Scale and Corrosion: What Bad Water Does to a Boiler
Scale and corrosion are the two ways water damages a boiler. Scale is a deposit that insulates the heating surface, wasting fuel and overheating metal. Corrosion is the water attacking the steel itself. Both are slow, invisible from outside, and expensive by the time they are found.
You'll learn
- Explain how scale forms and why it wastes fuel
- Read the published fuel cost of scale
- Describe oxygen pitting and other corrosion mechanisms
- Know how each is prevented and found
Before this: Raw water quality, Deaeration and feedwater temperature
Two kinds of damage
Scale is a hard mineral layer — mostly calcium and magnesium compounds, sometimes silica — that builds up on the water side of heating surfaces. It forms because these minerals become less soluble as water gets hotter, so they come out of solution exactly where the water is hottest: on the tubes and furnace.
Corrosion is the water attacking the steel. Dissolved oxygen causes deep pits; acidic water dissolves metal evenly; very concentrated alkali can attack steel under deposits. Corrosion products — iron oxides — travel with the water and deposit elsewhere, adding to the scale problem.
What scale costs
Scale conducts heat far worse than steel. A thin layer forces more of the furnace’s heat to stay in the gas and leave up the stack, so the boiler burns more fuel for the same steam. Published figures:
| Scale thickness | Normal scale | High iron scale | Iron plus silica |
|---|---|---|---|
| 0.4 mm | 1.0% | 1.6% | 3.5% |
| 0.8 mm | 2.0% | 3.1% | 7.0% |
| 1.2 mm | 3.0% | 4.7% | — |
| 1.6 mm | 3.9% | 6.2% | — |
US DOE Steam Tip Sheet #7, from NIST Handbook 115 Supplement 1. Thickness converted from inches.
Less than half a millimetre of ordinary scale costs about 1% of fuel; iron and silica scales cost far more for the same thickness, because they insulate better. On a boiler burning several lakh rupees of fuel a month, 2% is a large and entirely avoidable bill.
Overheating, and the corrosion mechanisms
The bigger danger of scale is not fuel but metal temperature. A tube is designed to run a little above the water temperature because the water cools it. Scale insulates the tube from that water, so the metal runs hotter — and at high heat flux, enough hotter to lose strength, bulge and rupture. Furnace tubes and crowns, where heat flux is highest, fail first.
- Oxygen pitting: localized, deep pits, worst in the feed system, economizer and where water first enters the boiler. Prevented by deaeration and oxygen scavenger.
- Acid corrosion: low pH dissolves steel; also the cause of grooving in condensate lines from carbon dioxide (carbonic acid). Prevented by pH control and neutralizing amines in condensate.
- Caustic attack and under-deposit corrosion: concentrated alkali under deposits attacks steel. Prevented by keeping surfaces clean and alkalinity within limits.
- Standby corrosion: an idle boiler partly drained, with air getting in, corrodes fast. Idle boilers are stored either completely dry or completely full of treated water.
Key takeaways
- Scale forms where water is hottest, because hardness salts are less soluble when hot.
- Under half a millimetre of normal scale costs about 1% of fuel; iron and silica scales cost much more.
- Scale’s worst effect is overheating the metal it insulates.
- Oxygen pits, acid dissolves, concentrated alkali attacks under deposits; idle boilers corrode fast.