Why Does a Galvanized Steel Pipe Corrode? Expert Analysis by FILAB

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Understanding the origin of corrosion in a galvanized steel pipe

Corrosion on a galvanized steel pipe appears when the zinc layer, intended to act as a sacrificial anode, no longer provides lasting protection for the steel. In a hydraulic system, several factors can destabilize this protection: unsuitable water quality, the presence of dissolved oxygen, chlorides, unfavorable pH, stagnation, deposits, galvanic couples between different materials, or a local coating defect. The zinc may then dissolve abnormally, form white rust, and leave the bare steel to oxidize. In some cases, a zinc polarity reversal can abruptly accelerate attack on the substrate.

The phenomenon does not remain local: it generates corrosion products and sludge that foul heat exchangers, clog flow sections, and degrade circulators. Ignoring corrosion means condemning all the building’s hydraulic equipment.

Role of zinc, passivation, and loss of protection

The galvanized coating protects the steel through both barrier effect and sacrificial protection. In service, the zinc normally forms a passivation layer that limits its dissolution.

If this layer is unstable, porous, or destroyed by the water chemistry, the zinc is consumed more quickly. The presence of oxygen, dissolved salts, temperature variations, or stagnant zones can promote this degradation.

The first signs are often whitish deposits, known as white rust, followed by deeper attack.

Visible signs on the network and hydraulic consequences

A galvanized steel failure analysis often begins with operating symptoms: cloudy water, grayish or whitish sludge, repeated filter clogging, reduced flow, abnormal pressure losses, noise in pumps, localized corrosion on fittings, or the appearance of leaks.

White powdery deposits may indicate zinc attack, while red-brown oxides signal damage to the underlying steel.

Corrosion and metallurgy investigation approach

The investigation relies on a multi-scale approach: visual examination of the affected areas, metallographic observation, characterization of deposits, and comparison between sound and failed areas. The objective is to identify the corrosion mode, assess the degree of oxidation, verify the condition of the galvanized coating, and identify possible precursors such as halogens, contaminants, or incrusted deposits.

This methodology makes it possible to link the observed damage to the actual service conditions.

Investigating the failure to stop network degradation

The issue is not only to note the presence of rust or deposits, but to identify the root cause of the failure. A relevant investigation combines observation of the affected areas, analysis of deposits, assessment of the condition of the zinc and steel, and study of the service environment.

This approach makes it possible to distinguish galvanic corrosion in a network, under-deposit corrosion, attack by overly aggressive water, a passivation defect, or an electrochemical imbalance linked to operation. The goal is to provide a construction corrosion diagnosis that can be used to decide on corrective actions: water treatment, material changes, elimination of a galvanic couple, network cleaning, or targeted replacement of degraded sections.

Galvanic coupling and polarity reversal

In a network containing several metals, the difference in electrode potential can create a galvanic cell. The least noble metal becomes anodic and corrodes preferentially. If local conditions change, the behavior of the zinc can shift and lead to a zinc polarity reversal, a particularly critical situation for the durability of the coating. This scenario is common when water composition, deposits, or contact surfaces alter electrochemical balances.

When corrosion becomes a risk for the installation

When corrosion products circulate through the system, they accumulate in low points, heat exchangers, valves, and pumps. This solid contamination disrupts hydraulic balancing and can cause premature wear of components. The later the intervention, the greater the risk of widespread degradation. In heating and chilled water systems, the indirect cost linked to shutdowns, replacements, and cleaning can quickly exceed the cost of the investigation.

Technical methods that can be used depending on the case

Selon la problématique, différents moyens techniques peuvent être mis en œuvre : essais électrochimiques avec mesure du potentiel libre (OCV), étude de la vitesse de corrosion (LSV), mesure d’impédance électrochimique (EIS) et essai de couplage galvanique ; analyses de surface et de dépôts par MEB-EDX, XPS, DRX et microscopie optique ; analyses chimiques par ICP ; simulations de milieux spécifiques, vieillissement accéléré et essais comparatifs de matériaux ou traitements de surface. Cette combinaison permet d’établir un diagnostic robuste et de hiérarchiser les causes probables.

Moving from assessment to corrective action

FILAB supports operators, property managers, fluid engineering firms, and installers in the FILAB metallurgical expertise of network failures.

The value of a laboratory approach for a pipe analysis is to turn a visible issue into an evidence-based diagnosis, then into an action plan. By combining electrochemical testing, surface analysis, chemical characterization, and environmental simulations, it becomes possible to identify the dominant cause, validate technical hypotheses, and guide lasting corrective measures.

Frequently asked questions

Why does a galvanized steel pipe corrode in a heating or chilled water system?

A galvanized pipe corrodes when the protective zinc loses its electrochemical stability in the circulating medium. As long as the zinc layer remains continuous and passivated, it protects the steel. But if the electrode potential changes unfavorably, if the pH is unsuitable, if chlorides or oxygen are present, or if a coupling with a more noble metal is established, the zinc dissolution rate increases. Once the protective barrier is broken, the steel oxidizes in turn, producing deposits, sludge, and loss of thickness.

What electrochemical mechanisms cause the degradation of zinc and then steel?

The degradation results from an electrochemical imbalance. The zinc first dissolves when it can no longer maintain a stable passive layer. Then the exposed steel becomes susceptible to oxidation. The most common mechanisms are accelerated zinc dissolution, galvanic corrosion between dissimilar materials, differential corrosion under deposits, and, in some cases, polarity reversal that changes the expected protective behavior of the coating.

What signs should alert operators and building managers?

Warning signs include sludge, white or rust-colored deposits, reduced hydraulic performance, fouling of equipment, and localized leaks. These symptoms often indicate that corrosion is no longer superficial. A rapid investigation helps prevent network blockage and deterioration of pumps, heat exchangers, and control components.

How does the laboratory identify the root cause of corrosion on galvanized steel?

The laboratory identifies the root cause by cross-referencing the material’s metallurgical condition, the nature of the deposits, the electrochemical behavior of zinc and steel, as well as the environmental parameters. This approach makes it possible to distinguish a simple local defect from a systemic mechanism linked to water, sizing, material selection, or operating conditions in the network.

Why choose FILAB for a corrosion diagnosis on galvanized piping?

Calling on FILAB provides a technical diagnosis based on laboratory measurements and observations, rather than on mere assumption. The expertise aims to understand, prove, and correct: identify the corrosion mechanism, quantify its impact, secure hydraulic equipment, and prevent the phenomenon from recurring after repair.