From White Rust to Long-Term Protection Mechanisms:

Key Factors, Passivation Strategies and Field Applications for Galvanized Cooling Towers

Foreword:

Strategic decisions on passivation must be made based on site-specific factors, which this article does not cover in detail.

1. Introduction to Galvanized Steel

Galvanized steel is widely used in the construction of industrial cooling towers (CTs) due to its relatively low cost and its intrinsic
corrosion resistance, provided by a sacrificial layer of metallic zinc. However, this protection is not passive: it requires a controlled
oxidation phase to develop a durable, adherent film capable of shielding the zinc substrate from further corrosion. This passivation
process is critical during the first days following system start-up.

Two distinct passivation pathways exist, each with different chemical origins and long-term implications:

• Natural passivation leads to the formation of penta-zinc hydroxycarbonate (PZHC), a thermodynamically stable compound
  that forms spontaneously in mildly alkaline and aerated water, without chemical additives. The formation of PZHC is highly
  sensitive to water chemistry and system parameters and tends to be self-limiting once a coherent layer is established.
• In contrast, induced passivation by phosphoric passivation results in the formation of hopeite (Zn₃(PO₄)₂·4H₂O), a zinc
  phosphate compound precipitated deliberately through the addition of phosphate species. This approach, while potentially
  effective, shifts the corrosion control mechanism and must be carefully dosed to avoid destabilizing the surface layer or
  triggering undesirable interactions with other water treatment agents.

The confusion between these two mechanisms has led to widespread operational errors, particularly during commissioning, when the
introduction of corrosion inhibitors containing phosphonates or orthophosphates can interfere with the natural development of the
PZHC layer. As a result, many CT systems suffer from premature formation of white rust—a loosely adherent, porous corrosion product
that signals a failed passivation and significantly reduces the durability of the galvanized surface.

Operational confusion between these two mechanisms—natural versus induced passivation—has led to commissioning errors. The
early introduction of phosphonates or orthophosphates during startup can disrupt PZHC formation, triggering white rust: a loosely
adherent, porous zinc corrosion product indicative of failed passivation and reduced service life.

The objective of this article is clearly defined:

• Discuss exclusively natural passivation, based solely on mineral elements naturally present in the process water, excluding

any phosphate or additive-based intervention.

• Clarify the mechanism of PZHC formation

• Present a robust, field-validated commissioning strategy to promote enduring PZHC-driven protection of galvanized CT

components.

• Propose corrective measures strictly rooted in natural passivation principles.

2.Formation of the Protective Oxide Layer on Galvanized Steel

The natural passivation of galvanized steel relies on the progressive transformation of metallic zinc into a coherent, protective layer
of penta-zinc hydroxycarbonate (PZHC). This process is governed by electrochemical reactions occurring at the metal–water interface,
where oxygen availability, water chemistry, and thermodynamic stability collectively define the layer’s quality and durability.

2.1. Electrochemical Pathway to PZHC Formation

The oxidation of zinc begins at the anode:

Zn → Zn²⁺ + 2e⁻

These products combine to form an initial, metastable precipitate:

Zn²⁺ + 2OH⁻ → Zn(OH)₂

Protection is provided by PZHC, defined by the formula: Zn₅(CO₃)₂₋ₓ(OH)₆₊₂ₓ·H₂O, where x depends on the presence of inorganic carbon
(given by the P-Alk and M-Alk measurements). The most thermodynamically stable structure is obtained for x = 0

Under the influence of dissolved inorganic carbon—primarily in the form of bicarbonate (HCO₃⁻)—this hydroxide is gradually
transformed into PZHC, following the reaction:

5Zn(OH)₂ + 2HCO₃⁻ → Zn₅(CO₃)₂(OH)₆ + 2H₂O + 2OH⁻

The resulting PZHC layer is dense, adherent, and chemically stable under mildly alkaline conditions. It limits further zinc oxidation by
acting as a semi-permeable barrier, allowing for long-term corrosion control without external chemical intervention [3,4,5,6,7,8,9]

An infrared analysis makes it possible to assess the quality of the PZHC layer formed, based on the following criteria:

• Hydration: Peak at 3400 cm⁻¹ / Peak at 1390 cm⁻¹ (second peak)

The higher the value, the lower the quality of the PZHC.

• Crystallinity: Calculation = (Peak at 1507 cm⁻¹ + Peak at 1390 cm⁻¹) / Trough between the two peaks at 1446 cm⁻¹

The higher the value, the better the quality of the PZHC. [11]

2.2. Role of Water Chemistry Parameters

The formation and stability of the PZHC layer depend critically on a narrow envelope of water quality parameters during the initial
exposure period (typically the first 4 to 12 weeks). Depending on the parameter, manufacturer recommendations may vary. The values
presented here reflect a combination of OEM guidance, laboratory research, and field observations. Any passivation strategy must be
adapted to the equipment and the specific operating conditions.

PARAMETER	RECOMMENDED RANGE
pH	7.4 – 8.5
CONDUCTIVITY	250 – 2’400 µS/cm
M-ALKALINITY	100 – 500 ppm (CaCO₃)
TOTAL HARDNESS (TH)	50 ppm (Ca²⁺/Mg²⁺)
DISSOLVED OXYGEN	Saturated (>6 ppm at 20 °C)
TEMPERATURE	< 60 °C (140 °F)
CHLORIDES / SULFATES	< 250 ppm

These parameters must be monitored and maintained in real time during the passivation phase. Any deviation—such as high pH from
caustic leaks or insufficient buffering from low alkalinity—can result in non-adherent Zn(OH)₂ or amorphous carbonates, both
precursors to white rust

2.3. Factors to Avoid During Natural Passivation

To ensure the PZHC layer forms slowly and uniformly, the following must be strictly avoided:

CATEGORY	CRITICAL FACTORS TO AVOID	IMPACT ON PASSIVATION
CHEMICALS AND ADDITIVES	– Oxidizing biocides ( e.g., chlorine, bromine, FRC > 0.5 ppm) – Corrosion inhibitors: phosphonates, phosphates, orthophosphates – Molybdates – Polymers with corrosion inhibition properties	Disrupts electrochemical equilibirum, promotes porous oxide films or alters natural layer structure
HYDRAULIC AND OPERATIONAL DESIGN	– Presence of copper upstream or in recirculation loop – No pre-commissioning passivation protocol	Promotes galvanic corrosion anaccelerates zinc loss
WATER SYSTEM CONDITIONS	– Water stagnation – Deposits, especially at the bottom of the basin	Localized corrosion cells and heterogeneous layer formation
CONSTRUCTION PRATICES	– Exposing the CT system to weather before commissioning (e.g., rain ingress, UV, airborne dust)	Surface contamination and premature corrosion before controlled passivation
MATERIAL INTEGRITY	– Poor or non-standard galvanized steel quality (e.g., insufficient zinc layer thicknes	Limits the base material’s ability to form protective oxides
THERMAL CONDITIONS	Temperatures > 60 °C (140 °F) during startup	Accelerates Zn²⁺ release → uncontrolleprecipitation
WATER CHEMISTRY INSTABILITY	Cf. 2.2	Either prevents layer formation or induces non-coherent precipitates

Note: The PZHC layer must not form too quickly to be protective, nor too slowly. The use of effective corrosion inhibitors during the
layer formation phase should therefore be avoided.

3. White Rust: Misconceptions, Identification and Consequences

White rust is one form of natural surface oxidation, it refers to a white, powdery corrosion product that forms on freshly exposed
galvanized steel in humid or aqueous environments. It is primarily composed of zinc hydroxides, carbonates, and oxides, typically in
non-crystalline or poorly adherent forms. While often perceived as a superficial or aesthetic issue, its presence is a strong indicator of
failed natural passivation

The presence of ‘white rust’ can indicate:

• A non-homogeneous protective PZHC layer due to initial surface condition (hard and adherent deposit),

• A non-protective PZHC layer (less hard and non-adherent deposit).

3.1. Misconceptions and Common Errors

Despite its visual impact, white rust is frequently misunderstood in both field operations and OEM guidance. The following
misconceptions are widespread and dangerous:

• “White rust is only cosmetic”: In reality, it reveals localized corrosion and depletion of the protective zinc layer, particularly in areas of low oxygen or unstable pH.

• “Adding corrosion inhibitors will stop it”: If introduced too early, inhibitors (especially phosphates and molybdates) divert the electrochemical pathway, preventing coherent PZHC formation and favoring weak, mixed oxide layers.

• “It can be safely cleaned with acid”: Acid washing may remove the layer, but also dissolves partially formed passivation, leading to uncontrolled re-corrosion and deeper substrate attack.

This confusion is compounded by overlapping surface appearances between protective PZHC and non-protective white corrosion
products, making infrared or SEM/EDX analysis essential in determining the layer’s nature and structure. [12,13]

3.2. Mechanisms Leading to White Rust

White rust results from an imbalance or disruption in the early passivation reactions. The most common causes are:

• Excessive pH (> 8.5): Accelerates Zn²⁺ release, precipitating poorly adherent hydroxides.

• Low conductivity (< 250 µS/cm) or lack of carbonate alkalinity: Prevents full conversion to PZHC.

• Presence of oxidizing agents (e.g., chlorine): Creates unstable oxides or porous films.

• Water stagnation or non-aerated conditions: Leads to differential aeration corrosion.

• Surface contamination prior to startup: Dust, oils or atmospheric corrosion products inhibit uniform passivation.

These conditions often arise during construction, delayed commissioning, or when bypassing proper pre-commissioning cleaning and
conditioning protocols. [14,15]

3.3. Practical Identification and Field Implications

Visual signs of white rust include chalky or fluffy white spots, often localized on flat surfaces, welds, or areas with water retention.
However, IRM or SEM analysis is necessary to distinguish it from adherent PZHC layers that may have a similar hue but very different
protective behaviors (Figure 2).

Operational consequences include:

• Reduced lifespan of galvanized surfaces (accelerated zinc loss)

• Premature loss of hydrophobicity and surface uniformity

• Local under-deposit corrosion risks

• Difficulties in post-facto treatment and irreversible aesthetic degradation

In severe cases, white rust development within the first month of startup can lead to complete failure of the protective coating within
12–18 months, particularly in high-temperature, low-hardness systems [3,16]

4. Field-Based Passivation Strategies: Protocol and Operational Execution

Successful natural passivation of galvanized cooling systems requires more than chemical control: it is a structured, field-driven
protocol that ensures favorable electrochemical conditions are maintained long enough for the formation of a dense, adherent PZHC
layer. Below is a stepwise breakdown of best practices implemented during the passivation window.

4.1. Initial Assessment and System Baseline

Full system review and documentation must be conducted prior to startup. This includes a historical assessment of water treatment
performance, previous corrosion issues, and manufacturer specifications.

Photographic documentation of the tower structure, new coils, and tube bundles is recommended to serve as a baseline reference for
future visual assessments.

4.2. Deployment of Monitoring Coupons

Corrosion coupons should be installed in representative areas of the system to allow direct measurement of corrosion rates and layer
morphology.

If available, OEM reference coupons from the CT manufacturer can be used for comparative evaluation against standard protocols.

4.3. Water Chemistry Control and Concentration Ratio Adjustment

The concentration ratio must be adjusted during the passivation phase to maintain the parameters of 2.2.

Avoid active pH regulation systems, which often destabilize M-alkalinity and introduce variability detrimental to PZHC layer formation.

4.4. Real-Time Monitoring and Instrumentation

Ensure continuous online monitoring and data logging for the following critical parameters:

• pH

• Conductivity

• Redox potential

• Flow rate

• Temperature

These data serve both as a real-time control tool and a traceable record for validation and audit purposes.

4.5. Controlled Chemical Dosing

The formation of a durable and protective PZHC layer on galvanized surfaces depends not only on water chemistry but also on the
compatibility of additives used during the passivation window. This section summarizes experimental evidence showing how different
chemical agents influence the structure and quality of the passivation layer.

As a reminder, the formation of a PZHC passive layer is the primary objective during the first month of operation of a cooling tower.

Galvanized steel test coupons were immersed for 35 days (from 06/30/2016 to 08/04/2016) in 250 mL of solution (sealed bottles). At
the end of the test period, surface deposits were collected and analyzed using infrared absorption spectrometry to verify whether or
not an PZHC layer had properly formed.

Note regarding the photos: The surfaces were deliberately scraped to collect the deposit material, which explains their lack of uniform
coverage. For visual comparison, a new (unused) coupon was placed on the right in each photo.

4.5.1. Influence of Water Quality Alone

Test conditions showed that water without additives can still lead to PZHC formation, particularly in:

• Raw tap water (city water), even at 1.0× and 1.2× concentration: moderate to good layer quality

• Softened water: resulted in a high-quality PZHC layer

• Demineralized water: failed to form PZHC; instead, zinc oxide (ZnO) was detected, indicating the absence of protective passivation

This confirms that minimal mineral content (alkalinity, hardness) is essential for PZHC development.

4.5.2. Effects of Co-Passivating Agents

The addition of Co-Passivating Agents during passivation showed a positive impact on PZHC formation, when used alone and at
controlled concentrations.

Proprietary Blend of Co-Passivating Agents in city water produced a highly crystalline PZHC layer, comparable to that formed in
softened water.

These results suggest that the Blend of Co-Passivating Agents, when free from corrosion inhibition functions, is compatible with natural
passivation and may even enhance PZHC structural development, provided they do not contain interfering agents.

4.5.3. Impact of Additive Interferences

The formation of PZHC was disrupted or inhibited in the presence of:

• Phosphonate corrosion inhibitors: yielded poorly crystalline or amorphous deposits

• Biodispersants: led to layers rich in calcium carbonate (calcite) with no identifiable PZHC

• Certain phosphate-based blends: resulted in amorphous or mixed deposits with poor PZHC signature

This confirms that phosphorus-based inhibitors, multifunctional dispersants and biodispersants must be avoided during natural
passivation, as they can block or distort the electrochemical formation of the PZHC structure.

4.5.4. Optimal Observed Conditions

The best results were obtained:

• In terms of surface appearance: with city water + 100 ppm of the proprietary Blend of Co-Passivating Agent

• In terms of spectral analysis: with softened water and city water + 100 ppm of the proprietary Blend of Co-Passivating Agents. City water alone ranked third.

4.6. Routine Field Inspection

Weekly on-site inspections are strongly recommended throughout the 4–12-weeks passivation window. These should include:

• On-site water sampling and analysis

• Visual inspection of CT surfaces, coil exteriors, basin bottom

• Integrity checks of dosing and monitoring systems. The importance of continuous monitoring lies in the fact that even brief or isolated deviations can significantly impact the effectiveness of the passivation process.

4.7. End-of-Phase Validation

After 4 to 12 weeks, corrosion coupons must be retrieved and subjected to:

• Gravimetric weight loss analysis (per ASTM G1 or equivalent)

• Surface morphology evaluation using optical or infrared methods

• Optional SEM/EDX for composition and PZHC structure verification by experts

4.8. Documentation and Certification

A comprehensive passivation report must be drafted, including:

• Time-series data of monitored parameters

• Photographic records

• Laboratory and field analyses

• Interpretation of coupon results

• Visual assessment of galvanized surfaces

Upon successful review, issue a Passivation Certificate confirming the quality and durability of the PZHC layer. This document should
include:

• Coupon analysis results

• Layer validation outcomes

• Compliance with expected parameters

• Approval from technical field supervisor

4.9. Corrective Actions in Case of White Rust

Despite strict adherence to best practices, field conditions may deviate, and white rust can occasionally develop during or after the
passivation period. When early signs of localized corrosion or surface whitening appear, rapid and proportionate corrective action is
required to prevent irreversible damage to the galvanized surface.

The type and extent of remediation must be guided by the severity and distribution of the observed corrosion.

4.9.1. Case 1: Slight White Rust (Localized, Powdery, Non-Adherent Spots)

Characterized by scattered, soft, superficial corrosion products, typically due to marginal deviations

Recommended action: Initiate a re-passivation cycle, including:

• Restabilization of water chemistry (see 2.2)

• Re-deployment of corrosion coupons for confirmation

No surface cleaning is necessary unless biofilm or debris is also present.

4.9.2. Case 2: Significant White Rust (Widespread, Adherent, or Crystalline Growth)

Usually observed when passivation has failed entirely

The corrosion layer is dense, layered, and may be slightly adherent but non-protective.

Recommended action:

• Perform acid cleaning:
  o This step must be validated case by case depending on the surface condition, galvanizing thickness, and environmental
  constraints.
  o Excessive cleaning may remove zinc coating entirely if poorly controlled.
• Immediately follow with a full natural passivation cycle, under strict monitoring conditions, with renewed water chemistry
  stabilization and coupon tracking.

Note: Acid cleaning is a last-resort solution, not part of a standard commissioning protocol. Any such intervention should be
documented and justified through photographic evidence and SEM/IR layer analysis.

After corrective actions, a full restart of the validation process is required:

• New coupon set deployment

• Analytical validation of surface integrity

• Issuance of a revised passivation certificate, with incident traceability included in the report.

5.Discussion and Perspectives

The implementation of natural passivation protocols for galvanized cooling systems presents both technical opportunities and
operational challenges. Based on field evidence and scientific principles, the approach proves effective when conditions are strictly
controlled—but its reproducibility and scalability remain subject to external variability and human factors.

5.1. Strengths of the Natural Passivation Approach

• Environmentally aligned: No phosphorus, halogens, or heavy metals involved. Meets sustainable water management objectives and avoids regulatory risks.

• Surface integrity: PZHC layers formed under controlled conditions show strong adhesion, structural coherence, and long-term resistance to corrosion.

• No chemical residuals: Unlike phosphate-based passivation, no risk of persistent chemical by-products or interactions with downstream treatments.

• Cost control: Lower chemical consumption and simplified startup protocols reduce commissioning overheads.

5.2. Identified Limitations and Risk Factors

• Narrow operational window: Natural passivation is highly sensitive to pH, alkalinity, conductivity, and oxygenation; even short deviations may compromise success.

• Human dependency: Execution relies on operator expertise, proper interpretation of data, and strict discipline in field practices.

• Incompatibility with fast-track projects: Construction delays or exposure to weather frequently lead to early corrosion that precludes natural layer formation.

• Lack of international standardization: No unified guidelines exist across OEMs, consultants, and operators for natural passivation workflows.

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