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The Case of the Acid Test
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, February 17, 2003
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The acid manufacturer looked like a sick puppy when he realized hsi mistake in selecting the wrong shipping drum. |
Acids are used in modern manufacturing processes for cleaning, etching, and a myriad of other purposes. These acids are highly corrosive and frequently produce toxic fumes.
In the present case, highly concentrated 90% fuming nitric acid was being shipped by truck in two 55-gallon stainless steel drums. On arrival at their final destination, there was an odor and a yellow cloud in the trailer. One drum was found to have a small leak near the bottom and was laid upon its side. The movement caused the bottom of the drum to separate nearly completely, as if it had been cut out by a giant can opener. The concentrated acid poured out, causing a major fire. Major litigation soon ensued.
I was called in to determine what had caused the drum to behave in such an anomalous and destructive manner.
The drum bottom had been attached to the body by a rolled joint, which had been resistance-welded to provide a liquid seal. The fracture was along a line next to the weld. I took sections of the barrel from the region containing the fracture and had the samples mounted in plastic and polished for microscopic examination. The accompanying figure shows a sample magnified to about 400 diameters (polished section normal to fracture surface). The light region is stainless steel, the dark region is plastic mount. The polyhedra are the polished surfaces of the metal grains, and black lines are disordered boundaries between the crystalline grains. The cause of failure is obvious: Intergranular corrosive attack.
Some corrosion was inevitable, but was meant to occur by a gradual, general attack. Instead, the acid attack is seen to have proceeded catastrophically along the grain boundaries. The intergranular attack was due to a highly corrosive environment and a sensitized stainless steel.
Chromium in concentrations above 12% renders steel stainless. The drums were type 304 stainless steel, which contains 19% Cr and 9% Ni. (The nickel strengthens the alloy and gives greater corrosion resistance.) Unfortunately the steel contains nearly 0.1% residual carbon from the iron and steel-making processes. Chromium has a strong affinity for carbon, and slow cooling through the red heat range allows chromium carbides to nucleate heterogeneously on the grain boundaries. The adjacent regions in the grains are depleted of chromium to far below the 12% threshold and are no longer as corrosion resistant. Thus, the steel is said to be "sensitized" and is susceptible to intergranular corrosive attack. The slow cool after the welding of the lid allowed such precipitation and ensuing sensitization.
Even when sensitized, the steel is adequate for many applications, such as household products and even containers for less concentrated nitric acid. However, the fine print in the DOT regulations states that while sensitized steel is adequate for 75% nitric acid, it could not be used for the 90% solution. The manufacturer/shipper of the acid looked like a sick puppy when he showed me this fine print.
The corrosive attack could have been prevented in several ways. Low carbon stainless steel, designated type 304L could have been used. Or, addition of niobium during steel making would have tied the carbon up as fine, harmless intragranular niobium carbides. Alternatively, given a large enough furnace, the welded drums could have been annealed at a bright-red heat to dissolve the carbides and water quenched to prevent their re-nucleation. Any of these techniques would have been effective, but of course each would have added to the cost of the drum.
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