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Coating Protection
Published in Karan Sotoodeh, Coating Application for Piping, Valves and Actuators in Offshore Oil and Gas Industry, 2023
Lead and chromate are no longer used to enhance coating inhibition since these two elements have been found to be very toxic. The unfavorable health effects of these two compounds are not limited to toxicity. It has been shown that long exposure to coating that contains lead can cause motor neuron disease. Chromate can cause lung and throat cancer. Thus, modern pigments are lead- and chromate-free and can be either organic or inorganic. Inorganic pigments are phosphates, molybdates, silicates and ferrites. Organic pigments are based on the carbon chain and carbon rings. The third and final protection coating provides cathodic protection, in which the coating sacrifices itself to protect the steel. Figure 3.3 illustrates the sacrificial or cathodic concept of coating protection, which is associated with the presence of zinc.
Practical Applications: Metals Contamination
Published in William J. Deutsch, Groundwater Geochemistry, 2020
At trace concentrations chromium appears to be an important nutrient in the human diet; however, as with other trace elements, it is also toxic at above average concentrations. In cases of very high exposure Cr can be toxic and carcinogenic.8 Cr(VI) is usually identified as the toxic form of chromium because it occurs as the fairly mobile anion Chromate (CrO42−), and its minerals are soluble in the natural environment. Relatively high concentrations of Cr(VI) are possible in the environment compared with Cr(HI), which forms an insoluble hydroxide mineral, Cr(OH)3. Many industries use the Chromate and dichromate (Cr2O72−) forms of chromium for such applications as corrosion inhibition because of the strong oxidizing ability of these species. It has been reported that the dust emitted from ferrochrome smelters contains on the order of 40% chromium as Cr(VI), whereas chromium in sewage sludge is predominantly in the Cr(III) valence state because of the reducing environment.8
Industry-viable metal anticorrosion application of polyaniline
Published in Ze Zhang, Mahmoud Rouabhia, Simon E. Moulton, Conductive Polymers, 2018
Fortunately, unlike sudden natural disasters such as earthquakes or severe weather, metal corrosion is a time-dependent damage that can be controlled and even prevented by various state-of-the-art techniques. One of such techniques is employing an organic coating containing corrosion-inhibiting pigments, by which the organic matrix offers the physical barrier, while corrosion like pinholes and small defects may be prevented by the inhibitory pigments. Chromate (Cr6+) exhibits excellent protection by forming a passive layer (Zhao et al. 2001; Pommiers et al. 2014). However, because chromate is toxic and carcinogenic, its use has been strictly limited by the Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (ROHS) Directives since 2005. Another pigment is zinc powder, which is used as a sacrificial material to prevent metal corrosion. But as a heavy metal, zinc can spoil soil or marine environment. Therefore, novel environmentally friendly anticorrosive pigments are critically needed to replace chromate and zinc.
Restricted substances for textiles
Published in Textile Progress, 2022
Arun Kumar Patra, Siva Rama Kumar Pariti
Chromium is abundantly-available in the earth’s crust and is widely used in leather tanning, electroplating, dyeing, cement formulations, mining, fertilizers and in the photographic industries. Because of widespread industrial use, chromium compounds may be discharged in various different forms into the environment leading to adverse ecological and biological effects. Chromium exists in a series of oxidation states from 2 to 6, the predominant stable states being zero (elemental metal), trivalent Cr (3) and hexavalent Cr (6). The bivalent form in most compounds is unstable as it tends to oxidise to the trivalent form in air. The trivalent form of chromium is the most stable among all oxidation states followed by the hexavalent one (Kaur et al., 2019). In fact, trivalent chromium is believed to be an important nutrient in human beings and has no negative effects, whilst metallic chromium also does not exhibit significant toxicity. However, the hexavalent chromium is very toxic and carcinogenic. This behaviour of Cr (VI) is due to its strong oxidant properties. On reaching blood stream, it damages kidney, liver and blood cells through oxidation reactions. The carcinogenity of chromate dust is well known and the first discovery of the fact came through a publication showing elevated cancer risk for workers in a chromate dye company (Langard, 1990). Chromium based dyes are useful in dyeing wool, silk and nylon. Chromium containing compounds are used in other textile wet processes as well. Hence the heavy metal is a matter of concern from textile point of view.
Remediation of lime-free roasting chromite ore processing residue (COPR) by water leaching and pyrolysis process
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2018
Cunfang Lu, Wenchang Xi, Xuejun Quan, Zhien Zhang
Chromium salt is one of the most critical materials widely used in pigments, ceramics, electronics, anticorrosion, metallurgy, etc. (Kleynhans et al. 2016; Li, Zhang, and Liu 2017; Matern et al. 2017; Zhang et al. 2018; Zhao et al. 2017). Chromite ore processing residue (COPR) is a byproduct of the chromium salt manufacturing process. In the process, chromite ore is roasted at high temperatures with fusion agent, oxidizing Cr(III) to Cr(VI), and then formed the sodium chromate chemically (Wu, Li, and Yang 2015). The manufacturing process can be classified into high calcium roasting, low lime roasting, and lime-free roasting process according to the used fusion agent (Jagupilla et al. 2015). The lime-based roasting process has been eliminated gradually because of some shortcomings, especially the carcinogenic CaCrO4 formation (Wang and Li 2004), the large amount of COPR production. The chromium contents in COPR can be as high as 46000 mg/kg, about 30% of which in the form of Cr(VI) (Liao et al. 2017). Thus, the chromium salt industry has become a major concern in recent decades from health and environment perspectives.
Electrochemical regeneration of hexavalent chromium from aqueous solutions in a gas sparged parallel plate reactor
Published in Environmental Technology, 2022
N. K. Amin, S. A. Nosier, M. H. Abdel-Aziz, M. S. Hassan, G. H. Sedahmed, M. A. El-Naggar
In the metal finishing industry [4,5] hexavalent chromium is used in chromium plating, chromate conversion coating of metals to guarantee subsequent paint adhesion to the metallic substrate and to give extra protection to the metal against corrosion. Hexavalent chromium is also used in printed circuit copper etching [6], plastic etching [7] before the metallization of the plastic surface, production of alloys, leather tanning, pigment making, electroplating, wood preserving, and refractory [8–10]. In all the above applications the waste solution contains Cr3+ in a high concentration and some residual Cr6+, both ions are lethal especially Cr6+ which is carcinogenic.