presentation 2b whittaker

Biologically Induced Corrosion of Zirconium 702 Heat Exchanger Tubes

Gary Whittaker, PE

Eastman operates a proprietary process utilizing a glass lined tank to process an organic stream containing a small amount of mineral acid. The condenser on top of the GLT has been plagued by corrosion problems since it was added to the system many years ago.

GLT – Condenser Layout

filtered water out

filtered water in

GLT

heat in

Vapor out

process in

condenser

sludge out

vapor out

History Process was designed without a condenser First condenser was 316L, failed quickly Second condenser was alloy C-276, failed in two years Current condenser is Zirconium 702, fails in 18 months

Alloy C-276 Tubesheet

Zr 702 Tubesheet

Zr 702 Tubesheet

Eddy Current Results

Window in shell pre-cleaning

Window in shell post-cleaning

Proposed Corrosion Mechanism 1. Low flow rates through the condenser shell allow biological infestation to occur. 2. Iron oxidizing bacteria present in the bio-mass oxidize ferrous ion (Fe+2) in the water to ferric ion (Fe+3), a strong oxidizer. 3. Chloride Ion (Cl-) is attracted to the bio-mass to maintain charge balance. 4. Chloride ion combined with a strong oxidizing agent is known to pit zirconium.

River Water Chemistry Iron – 0.03 mg/L Chloride – 9.5 mg/L

Bio Analysis Samples of the bio-mass were collected by the plant micro-biologist Analysis showed the presence of filamentous bacterium called sphaerotilus, a well-known ferrous oxidizer Identity was confirmed by both a staining method and genetic analysis

Sphaerotilus collected from the condenser.

Sphaerotilus from literature

Laboratory Simulation Bio-mass samples were collected and placed in a jar with fresh river water After several weeks exposure samples were taken to measure iron and chloride levels in the bio-mass

Simulation Results Chloride – 1100 ppm Iron – 4000 to 12000 ppm

Zirconium oxide nodules were found in the pits. Zirconium chloride complexes are the primary corrosion product of ferric chloride pitting The complexes are hydrolyzed to zirconium oxide forming nodules

Conclusions 1. Zr 702 performs better than alloy C-276 in this mix of hot organic and mineral acids. 2. Biological films composed of iron oxidizing sphaerotilus form on the shell side of the exchanger. 3. Biologically produced ferric ion combines with chloride in the river water to produce a strong pitting environment under the bio-mass. 4. Low flow through the condenser shell promotes bio-mass formation and pitting of the tubes.

Questions?