{"id":21852,"date":"2025-11-19T18:18:02","date_gmt":"2025-11-19T17:18:02","guid":{"rendered":"https:\/\/www.odyssee-environnement.fr\/?p=21852"},"modified":"2025-12-18T14:02:11","modified_gmt":"2025-12-18T13:02:11","slug":"awt-2025-natural-passivation-white-rust","status":"publish","type":"post","link":"https:\/\/www.odyssee-environnement.fr\/en\/awt-2025-natural-passivation-white-rust\/","title":{"rendered":"From White Rust to Long-Term Protection Mechanisms:"},"content":{"rendered":"\n

Key Factors, Passivation Strategies and Field Applications for Galvanized Cooling Towers<\/h2>\n\n\n\n

Foreword:<\/mark><\/h3>\n\n\n\n

Strategic decisions on passivation must be made based on site-specific factors, which this article does not cover in detail.<\/p>\n\n\n\n

1. Introduction to Galvanized Steel<\/mark><\/h2>\n\n\n\n

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.<\/p>\n\n\n\n

\"\"
<\/figcaption><\/figure>\n\n\n\n

Two distinct passivation pathways exist, each with different chemical origins and long-term implications:<\/p>\n\n\n\n

    \n
  • 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.<\/li>\n\n\n\n
  • In contrast, induced passivation by phosphoric passivation results in the formation of hopeite (Zn\u2083(PO\u2084)\u2082\u00b74H\u2082O), 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.<\/li>\n<\/ul>\n\n\n\n

    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\u2014a loosely adherent, porous corrosion product
    that signals a failed passivation and significantly reduces the durability of the galvanized surface.<\/p>\n\n\n\n

    Operational confusion between these two mechanisms\u2014natural versus induced passivation\u2014has 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.<\/p>\n\n\n\n


    The objective of this article is clearly defined:<\/p>\n\n\n\n

    \u2022 Discuss exclusively natural passivation, based solely on mineral elements naturally present in the process water, excluding<\/p>\n\n\n\n

    any phosphate or additive-based intervention.<\/p>\n\n\n\n

    \u2022 Clarify the mechanism of PZHC formation<\/p>\n\n\n\n

    \u2022 Present a robust, field-validated commissioning strategy to promote enduring PZHC-driven protection of galvanized CT<\/p>\n\n\n\n

    components.<\/p>\n\n\n\n

    \u2022 Propose corrective measures strictly rooted in natural passivation principles.<\/p>\n\n\n\n


    2.Formation of the Protective Oxide Layer on Galvanized Steel<\/mark><\/h2>\n\n\n\n

    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\u2013water interface,
    where oxygen availability, water chemistry, and thermodynamic stability collectively define the layer\u2019s quality and durability.<\/p>\n\n\n\n


    2.1. Electrochemical Pathway to PZHC Formation<\/strong><\/p>\n\n\n\n


    The oxidation of zinc begins at the anode:<\/p>\n\n\n\n

    Zn \u2192 Zn\u00b2\u207a + 2e\u207b<\/strong><\/p>\n\n\n\n

    These products combine to form an initial, metastable precipitate:<\/p>\n\n\n\n

    Zn\u00b2\u207a + 2OH\u207b \u2192 Zn(OH)\u2082<\/strong><\/p>\n\n\n\n

    Protection is provided by PZHC, defined by the formula: Zn\u2085(CO\u2083)\u2082\u208b\u2093(OH)\u2086\u208a\u2082\u2093\u00b7H\u2082O, 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<\/p>\n\n\n\n

    Under the influence of dissolved inorganic carbon\u2014primarily in the form of bicarbonate (HCO\u2083\u207b)\u2014this hydroxide is gradually
    transformed into PZHC, following the reaction:<\/p>\n\n\n\n

    5Zn(OH)\u2082 + 2HCO\u2083\u207b \u2192 Zn\u2085(CO\u2083)\u2082(OH)\u2086 + 2H\u2082O + 2OH\u207b<\/strong><\/p>\n\n\n\n

    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]<\/p>\n\n\n\n

    \"\"
    <\/figcaption><\/figure>\n\n\n\n

    An infrared analysis makes it possible to assess the quality of the PZHC layer formed, based on the following criteria:<\/p>\n\n\n\n

    \u2022 Hydration: Peak at 3400 cm\u207b\u00b9 \/ Peak at 1390 cm\u207b\u00b9 (second peak)<\/p>\n\n\n\n

    The higher the value, the lower the quality of the PZHC.<\/p>\n\n\n\n

    \u2022 Crystallinity: Calculation = (Peak at 1507 cm\u207b\u00b9 + Peak at 1390 cm\u207b\u00b9) \/ Trough between the two peaks at 1446 cm\u207b\u00b9<\/p>\n\n\n\n

    The higher the value, the better the quality of the PZHC. [11]<\/p>\n\n\n\n

    2.2. Role of Water Chemistry Parameters<\/strong><\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

    PARAMETER<\/th>RECOMMENDED RANGE<\/th><\/tr><\/thead>
    pH<\/td>7.4 \u2013 8.5<\/td><\/tr>
    CONDUCTIVITY<\/td>250 \u2013 2\u2019400 \u00b5S\/cm<\/td><\/tr>
    M-ALKALINITY<\/td>100 \u2013 500 ppm (CaCO\u2083)<\/td><\/tr>
    TOTAL HARDNESS (TH)<\/td>50 ppm (Ca\u00b2\u207a\/Mg\u00b2\u207a)<\/td><\/tr>
    DISSOLVED OXYGEN <\/td>Saturated (>6 ppm at 20 \u00b0C)<\/td><\/tr>
    TEMPERATURE<\/td>< 60 \u00b0C (140 \u00b0F)<\/td><\/tr>
    CHLORIDES \/ SULFATES<\/td>< 250 ppm<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    These parameters must be monitored and maintained in real time during the passivation phase. Any deviation\u2014such as high pH from
    caustic leaks or insufficient buffering from low alkalinity\u2014can result in non-adherent Zn(OH)\u2082 or amorphous carbonates, both
    precursors to white rust<\/p>\n\n\n\n

    2.3. Factors to Avoid During Natural Passivation<\/strong><\/p>\n\n\n\n

    To ensure the PZHC layer forms slowly and uniformly, the following must be strictly avoided:<\/p>\n\n\n\n

    CATEGORY<\/th>CRITICAL FACTORS TO AVOID<\/th>IMPACT ON PASSIVATION<\/th><\/tr><\/thead>
    CHEMICALS AND ADDITIVES<\/strong><\/td>– Oxidizing biocides ( e.g., chlorine, bromine, FRC > 0.5 ppm)
    – Corrosion inhibitors: phosphonates, phosphates, orthophosphates – Molybdates – Polymers with corrosion inhibition properties<\/td>    		Disrupts electrochemical equilibirum, promotes porous oxide films or alters natural layer structure<\/td><\/tr>
    HYDRAULIC AND OPERATIONAL DESIGN<\/strong><\/td>– Presence of copper upstream or in recirculation loop
    – No pre-commissioning passivation protocol<\/td>    		Promotes galvanic corrosion anaccelerates zinc loss<\/td><\/tr>
    WATER SYSTEM CONDITIONS<\/strong><\/td>– Water stagnation
    – Deposits, especially at the bottom of the basin<\/td>    		Localized corrosion cells and heterogeneous layer formation<\/td><\/tr>
    CONSTRUCTION PRATICES<\/strong><\/td>– Exposing the CT system to weather before
    commissioning (e.g., rain ingress, UV, airborne dust)<\/td>    		Surface contamination and premature
    corrosion before controlled passivation<\/td><\/tr>
    MATERIAL INTEGRITY<\/strong><\/td>– Poor or non-standard galvanized steel quality (e.g.,
    insufficient zinc layer thicknes<\/td>    		Limits the base material\u2019s ability to form
    protective oxides<\/td><\/tr>
    THERMAL CONDITIONS<\/strong><\/td>Temperatures > 60 \u00b0C (140 \u00b0F) during startup<\/td>Accelerates Zn\u00b2\u207a release \u2192 uncontrolleprecipitation<\/td><\/tr>
    WATER CHEMISTRY INSTABILITY<\/strong><\/td>Cf. 2.2<\/td>Either prevents layer formation or induces
    non-coherent precipitates<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    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.<\/p>\n\n\n\n

    3. White Rust: Misconceptions, Identification and Consequences<\/mark><\/h2>\n\n\n\n

    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<\/p>\n\n\n\n

    The presence of ‘white rust’ can indicate:<\/p>\n\n\n\n

    \u2022 A non-homogeneous protective PZHC layer due to initial surface condition (hard and adherent deposit),<\/p>\n\n\n\n

    \u2022 A non-protective PZHC layer (less hard and non-adherent deposit).<\/p>\n\n\n\n

    3.1. Misconceptions and Common Errors<\/strong><\/p>\n\n\n\n

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

    \u2022 \u201cWhite rust is only cosmetic\u201d: In reality, it reveals localized corrosion and depletion of the protective zinc layer, particularly in areas of low oxygen or unstable pH.<\/p>\n\n\n\n

    \u2022 \u201cAdding corrosion inhibitors will stop it\u201d: If introduced too early, inhibitors (especially phosphates and molybdates) divert the electrochemical pathway, preventing coherent PZHC formation and favoring weak, mixed oxide layers.<\/p>\n\n\n\n

    \u2022 \u201cIt can be safely cleaned with acid\u201d: Acid washing may remove the layer, but also dissolves partially formed passivation, leading to uncontrolled re-corrosion and deeper substrate attack.<\/p>\n\n\n\n

    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\u2019s nature and structure. [12,13]<\/p>\n\n\n\n

    3.2. Mechanisms Leading to White Rust<\/strong> <\/p>\n\n\n\n

    White rust results from an imbalance or disruption in the early passivation reactions. The most common causes are:<\/p>\n\n\n\n

    \u2022 Excessive pH (> 8.5): Accelerates Zn\u00b2\u207a release, precipitating poorly adherent hydroxides.<\/p>\n\n\n\n

    \u2022 Low conductivity (< 250 \u00b5S\/cm) or lack of carbonate alkalinity: Prevents full conversion to PZHC.<\/p>\n\n\n\n

    \u2022 Presence of oxidizing agents (e.g., chlorine): Creates unstable oxides or porous films.<\/p>\n\n\n\n

    \u2022 Water stagnation or non-aerated conditions: Leads to differential aeration corrosion.<\/p>\n\n\n\n

    \u2022 Surface contamination prior to startup: Dust, oils or atmospheric corrosion products inhibit uniform passivation.<\/p>\n\n\n\n

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

    3.3. Practical Identification and Field Implications<\/strong><\/p>\n\n\n\n

    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).<\/p>\n\n\n\n

    \"\"
    <\/figcaption><\/figure>\n\n\n\n

    Operational consequences include:<\/p>\n\n\n\n

    \u2022 Reduced lifespan of galvanized surfaces (accelerated zinc loss)<\/p>\n\n\n\n

    \u2022 Premature loss of hydrophobicity and surface uniformity<\/p>\n\n\n\n

    \u2022 Local under-deposit corrosion risks<\/p>\n\n\n\n

    \u2022 Difficulties in post-facto treatment and irreversible aesthetic degradation<\/p>\n\n\n\n

    In severe cases, white rust development within the first month of startup can lead to complete failure of the protective coating within
    12\u201318 months, particularly in high-temperature, low-hardness systems [3,16]<\/p>\n\n\n\n

    4. Field-Based Passivation Strategies: Protocol and Operational Execution<\/mark><\/h2>\n\n\n\n

    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.<\/p>\n\n\n\n

    4.1. Initial Assessment and System Baseline<\/strong><\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

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

    4.2. Deployment of Monitoring Coupons<\/strong><\/p>\n\n\n\n

    Corrosion coupons should be installed in representative areas of the system to allow direct measurement of corrosion rates and layer
    morphology.<\/p>\n\n\n\n

    If available, OEM reference coupons from the CT manufacturer can be used for comparative evaluation against standard protocols.<\/p>\n\n\n\n

    4.3. Water Chemistry Control and Concentration Ratio Adjustment<\/strong><\/p>\n\n\n\n

    The concentration ratio must be adjusted during the passivation phase to maintain the parameters of 2.2.<\/p>\n\n\n\n

    Avoid active pH regulation systems, which often destabilize M-alkalinity and introduce variability detrimental to PZHC layer formation.<\/p>\n\n\n\n

    4.4. Real-Time Monitoring and Instrumentation<\/strong><\/p>\n\n\n\n

    Ensure continuous online monitoring and data logging for the following critical parameters:<\/p>\n\n\n\n

    \u2022 pH<\/p>\n\n\n\n

    \u2022 Conductivity<\/p>\n\n\n\n

    \u2022 Redox potential<\/p>\n\n\n\n

    \u2022 Flow rate<\/p>\n\n\n\n

    \u2022 Temperature<\/p>\n\n\n\n

    These data serve both as a real-time control tool and a traceable record for validation and audit purposes.<\/p>\n\n\n\n

    4.5. Controlled Chemical Dosing<\/strong><\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

    As a reminder, the formation of a PZHC passive layer is the primary objective during the first month of operation of a cooling tower.<\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

    \"\"
    <\/figcaption><\/figure>\n\n\n\n

    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.<\/p>\n\n\n\n

    \n
    \n

    City Water
    <\/strong>Reference: BD817
    This sample contains PZHC as well as calcium
    carbonate in the form of aragonite (peaks
    labeled A). Due to spectral interference from
    the aragonite carbonate peak at 1481 cm\u207b\u00b9,
    it would be inadvisable to assign a
    crystallinity index value. However, the
    hydration index suggests the presence of a
    high-quality PZHC layer.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water Concentrated 1.2\u00d7
    <\/strong>Reference: BD824
    The spectral peaks clearly correspond to
    PZHC. The crystallinity is moderate. Traces of
    calcium carbonate in the form of aragonite
    cannot be ruled out, due to the small peak
    observed at 856 cm\u207b\u00b9.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water Concentrated 2\u00d7
    <\/strong>Reference: BD825
    PZHC is also present in this sample, but its
    quality is lower than that obtained with non-
    concentrated city water. All indices confirm
    this observation: the crystallinity index
    drops, and the hydration index is high.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    Softened Water
    <\/strong>Reference: BD837<\/p>\n\n\n\n


    The spectrum reveals the presence of high-
    quality PZHC. The hydration index and the
    crystallinity index indicate that the resulting
    corrosion product is effectively protective.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    Demineralized Water<\/strong>
    Reference: BD838<\/p>\n\n\n\n


    The spectrum clearly indicates the presence
    of poorly crystallized zinc oxide (ZnO) and
    the absence of PZHC.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 100 ppm Proprietary Blend of
    Co-Passivating Agents [17, 18,19]<\/strong>
    Reference: BD822<\/p>\n\n\n\n


    All spectral peaks correspond to PZHC,
    except for a few organic contamination
    signals around 2927 cm\u207b\u00b9. Here too, the
    quality appears good. It is comparable to
    that obtained with softened water, with
    similar crystallinity and hydration index
    values.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 100 ppm Proprietary Blend of
    Co-Passivating Agents \/ Corrosion Inhibitor<\/strong>
    Reference: BD819<\/p>\n\n\n\n


    The sample reveals a complex mixture in
    which PZHC is present (peaks at 1506, 1398,
    1043, and especially 825 cm\u207b\u00b9), but with poor
    crystallinity that could not be quantitatively
    assessed.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 100 ppm Proprietary Blend of
    Co-Passivating Agents \/ Isothiazolone<\/strong>
    Reference: BD820<\/p>\n\n\n\n


    In this sample, the spectrum shows that all
    major peaks can be attributed to PZHC, with
    the exception of signals related to organic
    matter. Index calculations indicate that the
    PZHC layer has relatively low crystallinity.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 100 ppm Proprietary Blend of
    Co-Passivating Agents \/ Isothiazolone + 20
    ppm Biodispersant<\/strong>
    Reference: BD826<\/p>\n\n\n\n


    The deposit formed consists primarily of
    calcium carbonate in the form of calcite
    (peaks labeled C) and shows significant
    organic contamination. No evidence of PZHC
    could be identified. The two broad signals
    centered around 1045 and 482 cm\u207b\u00b9 strongly
    suggest the presence of amorphous
    deposits.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 100 ppm Proprietary Blend of
    Phosphonates \/ Corrosion Inhibitor \/ Quats<\/strong>
    Reference: BD818<\/p>\n\n\n\n


    The peaks at 1632, 1063, and 592 cm\u207b\u00b9 do
    not exactly match those of standard PZHC.
    The spectrum suggests a complex mixture
    containing PZHC. Its complexity points to the
    presence of a phosphate-based matrix\u2014or
    possibly silicates, if the additive identity
    were unknown. In any case, given the
    hydration index, the PZHC present appears
    to be quasi-amorphous or very poorly
    crystallized.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 10 ppm Phosphonates<\/strong>
    Reference: BD823<\/p>\n\n\n\n


    The spectrum of this sample closely
    resembles that of the previous one. The
    overall structure appears to be amorphous
    or only very weakly crystallized.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    City Water + 100 ppm Proprietary Blend of
    Phosphonates<\/strong>
    Reference: BD821<\/p>\n\n\n\n


    The spectrum of this sample closely
    resembles the previous two. The overall
    deposit also appears to be amorphous or
    only very weakly crystallized.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n
    \n
    \n

    Untested Coupon (Blank)
    <\/strong>Reference: BD844

    The near absence of any deposit makes
    analysis difficult, resulting in a spectrum of
    average quality. However, no PZHC was
    detected.<\/p>\n<\/div>\n\n\n\n

    \n
    \"\"
    <\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n

    4.5.1. Influence of Water Quality Alone<\/strong><\/p>\n\n\n\n

    Test conditions showed that water without additives can still lead to PZHC formation, particularly in:<\/p>\n\n\n\n

    \u2022 Raw tap water (city water), even at 1.0\u00d7 and 1.2\u00d7 concentration: moderate to good layer quality<\/p>\n\n\n\n

    \u2022 Softened water: resulted in a high-quality PZHC layer<\/p>\n\n\n\n

    \u2022 Demineralized water: failed to form PZHC; instead, zinc oxide (ZnO) was detected, indicating the absence of protective passivation<\/p>\n\n\n\n

    This confirms that minimal mineral content (alkalinity, hardness) is essential for PZHC development.<\/p>\n\n\n\n

    4.5.2. Effects of Co-Passivating Agents<\/strong><\/p>\n\n\n\n

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

    Proprietary Blend of Co-Passivating Agents in city water produced a highly crystalline PZHC layer, comparable to that formed in
    softened water.<\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

    4.5.3. Impact of Additive Interferences<\/strong><\/p>\n\n\n\n

    The formation of PZHC was disrupted or inhibited in the presence of:<\/p>\n\n\n\n

    \u2022 Phosphonate corrosion inhibitors: yielded poorly crystalline or amorphous deposits<\/p>\n\n\n\n

    \u2022 Biodispersants: led to layers rich in calcium carbonate (calcite) with no identifiable PZHC<\/p>\n\n\n\n

    \u2022 Certain phosphate-based blends: resulted in amorphous or mixed deposits with poor PZHC signature<\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

    4.5.4. Optimal Observed Conditions<\/strong><\/p>\n\n\n\n

    The best results were obtained:<\/p>\n\n\n\n

    \u2022 In terms of surface appearance: with city water + 100 ppm of the proprietary Blend of Co-Passivating Agent<\/p>\n\n\n\n

    \u2022 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.<\/p>\n\n\n\n

    4.6. Routine Field Inspection<\/strong><\/p>\n\n\n\n

    Weekly on-site inspections are strongly recommended throughout the 4\u201312-weeks passivation window. These should include:<\/p>\n\n\n\n

    \u2022 On-site water sampling and analysis<\/p>\n\n\n\n

    \u2022 Visual inspection of CT surfaces, coil exteriors, basin bottom<\/p>\n\n\n\n

    \u2022 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.<\/p>\n\n\n\n

    4.7. End-of-Phase Validation<\/strong><\/p>\n\n\n\n

    After 4 to 12 weeks, corrosion coupons must be retrieved and subjected to:<\/p>\n\n\n\n

    \u2022 Gravimetric weight loss analysis (per ASTM G1 or equivalent)<\/p>\n\n\n\n

    \u2022 Surface morphology evaluation using optical or infrared methods<\/p>\n\n\n\n

    \u2022 Optional SEM\/EDX for composition and PZHC structure verification by experts<\/p>\n\n\n\n

    4.8. Documentation and Certification<\/strong><\/p>\n\n\n\n

    A comprehensive passivation report must be drafted, including:<\/p>\n\n\n\n

    \u2022 Time-series data of monitored parameters<\/p>\n\n\n\n

    \u2022 Photographic records<\/p>\n\n\n\n

    \u2022 Laboratory and field analyses<\/p>\n\n\n\n

    \u2022 Interpretation of coupon results<\/p>\n\n\n\n

    \u2022 Visual assessment of galvanized surfaces<\/p>\n\n\n\n

    Upon successful review, issue a Passivation Certificate confirming the quality and durability of the PZHC layer. This document should
    include:<\/p>\n\n\n\n

    \u2022 Coupon analysis results<\/p>\n\n\n\n

    \u2022 Layer validation outcomes<\/p>\n\n\n\n

    \u2022 Compliance with expected parameters<\/p>\n\n\n\n

    \u2022 Approval from technical field supervisor<\/p>\n\n\n\n

    4.9. Corrective Actions in Case of White Rust<\/strong><\/p>\n\n\n\n

    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.<\/p>\n\n\n\n

    The type and extent of remediation must be guided by the severity and distribution of the observed corrosion.<\/p>\n\n\n\n

    4.9.1. Case 1: Slight White Rust (Localized, Powdery, Non-Adherent Spots)<\/strong><\/p>\n\n\n\n

    Characterized by scattered, soft, superficial corrosion products, typically due to marginal deviations<\/p>\n\n\n\n

    Recommended action: Initiate a re-passivation cycle, including:<\/p>\n\n\n\n

    \u2022 Restabilization of water chemistry (see 2.2)<\/p>\n\n\n\n

    \u2022 Re-deployment of corrosion coupons for confirmation<\/p>\n\n\n\n

    No surface cleaning is necessary unless biofilm or debris is also present.<\/p>\n\n\n\n

    4.9.2. Case 2: Significant White Rust (Widespread, Adherent, or Crystalline Growth)<\/strong><\/p>\n\n\n\n

    Usually observed when passivation has failed entirely<\/p>\n\n\n\n

    The corrosion layer is dense, layered, and may be slightly adherent but non-protective.<\/p>\n\n\n\n

    Recommended action:<\/p>\n\n\n\n