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Graphitic Corrosion of Cast Iron Steam Boilers
Today’s water is not the water of the 1950’s, 1960’s and 1970’s
ByPaul Schuelke
Director, Technical ServicesWeil-McLain
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And effect of:
EPA Lead and Copper Rule in 1991
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Wisconsin Dept. of Natural Resources - 1988 Lakes in Madison, Wi Area
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Chloride concentrations in excess of 250 mg/L are widely distributed in Oakland County.
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They are commonly associated with high-density lakeshore communities using on-lot wells and septic systems.
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Chloride concentrations in excess of 250 mg/L are widely
distributed in Oakland County.
They are commonly associated with high-density lakeshore communities using on-lot wells and septic systems.
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1997Catskill-Delaware 8.6Croton 43.8Groundwater 48.9
2007Catskill-Delaware 9Croton 56Groundwater 97
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Pennsylvania IndiaWorld Health Organization
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Natural underground sources Road salt Road salt storage facilities Water softeners Septic systems Water treatment plants Industry Fertilizers Landfills Sea water intrusion
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Deterioration of gray cast iron in which the metallic constituents are selectively leached or converted to corrosion products leaving the graphite intact.
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Graphite is a form of carbon Graphite is in cast iron Within the cast iron matrix there is
an anode and a cathode
Anode Cathode
Voltage
Iron Graphite
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Fe Fe++ + 2e-
Fe++ + H2O Fe(OH)2
Fe(OH)2 + O2 Fe2O3 + H2O
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Rust (Fe2O3) stays in place Water (H2O) is present in the metal Needs oxygen (O2) to penetrate the rust to
continue the process As the rust thickness increases◦ Rust becomes a barrier
It is more difficult for the oxygen to penetrate the rust to the iron
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Corrosion rate is governed by◦ Voltage ≈ 2V max.◦ Conductivity of the water◦ Temperature of the water◦ Oxygen in the water
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Voltage is controlled by Metallurgy
Conductivity and Temperature control the rate of electron exchange
Oxygen controls the rate of reaction
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Conductivity is controlled by◦ Totally Dissolved Solids (TDS) Chlorides are part of TDS measurement Increasing Chlorides have increased TDS
Increasing TDS◦ Increases conductivity Resulting in higher corrosion rate
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Conductivity is affected by Temperature
Higher temperature ◦ Increases corrosion rate
IF TDS = 0 there is no conductivity
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Oxygen is in the water◦ Adding water adds oxygen Oxygen continues corrosion process
Make-up water use is controlled by:◦ Maintenance◦ Leaks in the heating system◦ System design
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Lead and Copper Rule or 1991 Rule
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June 7, 1991, The EPA issues “Lead & Copper Migration Rule”
To protect public health by reducing lead and copper entering drinking water from corrosion of older plumbing materials.
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20% to 25% of the public water systems use phosphate families of chemicals to comply
Polyphosphates react with soluble metals by sequestering (binding) the metal
Orthophosphates form a domestic water supply pipe coating
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Phosphate chemicals combined with dispersant
Used in conjunction with boiler blow down Are an economical solution for controlling
scale
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The phosphates added to comply with EPA Rule 1991 do not have the dispersant
Without the dispersant boiler water chemical treatment is out of balance
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In boiler water, phosphates combine with calcium to make calcium phosphate
Calcium phosphate can precipitate and form sludge
The sludge concentrates at the boiler water line Then bakes-on the heating surface at the water
line and above Allowing under deposit corrosion That increases the rate of corrosion
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Graphitic Corrosion &Baked-on SludgeGraphitic Corrosion
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