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Corrosion in Hydrochloric Acid |
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> Home > Technology > Corrosion Performance > Hydrochloric Acid Graphs |
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Corrosion in Hydrochloric Acid (HCl)
Hydrochloric acid is a solution of hydrogen chloride (HCl) in water, that is a highly corrosive, strong mineral acid with many industrial uses. Hydrochloric acid is used in the chemical industry as a chemical reagent in the large-scale production of vinyl chloride for PVC plastic, and MDI/TDI for polyurethane.
At lower temperatures plastics or polymer lining like PTFE or PVDF lining may be a cost efficient way to avoid corroion. At temperatures above 150 °C / 300° diffusion through the polymer materials becomes significant and plastic lining is in general not suitable. At these tempertaures the mechincal properties of polymer lining may be poor, forcing engineers to consider more expensive metalbased options.
Due to the potential strong reactions bewteen chloride ions and risk of rapid corrosion failure of many matels the concentration and tempertures of hydrochloric acid will have to be evaluated closely during material selection. The iso-corrosion chart and the tables below show the relative corrosion resistance of several specialty alloys and exotic metals in various concentrations and temperatures of hydrochloric acid (hydrogen chloride). Such alloys will typically be applied when stainless steel shows insufficent corrosion resistance in hydrochloric acid. It can be seen that the corrosion attack of hydrochloric acid, as with most acids, is highly dependent on the temperature. Chloride containing acids will in many situations show a corrosive nature similar to hydrochloric acid at comparable acid and/or chloride concentrations.
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Hydrochloric acid iso corrosion chart |
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| Hydrochloric Acid Iso Corrosion Curves for Hastelloy (nickel alloy), Titanium, Zirconium, Noibium and Tantalum. |
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Compared with other specialty metals and alloys like Hastelloy, niobium and zirconium, the corrosion resistance of tantalum metal is second to none in hydrochloric acid. Tantalum metal is an element (atomic number 73) and is considered to be the most corrosion resistant metals commercially available. At temperature less than 150C and a concentration less than 30%, tantalum is considered to have a nil corrosion rate or less than 1 mil/year in chlroride acid service.
For precision parts such as valves, fittings and instrumentation where tolerances are critical to their operation, a corrosion rate 5 mil/year or larger would typically be considered unacceptable in hydrochoric acid applications.
The table below shows the relative corrosion resistance in hydrochloric acid at concentrations in the range 5 to 35% and temperatures up to 200C. As can be seen, Tantaline has superior performance over niobium, zirconium and nickel based alloys at a lower relative price.
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