How can I eliminate the harmful gas produced from the heating/manipulation of galvanized steel? [closed]








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When welding on steel with a galvanized coating, hexavalent chromium is yielded from the chemical reaction. This gas is something that I would like to eliminate before I begin welding, possibly with some liquid or mixture. Any ideas?




Galvanizing by definition is zinc coating. The zinc boils off ( as 1700 F) is approached and immediately oxidizes to ZnO , a white powder. ZnO has the unusual characteristic of causing zinc chills = brass founders ague = oxide shakes, IF inhaled while less than one hour old . After that is is pretty harmless and used in various lotions and creams and rubber products. So avoid the fumes when welding. However , one recovers from "chills" in about 24 hours so not toxic ( I can verify no lasting affects as I had "chills" many decades ago). There is another potential problem ; zinc can cause liquid metal embrittlement of steel . This occurs where the zinc is not exposed to air as if two surfaces are placed together and the edges sealed by welding. I don't know where you think chromium is coming from other than steel made with scrap ( the majority) typically contains about 0.04 % tramp chrome ( and Ni ,and Cu , and Mo). So essentially all steel welders in the world are exposed to this tramp chrome.


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My speculation on the possible mechanics of redox reactions that could move Cr into higher oxidation states upon welding with galvanized-steel. Understanding the chemistry may be instrumental in providing guidance.


To begin, welding creates, in addition to heat, CO2 and water vapor (from the flame), which could act on the Zn coating creating hydrogen (and Zn(OH)2/ZnO). Some of the hydrogen may be ingressed on the metal surface forming atomic hydrogen (preferably cited as the hydrogen radical, $\ce{.H}$). Possible surface chemistry interaction of the latter directly with oxygen forming the hydroperoxyl radical :


$\ce{.H + O2 -> .HO2}$


Also, if there is a galvanic cell resulting in any solvated electrons, interaction with oxygen can produce the superoxide radical anion:


$\ce{O2 + e- -> .O2-}$


which exists in atmospheric conditions as the acidic hydroperoxyl (or called the perhydroxyl radical $\ce{.HO2}$). The latter could react with a transition metal (including Fe and Cr) as follows:


$\ce{M(n) + .HO2 -> M(n + 1) + HO2-}$


$\ce{H+ + HO2- -> H2O2}$


where any formed hydrogen peroxide could feed a classic Fenton reaction also moving $\ce{M(n) to M( n + 1)}$. Per this 2011 source, ["Advanced Oxidation Process Based on the Cr(III)/Cr(VI) Redox Cycle"](http://pubs.acs.org/doi/abs/10.1021/es2021704), by Alok D. Bokare and Wonyong Choi. To quote from the abstract:


"Oxidative degradation of aqueous organic pollutants, using 4-chlorophenol (4-CP) as a main model substrate, was achieved with the concurrent H2O2-mediated transformation of Cr(III) to Cr(VI). The Fenton-like oxidation of 4-CP is initiated by the reaction between the aquo-complex of Cr(III) and H2O2, which generates HO• along with the stepwise oxidation of Cr(III) to Cr(VI). The Cr(III)/H2O2 system is inactive in acidic condition, but exhibits maximum oxidative capacity at neutral and near-alkaline pH. Since we previously reported that Cr(VI) can also activate H2O2 to efficiently generate HO•, the dual role of H2O2 as an oxidant of Cr(III) and a reductant of Cr(VI) can be utilized to establish a redox cycle of Cr(III)–Cr(VI)–Cr(III). As a result, HO• can be generated using both Cr(III)/H2O2 and Cr(VI)/H2O2 reactions, either concurrently or sequentially. The formation of HO• was confirmed by monitoring the production of p-hydroxybenzoic acid from [benzoic acid + HO•] as a probe reaction and by quenching the degradation of 4-CP in the presence of methanol as a HO• scavenger. The oxidation rate of 4-CP in the Cr(III)/H2O2 solution was highly influenced by pH, which is ascribed to the hydrolysis of CrIII(H2O)n into CrIII(H2O)n-m(OH)m and the subsequent condensation to oligomers. The present study proposes that the Cr(III)/H2O2 combined with Cr(VI)/H2O2 process is a viable advanced oxidation process that operates over a wide pH range using the reusable redox cycle of Cr(III) and Cr(VI)."


Possible Implied Solution: As the Cr(III)/H2O2 system is inactive in acidic condition, try employing a sulfur-rich fuel gas mix, forming SO2 on combustion, to maintain an acidic pH.


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