|
FACTOR |
DEFINITION — CORROSION INFLUENCE
|
CONTROL |
|
Dissolved Oxygen (DO) |
Scale: 0-12 (mg/l). Defines oxygen gas
dissolved in water (normal range = 3-9 mg/l).
Corrosivity increases with
DO. |
Very difficult to control in open systems.
CAP water has higher levels (9-10 mg/l) than groundwater (~6 mg/l). |
|
pH |
Scale: 1-14 (s.u). Defines amount of free
acidity in water. Log scale of acid concentration. One (1) unit decrease
corresponds to a 10-fold increase in acid concentration. Normal range in
groundwater and surface water: 6.5-8.5. Usually, higher pH means higher
alkalinity.
Corrosivity increases with decreasing pH. |
Relatively easy to control in water with low
to moderate alkalinity. Requires the addition of chemicals such as lime.
|
|
Total Dissolved
Solids (TDS) |
Scale: 0-? (mg/l) Refers to amount of
dissolved minerals in water: ninety percent or more of TDS in natural waters
includes the following elements:
Sodium, calcium, magnesium, potassium,
chloride, sulfate, carbonate.
These elements are from table salt, gypsum,
calcite and other minerals.
In general, corrosivity increases with TDS,
depending on alkalinity. |
Moderately easy to very difficult to change.
Dilution: easy.
Removal by filtration: moderately difficult and
expensive. |
|
Alkalinity |
Scale: 0-? (mg/l as CaCO3). Refers to
concentration of carbonates and bicarbonates present in water.
In general,
corrosivity decreases with increasing alkalinity. Note: excessive calcium
carbonate scale can clog pipes. |
Moderately easy to change. Requires addition of acid or base
(alkaline) chemicals. |
|
Temperature |
(Degrees Centigrade). Varies with season and
location.
Corrosivity can increase or decrease, depending on alkalinity and
temperature range. Note: higher temperatures increase metal oxidation rates,
but they also increase scale formation by calcite precipitation. |
Cannot be controlled.
Groundwater
temperature is more constant (~20-30°C)
CAP fluctuates with ambient
(~10-40°C) |
|
Type of pipe |
In general, metal pipes corrode and plastic
pipes do not.
Most corrosive pipe: galvanized iron, galvanized steel,
aluminum, iron.
Least corrosive pipe: plastic, stainless steel, copper.
Note: zinc metal corrodes much faster than iron, which helps protect
iron
from corrosion in galvanized iron pipes. |
Controllable in new homes.
Use of plastic
pipes restricted by codes.
Can be expensive to retrofit.
Use of
corrosion inhibitors such as polyphosphate. |
|
Water Velocity |
Corrosivity may increase at high velocity due
to turbulence that helps DO react faster with the metal surface. However, at
high velocity, faster scale formation of CaCO33 (a corrosion
inhibitor) may also occur. |
May be controllable through pipe design
changes and use of low water use appliances, faucets, shower heads, etc.
|
|
Hardness |
Hardness refers to the concentrations of
calcium and magnesium ions, but is usually reported in mg/l of CaCO3. Water
hardness is linked to scale formation and the reduced cleaning efficiency of
soaps.
Hardness is linked to scale formation; therefore, in pipes hard
water is less corrosive than soft water.. |
Through water softening and salt removal
techniques. |