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Background |
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As part of a remediation
site project, activated carbon, resin bed adsorption units are used to
remove chlorinated hydrocarbons and carbon disulfide from soil. |
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The resin bed is inserted,
and gasket-sealed inside, in an open top, carbon steel chamber. The resin
is retained with a 3-foot x 4-foot (0.9 m x 12 m) aluminum frame about
6-inches (15-cm) deep. The frame is bolted with stainless steel nuts and
bolts to a type 304 stainless steel (UNS S30400) top plate that provides a
platform for externally mounted inlet and outlet nozzles, valves, solenoids,
and controllers. |
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The open faces of the frame
are covered with a layer of fine mesh (80 mesh) type 304 SS wire screen
which is, in turn, covered with a 1/16-in. (1.6-mm) thick, perforated type
304 SS plate. For added stiffness, aluminum bars are bolted with SS nuts
and bolts across the perforated plate. |
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The bed assembly is covered
with aluminum shrouds that form chambers for solvent laden air to uniformly
access the face of the bed. Electric strip heaters are attached to the
outside of the aluminum shrouds to prevent condensation on their inner
surfaces. |
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Before early 1996, the
recovery design used recycled nitrogen atmosphere in an attempt to reduce
the presence of moisture. Due to cost considerations and other reasons, in
late 1996 the N2 atmosphere was replaced with outside air
resulting in an additional nominal moisture content to that already in the
solvent. |
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Problem |
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The aluminum shrouds had corroded through and were ineffective |
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The stainless steel components had significantly corroded.
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Resin escaped from the bed – indicating the wire screens, which retained the carbon-based resin, were breached. |
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What happened |
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The additional moisture in
the air atmosphere increased the acidity of the solvent-laden carbon bed.
During regeneration, warm dilute (~8%) hydrochloric acid was formed. |
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Type 304 SS and aluminum
are not resistant to corrosion by aqueous, acidic, chloride-containing
compounds – at near ambient temperatures. Aluminum can corrode in dilute
hydrochloric acid (HCl) at a rate greater than 1/16-inch per year (1.6
mm/y). Type 300 series SS will pit and stress corrosion crack in chloride
solutions. |
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Carbon tetrachloride, one
of the chlorinated hydrocarbons to be removed, and carbon disulfide, once
adsorbed in the carbon resin bed, decompose, react with residual moisture,
and acidify during the desorption portion of the bed regeneration cycle.
This acidified vapor corrodes the aluminum and SS surfaces with which it
comes in contact with. |
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The carbon-based resin bed
is in contact with the aluminum frame, and stainless steel screen and
support plates, forming a tri-galvanic couple. Aluminum is the least noble,
and carbon is the noblest of the three materials. Under nitrogen
atmosphere there in insufficient moisture to create a continuous electrolyte
between these three materials, however under redesign, using outside air,
sufficient moisture is present to “wet” the components. This creates the
fourth component of the galvanic corrosion cell, the common electrolyte. |
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Gene Liening, of the Dow
Chemical Company, reported on a similar case history in ASTM STP 908 (1986),
and the special problems associated with carbon bed absorber vessels due to
organics, temperature and galvanic effects. At Corrosion/91 (Paper Number
165), he addressed the use of alternative materials for carbon bed
absorbers, but stressed that each application be tested and verified. |
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Laboratory Studies |
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Due to the organics present
in this case study, only metallic components were deemed useable.
Therefore, following the guidelines of Liening, electrochemical tests were
performed to evaluate several candidate materials of construction to dilute
HCl (7.67 g/L) when galvanically coupled to activated carbon. Initial tests
were performed at 32oC (90oF) under process flow
conditions and at 90oC (194oF) under regeneration
conditions. Materials evaluated were type 304 as the control; alloys C-276
(UNS N10276), B-3 (UNS N10675), and 825 (UNS N08825); titanium, unalloyed
grade 2 (UNS R50400), and zirconium 702 (UNS R60702). |
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Laboratory Findings |
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Galvanically coupled to
activated carbon, |
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Type 304SS exhibits an unacceptably but predictably high
corrosion rate of over 25 mpy (0.64 mm/y), |
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Alloys B-3 and 825 and zirconium have high corrosion rates and
cannot be recommended for this service, while |
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Alloys C-276 and unalloyed titanium have acceptable rates and
can be considered suitable replacement materials for SS in this system. |
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Lessons Learned |
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This case continues to
illustrate the need to recognize the effect of mixed materials in contact
with each other in hostile environments. It should be obvious that mixed
metals must be electrically isolated from each other, but also to recognize
that carbon, as graphite, is capable of conducting electricity and can
therefore enter into the electrochemical process of corrosion. If graphite,
being on the noble end of the galvanic series, contact with other metals is
unavoidable, such as in the carbon bed absorber unit, then these metals must
be similarly noble. Only through testing of compatibility will the effects
of galvanic corrosion be known. |
