Corrosion, Minerals & Chemical Reactions

Learn corrosion, mineral chemistry, hardness, scale formation, precipitation, iron and manganese reactions, oxidation-reduction, and practical water-quality interpretation for operators.

Corrosion, mineral precipitation, and chemical reactions affect pipes, tanks, treatment equipment, water quality, and chemical-feed systems. Operators should understand why metals dissolve, why minerals form scale, how pH and alkalinity affect water stability, and how oxidation changes substances such as iron and manganese.

These reactions are connected. A change in pH, alkalinity, dissolved oxygen, temperature, or chemical dose can change whether a substance remains dissolved, precipitates as a solid, or reacts with system materials.

What Is Corrosion?

Corrosion is the deterioration of a material through chemical or electrochemical reactions with its environment.

For metal systems, corrosion commonly involves oxidation of the metal.

Why Corrosion Matters

Corrosion can cause:

pipe-wall loss;
leaks;
equipment failure;
discolored water;
metal release;
reduced hydraulic capacity;
higher maintenance cost.

Corrosion Is an Electrochemical Process

Many metal-corrosion reactions involve separate oxidation and reduction reactions.

At one location, metal atoms may lose electrons and enter solution as ions.

At another location, those electrons are consumed by a reduction reaction.

Oxidation

Oxidation means loss of electrons.

When a metal corrodes, metal atoms may be oxidized to dissolved metal ions.

Reduction

Reduction means gain of electrons.

Oxidation and reduction occur together in a redox reaction.

Factors Affecting Corrosion

Corrosion can be influenced by:

pH;
alkalinity;
dissolved oxygen;
temperature;
chloride;
sulfate;
water velocity;
metal type;
protective scale or coating;
contact with dissimilar metals.

pH and Corrosion

Low pH can increase corrosion of many metals because acidic conditions can promote metal dissolution.

However, corrosion cannot be predicted from pH alone.

Operators should also consider:

alkalinity;
mineral content;
dissolved gases;
temperature;
pipe material.

Alkalinity and Corrosion

Alkalinity provides buffering and helps resist rapid pH changes.

Low-alkalinity water may be more sensitive to chemical changes and may provide less ability to maintain stable carbonate chemistry.

Dissolved Oxygen and Corrosion

Dissolved oxygen can participate in corrosion reactions.

Changes in oxygen concentration can influence corrosion rates and corrosion patterns.

Carbon Dioxide and Corrosion

Dissolved carbon dioxide contributes to carbonic-acid chemistry and can lower pH.

Water containing significant carbon dioxide and low buffering capacity may be more corrosive toward some materials.

Temperature and Corrosion

Increasing temperature can accelerate many chemical reactions.

Temperature also changes:

gas solubility;
mineral solubility;
reaction rates.

Chloride and Corrosion

Chloride can contribute to corrosion of some metals and can interfere with protective surface films under certain conditions.

High chloride should therefore be considered together with other water-quality characteristics.

Galvanic Corrosion

Galvanic corrosion can occur when dissimilar metals are electrically connected while exposed to an electrically conductive environment.

One metal can become more anodic and corrode faster.

Examples of Dissimilar-Metal Contact

Potential galvanic couples may occur where different metals are connected in:

piping;
valves;
fasteners;
fittings;
equipment assemblies.

Material compatibility and proper isolation are important.

Pitting Corrosion

Pitting corrosion is highly localized corrosion that forms small but sometimes deep cavities.

Pitting can cause failure even when average metal loss appears small.

Uniform Corrosion

Uniform corrosion occurs relatively evenly over a larger surface.

Over time, it can reduce overall wall thickness.

Corrosion Products

Corrosion reactions may produce deposits on metal surfaces.

Depending on conditions, these deposits may:

provide some protection;
remain loose;
restrict flow;
contribute to discolored water.

Protective Scale

Some water systems develop mineral films that reduce direct contact between water and metal surfaces.

A stable protective film can reduce corrosion.

However, excessive scale can create operational problems.

What Is Scale?

Scale is a mineral deposit that forms when dissolved substances precipitate onto surfaces.

Scale can form on:

pipes;
valves;
meters;
chemical-feed equipment;
membranes;
heat-transfer surfaces.

Common Scale-Forming Minerals

Common mineral deposits can contain:

calcium carbonate;
iron compounds;
manganese compounds;
other mineral precipitates.

Hardness

Hardness is caused primarily by dissolved calcium and magnesium.

Hardness is commonly reported as:

mg/L as CaCO3

Hardness Does Not Mean Calcium Carbonate Is the Only Mineral Present

Reporting hardness as CaCO3 provides a common equivalent basis.

The measured hardness may come from calcium, magnesium, and other multivalent ions.

Temporary and Carbonate Hardness

Hardness associated with carbonate and bicarbonate alkalinity is commonly described as carbonate hardness.

In traditional terminology, much of this may also be called temporary hardness because heating can promote precipitation of calcium carbonate.

Noncarbonate Hardness

Hardness associated with anions other than carbonate and bicarbonate is called noncarbonate hardness.

Examples may involve:

sulfate;
chloride;
nitrate.

Hardness and Alkalinity Relationship

Hardness and alkalinity are different measurements.

A useful operator relationship is:

carbonate hardness is limited by either total hardness or total alkalinity, whichever is lower when both are expressed as CaCO3;
hardness above the carbonate-hardness portion is noncarbonate hardness.

Carbonate Hardness Example

Suppose:

total hardness = 180 mg/L as CaCO3;
total alkalinity = 120 mg/L as CaCO3.

Carbonate hardness is approximately:

120 mg/L as CaCO3

Noncarbonate hardness is:

180 - 120 = 60 mg/L as CaCO3

Second Hardness Example

Suppose:

total hardness = 90 mg/L as CaCO3;
total alkalinity = 140 mg/L as CaCO3.

Carbonate hardness is limited by total hardness:

90 mg/L as CaCO3

Noncarbonate hardness is therefore approximately zero.

Calcium Carbonate Equilibrium

Calcium carbonate behavior is influenced by:

calcium concentration;
alkalinity;
pH;
temperature;
dissolved carbon dioxide;
other dissolved substances.

Precipitation

Precipitation occurs when dissolved substances form an insoluble or less-soluble solid.

Precipitation can be useful in treatment, but uncontrolled precipitation can create scale.

Dissolution

Dissolution is the process by which a solid enters solution.

Minerals can dissolve when water chemistry favors the dissolved form.

Solubility

Solubility describes how much of a substance can remain dissolved under specified conditions.

Solubility can change with:

pH;
temperature;
chemical species;
concentration of other substances.

Saturation

A solution is approximately saturated with a mineral when it is at chemical equilibrium with that mineral under the existing conditions.

If water becomes supersaturated, precipitation may become favorable.

If water becomes undersaturated, dissolution may become favorable.

Supersaturation

Supersaturated water contains more dissolved mineral components than would normally remain at equilibrium with the solid mineral.

This can increase the tendency for precipitation and scale formation.

Undersaturation

Undersaturated water can dissolve additional mineral material.

Depending on system chemistry, this can contribute to dissolution of existing mineral films.

Water Stability

Water stability describes the tendency of water to remain chemically balanced with respect to minerals and system materials.

Operators should avoid oversimplifying water as either simply corrosive or simply scale-forming based on one measurement.

Langelier Saturation Index

The Langelier Saturation Index, or LSI, is a calculated indicator related to calcium carbonate saturation.

In simplified terms:

positive LSI suggests calcium carbonate precipitation may be favored;
negative LSI suggests calcium carbonate dissolution may be favored;
LSI near zero suggests approximate calcium carbonate equilibrium.

LSI Is Not a Complete Corrosion Index

LSI describes calcium carbonate saturation tendency.

It does not directly measure all forms of corrosion.

Corrosion also depends on:

pipe material;
dissolved oxygen;
chloride;
sulfate;
temperature;
flow;
other chemical conditions.

Why Scale Can Be Helpful and Harmful

A thin, stable mineral film can sometimes reduce metal-water contact.

Excessive scale can:

reduce pipe diameter;
increase head loss;
restrict valves;
interfere with meters;
plug chemical-feed lines;
reduce heat transfer.

Scale and Hydraulic Capacity

As mineral deposits reduce effective pipe diameter, velocity and friction loss can increase for the same flow.

Severe scale can therefore reduce system capacity.

Iron Chemistry

Iron can exist in different oxidation states and chemical forms.

Under reducing conditions, dissolved ferrous iron may remain relatively soluble.

When it is oxidized, iron can form ferric compounds that are less soluble and may precipitate.

Iron Oxidation

Iron oxidation may be promoted by:

oxygen;
chlorine;
other oxidizing agents.

Oxidized iron can form particles that may be removed through:

sedimentation;
filtration;
other solids-separation processes.

Iron and Discolored Water

Iron corrosion products or precipitated iron can contribute to:

red;
brown;
orange

water or deposits.

Manganese Chemistry

Manganese can also exist in different oxidation states.

Dissolved reduced manganese can be oxidized into less-soluble forms that can precipitate.

Manganese Deposits

Manganese deposits may appear dark brown or black.

Deposits can accumulate in:

pipes;
filters;
tanks;
distribution systems.

Oxidation Must Be Followed by Removal

Oxidizing dissolved iron or manganese converts them toward particulate forms.

The treatment process must then remove those particles.

Oxidation without adequate solids removal can transfer the problem downstream.

pH and Metal Oxidation

Reaction rates and solubility of iron and manganese species depend strongly on pH.

Operators should consider pH when evaluating oxidation treatment.

Oxidant Dose

Oxidant demand can be created by multiple substances in the water.

An oxidant may react with:

iron;
manganese;
organic matter;
reduced sulfur compounds;
other reducing substances.

Oxidant Residual

If a measurable oxidant residual is required, the applied dose must first satisfy chemical demand.

A general relationship is:

Demand = Dose - Residual

Hydrogen Sulfide

Hydrogen sulfide can occur under reducing conditions.

It can cause:

odor;
corrosion;
worker-safety hazards;
oxidant demand.

Oxidation of Hydrogen Sulfide

Oxidants can convert reduced sulfur compounds into more oxidized forms.

Actual products depend on:

oxidant dose;
pH;
reaction conditions.

Mineral Precipitation in Chemical-Feed Systems

Improper chemical mixing or incompatible chemicals can form precipitates inside:

feed lines;
injectors;
valves;
storage or day tanks.

Chemical Compatibility

Chemicals should not be mixed unless compatibility is established.

Incompatible materials can cause:

precipitation;
heat;
toxic gases;
violent reactions;
equipment damage.

Concentration Effects

A reaction that does not cause significant precipitation at low concentration may behave differently when concentrated chemical solutions contact each other before adequate dilution.

Proper injection location and mixing are therefore important.

Temperature and Mineral Solubility

Temperature can change mineral solubility.

For some important mineral systems, increasing temperature can promote precipitation.

This is one reason scale can develop on warm surfaces.

Evaporation and Scale

Evaporation removes water while leaving dissolved minerals behind.

This increases mineral concentration and can promote precipitation.

Hard Water

Hard water contains relatively high concentrations of calcium and magnesium.

Operational effects can include:

scale;
soap consumption;
mineral deposits.

Soft Water

Soft water contains relatively low hardness.

Low hardness alone does not prove that water is corrosive.

Corrosion depends on the complete water chemistry and material conditions.

Ion Exchange Softening

Ion exchange can remove hardness ions such as calcium and magnesium by exchanging them for other ions, commonly sodium in conventional softening applications.

Lime Softening

Lime softening changes water chemistry to precipitate hardness compounds.

The process may involve precipitation of:

calcium carbonate;
magnesium compounds.

Precipitation Requires Solids Removal

When hardness is chemically precipitated, the resulting solids must be removed through appropriate clarification, settling, or filtration processes.

Chemical Equilibrium Can Shift

Water chemistry is dynamic.

Changing:

pH;
temperature;
carbon dioxide;
chemical dose;
concentration

can shift chemical equilibrium and change whether minerals dissolve or precipitate.

Aeration and Carbon Dioxide

Aeration can remove dissolved carbon dioxide.

As carbon dioxide decreases, pH may increase.

This can shift carbonate chemistry and change calcium carbonate precipitation tendency.

Corrosion Versus Erosion

Corrosion is chemical or electrochemical material deterioration.

Erosion is physical material removal caused by fluid, solids, bubbles, or mechanical forces.

The two can occur together.

Erosion-Corrosion

High velocity or turbulence can remove protective films and expose fresh metal to corrosion.

This combined mechanism is sometimes called erosion-corrosion.

Locations Vulnerable to Erosion-Corrosion

Potential locations include:

elbows;
partially open valves;
reducers;
pump components;
areas of high turbulence.

Stagnant Water and Corrosion

Low-flow or stagnant locations can develop different chemistry from the main flow stream.

Possible effects include:

oxygen differences;
deposit formation;
localized corrosion;
water-quality deterioration.

Deposits Can Hide Corrosion

A mineral or corrosion deposit may appear solid while severe metal loss exists underneath.

Do not assume a thick deposit means the pipe wall remains strong.

Laboratory Data Useful for Mineral and Corrosion Review

Useful parameters may include:

pH;
alkalinity;
hardness;
calcium;
conductivity;
chloride;
iron;
manganese;
temperature.

Operational Evidence of Corrosion

Possible signs include:

metallic staining;
rust-colored water;
pin-hole leaks;
increasing pipe failures;
pitting;
metal release;
corrosion products in tanks or filters.

Operational Evidence of Scale

Possible signs include:

mineral deposits;
reduced pipe diameter;
increased differential pressure;
plugged chemical lines;
restricted valves;
reduced equipment performance.

Trend Water Chemistry

Gradual changes in water chemistry may explain gradual changes in corrosion or scaling behavior.

Operators should review:

seasonal source-water changes;
chemical-feed changes;
pH trends;
alkalinity trends;
hardness trends;
temperature trends.

Do Not Diagnose Corrosion from One Number

No single field measurement can completely describe corrosion risk for every system.

Evaluate:

water chemistry;
materials;
operating conditions;
historical failure patterns;
actual corrosion evidence.

Do Not Eliminate All Scale Without Considering Its Function

Heavy scale is undesirable, but changing water chemistry aggressively can also disturb existing protective mineral films.

Treatment changes should be evaluated carefully and implemented according to approved procedures.

Sampling Location Matters

Water chemistry can change through a treatment and distribution system.

A sample from the source, treatment plant effluent, storage tank, or distant distribution location may represent different chemical conditions.

Common Corrosion and Mineral Chemistry Mistakes

Assuming low pH is the only cause of corrosion.
Assuming hard water is automatically noncorrosive.
Confusing hardness with alkalinity.
Assuming LSI measures every form of corrosion.
Ignoring chloride, dissolved oxygen, temperature, and material type.
Ignoring pitting because average corrosion appears low.
Assuming all scale is beneficial.
Oxidizing iron or manganese without providing adequate particle removal.
Mixing incompatible chemicals and creating precipitates.
Ignoring changes in source-water chemistry.

A Practical Corrosion Review

Review pH.
Review alkalinity.
Review hardness and calcium where relevant.
Review temperature.
Review chloride and other important dissolved ions.
Consider dissolved oxygen and carbon dioxide conditions.
Identify pipe and equipment materials.
Inspect for pitting, leaks, deposits, and metal release.
Review historical corrosion failures.
Evaluate trends before making major treatment changes.

A Practical Scale Review

Identify the location and appearance of deposits.
Review hardness and alkalinity.
Review pH and temperature.
Review chemical-feed changes.
Check differential pressure or hydraulic restriction.
Consider calcium carbonate saturation tendency.
Determine whether deposits are protective, excessive, or process-interfering.
Correct the underlying chemistry or operating condition according to approved procedures.

A Practical Iron or Manganese Review

Review raw-water iron or manganese concentration.
Review pH.
Review oxidant dose.
Review oxidant demand and residual.
Confirm adequate mixing and reaction time.
Review clarification or filtration performance.
Check for downstream deposits or discoloration.
Verify laboratory and online measurements when results are unexpected.

What to Remember for the Exam

Corrosion is chemical or electrochemical deterioration of a material.
Metal corrosion commonly involves oxidation of metal atoms.
Oxidation is loss of electrons, while reduction is gain of electrons.
Corrosion depends on multiple factors including pH, alkalinity, dissolved oxygen, temperature, chloride, material type, and flow conditions.
Galvanic corrosion can occur when dissimilar metals are electrically connected in a conductive environment.
Pitting is localized corrosion that can cause deep metal loss in a small area.
Hardness is caused mainly by calcium and magnesium and is commonly reported as mg/L as CaCO3.
Hardness and alkalinity are different measurements.
Carbonate hardness is limited by either total hardness or total alkalinity, whichever is lower when both are expressed as CaCO3.
Scale forms when dissolved minerals precipitate onto surfaces.
Supersaturation favors mineral precipitation, while undersaturation can favor dissolution.
LSI is an indicator of calcium carbonate saturation tendency, not a complete corrosion measurement.
A thin stable mineral film can sometimes be protective, while excessive scale can restrict pipes and equipment.
Oxidation can convert dissolved iron and manganese into less-soluble particulate forms.
After oxidation, the resulting particles must be removed by an appropriate solids-separation process.
pH strongly influences mineral solubility and many oxidation reactions.
Chemical incompatibility can cause precipitation, heat, toxic gas, or violent reaction.
Corrosion and erosion are different mechanisms but can occur together.
Deposits can hide serious corrosion underneath.
Corrosion and scale problems should be evaluated using water chemistry, system materials, operating conditions, and actual field evidence together.