Corrosion Basics – Water Constituents

WATER CHEMISTRY · CORROSION BASICS · INDUSTRIAL WATER · SCALE

Water is rarely just H₂O. Dissolved gases, salts, minerals, organic matter, microorganisms, temperature, and pH can all change the way a metal behaves in service.

For corrosion professionals, understanding water chemistry is often essential because even relatively small concentrations of certain constituents can influence corrosion rate, localized attack, deposit formation, scale, heat transfer, and materials compatibility.

What’s Actually in Water?

The concentration of individual substances in water may be relatively small, but the significance of those substances depends heavily on the application.

A constituent that is acceptable in a municipal water supply may be unacceptable in a high-pressure boiler, process-water system, cooling circuit, or another application where very small concentrations can affect corrosion, deposition, heat transfer, or equipment performance.

The original corrosion reference groups important water constituents into four broad categories.

Why Dissolved Oxygen Matters So Much

From a corrosion standpoint, dissolved oxygen is often one of the most important constituents in water.

In many aqueous corrosion systems, oxygen participates in the cathodic reaction. As oxygen is consumed at cathodic areas, the electrochemical corrosion process can continue as long as the necessary reactants and conductive pathways remain available.

This is one reason dissolved-oxygen control can be important in hot-water systems, boilers, and other closed industrial systems where oxygen-assisted corrosion may otherwise become severe.

Dissolved Oxygen Is Not the Whole Story

The effect of oxygen depends on the complete system. pH, temperature, flow, alloy composition, deposits, chlorides, microbial activity, scale formation, and inhibitors can all change the observed corrosion rate.

Local differences in oxygen concentration can also create differential-aeration cells, where areas exposed to less oxygen become anodic relative to more highly aerated regions and experience localized attack.

The Role of pH

pH is another major variable because it influences electrochemical reactions, solubility, passive-film stability, mineral precipitation, and the behavior of many dissolved species.

The original reference notes that many natural waters fall within a moderately acidic to moderately alkaline range. Within that range, the corrosion behavior of steel and other metals can change substantially as oxygen concentration, alkalinity, scale formation, and other factors change.

At sufficiently low pH, acid-driven corrosion can become dominant. At higher pH values, some metals may develop more protective oxide or hydroxide films, although the effect is material- and environment-specific.

Why Copper Can Create Problems for Other Metals

Copper chemistry provides a useful example of why water constituents must be considered as part of a system rather than one material at a time.

Under some water conditions, copper can dissolve into the water as ionic species. Those dissolved copper species can then travel downstream and deposit on more active metals such as aluminum or zinc-coated surfaces.

Once deposited, the copper can create small galvanic sites and promote severe localized attack of the more active substrate.

Chlorides Can Increase Corrosion Risk

Chloride ions are important in many corrosion problems because they can increase water conductivity and destabilize protective films on susceptible metals.

For passive alloys, chlorides can be particularly important in localized corrosion mechanisms such as pitting or crevice corrosion when other environmental conditions also favor attack.

The significance of a given chloride concentration depends on the alloy, pH, temperature, oxygen content, flow, deposits, other ions, and the specific system.

Carbon Dioxide Changes Water Chemistry

Dissolved carbon dioxide can react with water to affect carbonate chemistry and pH. Depending on the system, this can influence both corrosion and scale formation.

In industrial water systems, carbon dioxide can therefore be important not only as a dissolved gas but also because it changes the chemical environment surrounding the metal.

Sulfides and Ammonia Can Matter Too

Industrial and natural waters may contain sulfide species or ammonia, both of which can affect corrosion behavior depending on the materials present and the surrounding chemistry.

Their importance is highly material-specific, which is why water analyses used for corrosion assessment should be interpreted in the context of the actual metallurgy and operating environment.

Temperature Usually Speeds Chemical Reactions — But Water Systems Are Complicated

As with many chemical processes, increasing temperature often increases reaction rates and can accelerate corrosion.

But temperature can also change gas solubility, scale precipitation, inhibitor performance, oxygen availability, microbial activity, and water chemistry.

That means the relationship between temperature and corrosion is not always a simple straight line in an operating system.

Scale Can Protect — or Create New Problems

Minerals dissolved in water can precipitate onto metal surfaces and form scale.

A thin, uniform, adherent deposit can sometimes reduce direct contact between the metal and the water and therefore provide a degree of protection.

But nonuniform scale can create differential conditions on the surface. Cracks, pores, deposits, and under-deposit environments can promote localized corrosion even while other areas appear protected.

Thick scale can also interfere with heat transfer, reduce flow area, increase pressure drop, and create operational problems independently of corrosion.

Water Hardness and Corrosion Are Not the Same Thing

Water hardness generally reflects dissolved calcium and magnesium concentrations. Hardness can influence scale formation, but it should not be used by itself as a direct measure of whether water is corrosive.

Two waters with similar hardness can behave very differently if their alkalinity, pH, chloride concentration, dissolved oxygen, temperature, flow, or other constituents differ.

Microorganisms Can Change Local Conditions

Water systems can also support bacteria, algae, biofilms, and other biological growth.

Microorganisms may create deposits, consume or produce chemical species, alter local pH, establish differential aeration, or contribute to microbiologically influenced corrosion.

Their influence is another reason bulk-water chemistry alone may not fully describe the environment directly at the metal surface.

Bulk Water and Surface Water Can Behave Differently

A laboratory water sample usually describes the bulk fluid. Corrosion, however, occurs at an interface.

Under deposits, inside crevices, within biofilms, or near heat-transfer surfaces, the chemistry immediately adjacent to the metal can differ substantially from the chemistry measured in a bottle taken elsewhere in the system.

Effective corrosion assessment therefore combines water analysis with knowledge of operating conditions and evidence from the asset itself.

What to Look at in an Industrial Water Analysis

The exact analytical program depends on the system, but corrosion investigations may consider parameters such as:

The most useful parameters are those connected to a specific corrosion mechanism or operating question. More data are not automatically better unless the results can be interpreted in the context of the system.