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

And effect of:

EPA Lead and Copper Rule in 1991

Wisconsin Dept. of Natural Resources - 1988 Lakes in Madison, Wi Area

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.

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.

1997Catskill-Delaware 8.6Croton 43.8Groundwater 48.9

2007Catskill-Delaware 9Croton 56Groundwater 97

Pennsylvania IndiaWorld Health Organization

Natural underground sources Road salt Road salt storage facilities Water softeners Septic systems Water treatment plants Industry Fertilizers Landfills Sea water intrusion

Deterioration of gray cast iron in which the metallic constituents are selectively leached or converted to corrosion products leaving the graphite intact.

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

Fe Fe++ + 2e-

Fe++ + H2O Fe(OH)2

Fe(OH)2 + O2 Fe2O3 + H2O

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

Corrosion rate is governed by◦ Voltage ≈ 2V max.◦ Conductivity of the water◦ Temperature of the water◦ Oxygen in the water

Voltage is controlled by Metallurgy

Conductivity and Temperature control the rate of electron exchange

Oxygen controls the rate of reaction

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

Conductivity is affected by Temperature

Higher temperature ◦ Increases corrosion rate

IF TDS = 0 there is no conductivity

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

Lead and Copper Rule or 1991 Rule

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.

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

Phosphate chemicals combined with dispersant

Used in conjunction with boiler blow down Are an economical solution for controlling

scale

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

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

Graphitic Corrosion &Baked-on SludgeGraphitic Corrosion