures that will be compatible with the process stream. 

Koszman, I., Antifoulant Additive for Steam-Cracking Process, U.S. Patent 3,531,394, Sept. 29, 1970. Hochman, R. F, Fundamentals of the Metal Dusting Reaction, Proceedings, Fourth International Congress on Metallic Corrosion, NACF (1971). 

This approach has not been tested for any dusts that burn heterogeneously (A-6-1.2), such as some metal dusts. The equation should not be applied for gas concentrations greater than the LEE [11] otherwise extrapolation might be made into region Q shown on Eigure 6-1.3.1, where the predicted HMIE is greater than the gas MIE. The MIE of dust, D, must be determined by test using a conservatively fine dust sample to represent particles in the hybrid mixture. Values for G and Cg can be found in Appendix B. Where G is not... 

This approach has not been tested for any dusts that bum heterogeneously such as some metal dusts (A-6-1.2). Also, this large an effect of temperature has not been found by some other workers, it has yet to be confirmed that the effect is not due in part to loss of moisture (6-1.6) or evolution of flammable gas (6-1.3), both of which can have large effects on MIE. 

Coal tar pitch volatiles, see Particulate polycyclic aromatic hydrocarbons (PPAH), as benzene solubles Cobalt metal, dust and fume (as Co)... 

Other methods for removing scale include salt pickling, electrolytic pickling, and blasting blasting is environmentally desirable, where feasible. EAFs produce metal dusts, slag, and gaseous emissions. 

Combustible dusts include metal dust (e.g., aluminum, magnesium, and their commercial alloys), carbonaceous dust (e.g., carbon black, charcoal, and coal), flour, grain, wood, plastics, and chemicals. 

N. Maddison, Explosion Hazards in Large Scale Purification by Metal Dust, Hazards XII—European Advances in Process Safety, Symposium Series No. 134, Institution of Chemical Engineers, Rugby, UK, 1994. 

Group E Gonductive dust and metal dust aluminum, magnesium, and their commercial alloys... 

Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment. 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

Fig. 7.52 Optical micrograph of a 5< o Cr steel after service in a petrochemical plant showing typical metal dusting behaviour (after Hochmann ")...

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