Is water glycol oil create corrosion in components?

Water-glycol fluids can cause corrosion, but it's not just because of their base chemistry. It happens when the water content increases, the additive package degrades, there are dissimilar metals present, or the fluid isn’t kept within the recommended pH and inhibitor limit. Water-glycol hydraulic fluids that are properly formulated and maintained contain corrosion inhibitors designed to protect ferrous and non-ferrous metals. Most corrosion problems can be traced back to poor fluid maintenance rather than fluid type.

Because they are fire-resistant, water-glycol is used in hydraulic systems near ignition sources, such as foundries, steel mills, mining equipment, and die-casting machines. The same water content that makes these fluids fire resistant (typically between 35 and 45 percent) also makes corrosion control more important than with mineral oil-based liquids. Anyone specifying or maintaining water-glycol systems must understand why corrosion occurs and how to avoid it.

Why are water-glycol liquids more corrosive than mineral oils?

Mineral hydraulic oils form a protective layer on metal surfaces because they are hydrophobic by nature. Water-glycol liquids contain a large percentage of water, but not as a contamination. Water is a good conductor for electrochemical reactions. It's when water and oxygen are combined that rust forms on ferrous surfaces, while corrosion occurs on yellow metals such as brass, bronze, and copper.

Water-glycol formulations are heavily reliant on corrosion inhibitors—usually borate, nitrogenite, or amine-based substances—to neutralize this tendency. The corrosion protection of fluids is excellent when these inhibitors are in the right concentration, and the pH level is kept within the recommended range (9.0-9.5 in most formulations). When that chemical balance shifts, problems begin.

Common causes of corrosion within water-glycol systems

Inhibitor depletion over time

Consumable corrosion inhibitors. The concentration of corrosion inhibitors drops as the fluid ages, and inhibitors react with metal surfaces to neutralize acidic byproducts. Once the fluid is depleted to a certain level, it loses its ability to protect components. Corrosion can start quickly, sometimes without any visible signs until rust or pitting appears on valve spools or cylinder rods.

pH drift

Fluids containing water-glycol are designed to work within a narrow pH range. Over time, oxidation, contamination, and breakdown of glycol can cause the pH to move towards neutral or acidic. A declining pH is a reliable indicator that corrosion protection has weakened. This is why routine pH tests are considered an important maintenance task.

Water content imbalance

Even though water is an integral component of the fluid, it can cause problems if its content drifts out of the range specified by the manufacturer. The excessive evaporation of glycol (common with high-temperature systems or those that use an open reservoir) can cause the fluid to become unstable. Water dilution due to condensation or leaks in the system can also reduce the inhibitor concentration. Both directions can increase corrosion.

Contact metals that are similar.

In a fluid-based environment, systems that combine metals, such as zinc, cadmium, or certain aluminum alloys, with ferrous components are more susceptible to galvanic corrosive corrosion. Some water-glycol formulas are incompatible with zinc- and cadmium-plated components. The fluid's alkaline chemical composition can cause the plating to degrade faster.

Contamination of water and bacteria

Water-glycol fluids are more susceptible to microbial contamination than mineral oil in systems that have been poorly maintained because they contain a significant amount of water. Byproducts of bacteria can be acidic. This further damages pH balance and accelerates localized corrosion.

Components most at risk

Not all components in a water/glycol system are equally susceptible to corrosion. Most commonly affected parts include:

• Cylinder rods—prolonged exposure, particularly in humid environments, can increase the risk of pitting on rod surfaces.
• Valve bodies and spools—even minor surface corrosion may cause erratic or sticking operation.
• Pump internals—vanes, gears, and pistons that operate under high pressure and shear are sensitive to any film loss.
• Water separation and bacterial development are more likely to occur in low-flow areas and reservoirs.
• Fittings and fasteners with zinc or cadmium plating are chemically incompatible.

How can water-glycol systems be protected from corrosion?

Test fluid condition regularly.

Fluid analysis should be performed at regular intervals, usually monthly or quarterly, depending on the duty cycle. Early detection of pH drift and inhibitor depletion allows corrective additives to be applied before actual corrosion occurs.

Keep water content within specified limits.

Water content is built into fluids, so it's important to manage the water rather than just avoid it. Reservoirs must be sealed to prevent excessive evaporation. Any water lost should be replaced with deionized water or distilled water as specified by the fluid maker. Never use tap water because it contains minerals and contaminants, which can upset the chemical balance of the fluid.

Check metal compatibility prior to fluid selection.

Audit the system before switching to a water-glycol or specifying one. Check for components containing zinc, cadmium, and certain types of aluminum. If incompatible metals cannot be avoided, ask the fluid manufacturer for alternatives or protective coatings.

Keep your system clean and sealed.

By reducing the amount of contaminants in fluids, preventing process water from entering reservoirs, and controlling reservoir breathers, you can reduce the load.

Replace fluid according to a schedule.

Water-glycol has a limited service life, even with good maintenance. This is because the inhibitors gradually and irreversibly deplete. The best way to prevent corrosion is to follow the manufacturer's recommended interval for fluid replacement.

When properly maintained, water-glycol fluids are not more corrosive. The fire resistance they offer is a tradeoff for the maintenance, not a corrosion risk that can't be avoided. Water-glycol hydraulic fluid systems that have corrosion issues are usually dealing with depleted inhibitors, pH drifts, water imbalances, incompatible metals, or contamination. All of these problems can be prevented by following disciplined fluid management procedures.

1. Is water-glycol a more corrosive fluid than mineral oil or hydraulic oil?

Water-glycol liquids do not contain corrosion inhibitors by default. They are added to counteract the tendency for water-glycol mixtures to corrode. Corrosion problems are usually caused by poor maintenance and not the fluid chemistry.

2. How often should water-glycol be tested for corrosion resistance?

Most manufacturers recommend testing pH levels, water content, and inhibitor levels every month to quarter, depending on the system duty cycle and operating conditions.

3. Can I add tap water to water-glycol liquid?

No. Only use distilled or deionized drinking water that meets the manufacturer's specifications. Tap water contains minerals and contaminants, which can disrupt the fluids' chemical balance.

4. What metals are not compatible with water-glycol fluid?

Certain aluminum alloys, zinc, and cadmium are incompatible with each other and can cause fluid degradation and component corrosion to accelerate.

5. What is the first warning sign that corrosion may be present in a water/glycol system?

The first reliable sign that corrosion inhibitors have been depleted and the protection has weakened is a downward trend in fluid pH.