Source: Liu Y., Gu T., Liu M., Wang Z., Cao G., Guo Q., Wang C. (2024). Corrosion mechanism of a high corrosion-resistance Zn. Al. Mg coating in typical extremely harsh marine and cold environments. Journal of Materials Research and Technology, 33, 4290. 4302. https://doi.org/10.1016/j.jmrt.2024.10.080. Open access, CC BY-NC-ND 4.0.
ZAM (Zinc-Aluminum-Magnesium) alloy coating is now the default specification for solar mounting steel in C3. C5 corrosion environments. Lab data and accelerated salt spray tests tell one part of the story. This 2024 paper from the Corrosion Centre, Institute of Metal Research, Chinese Academy of Sciences tells another: what actually happens to a ZAM-coated steel panel over 24 months in two of the most severe real-world environments on record. A tropical marine coast and a sub-arctic city.
All data below is taken directly from the paper.
1. Test Material
The coating was produced by HBIS (Hebei Iron & Steel Group) using a hot-dip process on cold-rolled carbon steel sheet (SPCC substrate).
| Parameter | Value |
|---|---|
| Coating mass | 275 g/m² |
| Coating thickness | ~20 µm |
| Substrate | SPCC cold-rolled carbon steel |
| Phases present | Zn-rich (η), binary Zn-MgZn₂ eutectic, ternary Zn-Al-MgZn₂ eutectic |
2. Test Sites and Exposure Setup
Panels were mounted on racks at 45° tilt. One side facing the sky (skyward), the other facing the ground (field-ward). And exposed for 6, 12, and 24 months at both locations simultaneously.
| Site | Location | Climate Type | Conditions |
|---|---|---|---|
| Wanning | Hainan Province, South China Sea coast | Hot tropical marine | High temperature and humidity, significant chloride deposition from sea spray |
| Mohe | Heilongjiang Province, ~53°N | Extremely cold urban | Sub-arctic, winter temperatures to −40°C, low chloride, sulfate-dominated atmosphere |
3. Mass Loss and Corrosion Rate. 24 Months
The Wanning site is substantially more corrosive. Mass loss at 24 months in Wanning is approximately 2.4× that of Mohe:
| Site | 6 months (g/m²) | 12 months (g/m²) | 24 months (g/m²) |
|---|---|---|---|
| Wanning (marine) | ~4 | ~12.5 | ~24 |
| Mohe (cold) | ~2 | ~3.5 | ~10 |
The corrosion rate curves are not monotonic. At Wanning, the rate peaks sharply at 12 months (~17 g/m²/a) before declining to ~11.5 g/m²/a at 24 months. This peak reflects the transition from rapid early corrosion of the MgZn₂ eutectic to formation of a stabilizing protective product layer. At Mohe, the rate dips at 12 months (~2.6 g/m²/a) then rises again to ~6.5 g/m²/a at 24 months. A different kinetic pattern linked to cold-climate electrochemical suppression and its eventual lifting as the protective layer matures.
4. Marine Atmosphere. Wanning
4.1 Surface Morphology (SEM)
At 6 months (skyward), the surface shows initial bright corrosion product deposits. By 12 months, granular crystalline particles cover more uniformly. By 24 months, a dense, cracked product layer has formed. On the field-ward side the progression is more aggressive: significant pitting visible at 6 months, with larger irregular particle clusters and deeper corrosion at 12 and 24 months.
4.2 Cross-Section: Where Corrosion Initiates and Propagates
Corrosion initiates in the ternary eutectic phase (the MgZn₂-containing region) and propagates inward along the eutectic network toward the zinc-rich phase boundary. The Zn-rich dendritic phase is electrochemically noble and remains structurally intact for longer, serving as a scaffold while the eutectic is consumed. By 12 months (field-ward panel e), the paper identifies “preferential corrosion” of the eutectic phase with a clear pit at the eutectic-Zn-rich boundary. By 24 months, internal cracking and extensive void formation are visible within the coating.
4.3 Elemental Distribution. EDS Maps
Key findings from the EDS maps:
- Zn: Fills the bulk coating matrix (Zn-rich phase)
- Al: Concentrated in large rounded grains (Al-rich phase within eutectic)
- S: Present in matrix and surface pockets. SO₂-derived sulfate products
- Cl: Concentrated in localized pockets near the outer surface, more pronounced field-ward. Signature of simonkolleite and chloride products from sea spray
The Cl concentration difference between skyward and field-ward is critical: the field-ward side accumulates chloride from surface runoff and retained moisture, driving faster eutectic dissolution.
4.4 XRD. Corrosion Product Identification
The XRD data identifies chloride-dominated corrosion products at Wanning, reflecting the marine atmosphere:
| Symbol | Phase | Formula | Role |
|---|---|---|---|
| α | Simonkolleite | Zn₅(OH)₈Cl₂·H₂O | Primary barrier. Dominant skyward at 24m |
| ε | Zinc sodium chloride sulfate | NaZn₄SO₄(OH)₆Cl·6H₂O | Early-forming product |
| β | Zinc hydroxysulfate | Zn₄SO₄(OH)₆·H₂O | Protective, SO₂-derived |
| δ | Gordaite-like phase | Zn₄SO₄(OH)₆·5H₂O | Prominent skyward at 12m |
| γ | Pentahydrite | MgSO₄·5H₂O | Mg-derived from MgZn₂ dissolution |
| χ | Zinc sulfate chloride hydroxide | Zn₁₂(SO₄)₃Cl₃(OH)₁₅·xH₂O | Field-ward, early formation |
Simonkolleite (Zn₅(OH)₈Cl₂·H₂O) becomes dominant by 24 months on the skyward surface. It is a dense, low-solubility zinc hydroxide chloride that fills pits and micro-cracks, blocking further ingress. The Mg²⁺ released from MgZn₂ dissolution stabilizes this compound and also participates in forming MgAl-LDH (Mg₆Al₂(OH)₁₆CO₃·4H₂O) and ZnAl-LDH (Zn₂Al(OH)₆(CO₃)₀.₅·xH₂O). Layered double hydroxides with strong barrier properties.
4.5 Electrochemical Analysis. Polarization Resistance
EIS analysis quantifies the difference between skyward and field-ward protection at Wanning. The skyward side maintains 3. 6× higher polarization resistance (Rp) throughout the 24-month exposure:
| Side | 6 months (kΩ·cm²) | 12 months (kΩ·cm²) | 24 months (kΩ·cm²) |
|---|---|---|---|
| Skyward | ~97 | ~63 | ~76 |
| Field-ward | ~16 | ~4 | ~23 |
The skyward Rp arc diameter at 6 months is approximately 80 kΩ·cm² in the Nyquist plot, declining over time as the outer product layer gradually becomes more porous. The partial recovery at 24 months corresponds to stabilization of the simonkolleite barrier layer. Field-ward Rp drops to just 4 kΩ·cm² at 12 months. The lowest point in the dataset. Before partially recovering at 24 months.
5. Cold Urban Atmosphere. Mohe
5.1 Surface Morphology (SEM)
At Mohe, the degradation mode is localized pitting rather than the eutectic network dissolution seen at Wanning.
- 6m skyward: Oxide film spalling. Localized detachment of the thin surface oxide
- 12m skyward: Dendritic crystalline structures; linear cracks appear
- 24m skyward: Prominent cracks at 10.95 µm × 0.83 µm
- 12m field-ward: Dense particle clusters; inset reveals crack at 2 µm scale
- 24m field-ward: Multiple elongated cracks at 5.32 µm × 2.32 µm
5.2 Cross-Section: Pit Dimensions Over Time
Pits form in the eutectic phase near the Zn-rich phase boundary. Measured dimensions:
| Condition | Pit width (µm) | Pit depth (µm) |
|---|---|---|
| Skyward, 6m | . | Initial product layer: 0.25 µm |
| Skyward, 12m | 4.56 | 4.37 |
| Skyward, 24m | 14.23 | 6.06 |
| Field-ward, 12m | 3.83 | 6.51 |
| Field-ward, 24m | 12.18 | 7.43 |
5.3 Pit Shape Analysis. Depth/Width Ratio (β)
The β (depth/width) ratio tracks whether pits are growing deeper (β > 1, penetrating) or broadening (β < 1, shallow):
| Condition | Depth (µm) | Width (µm) | β | Shape |
|---|---|---|---|---|
| 12m skyward | ~4.4 | ~4.6 | ~0.95 | Roughly equiaxed |
| 12m field-ward | ~6.5 | ~3.9 | ~1.7 | Deep, narrow. Highest penetration risk |
| 24m skyward | ~6.1 | ~14.2 | ~0.45 | Wide, shallow |
| 24m field-ward | ~7.4 | ~12.2 | ~0.65 | Broader, less penetrating |
The field-ward sample at 12 months has β = 1.7. Pits significantly deeper than wide. This is the highest substrate-penetration risk morphology in the dataset. By 24 months, all β values are below 1. The protective product layer has filled the pit bases and redirected attack laterally rather than vertically.
5.4 XRD. Corrosion Products at Mohe
Mohe’s corrosion products are sulfate-dominated with no chloride compounds. Consistent with an inland cold atmosphere:
| Phase | Formula | First detected |
|---|---|---|
| Zinc (metallic) | Zn | 6m (dominant throughout) |
| Magnesium zinc intermetallic | MgZn₂ | 6m; visible field-ward at 24m |
| Zinc hydroxysulfate hydrate | Zn₄SO₄(OH)₆·4H₂O | 12m |
| Zinc sulfate | ZnSO₄ | 12m |
| Aluminium oxide | Al₂O₃ | 24m (skyward) |
| Boehmite | AlOOH | 24m (skyward) |
The appearance of Al₂O₃ and AlOOH at 24 months indicates the Al-rich phase is contributing to product formation at cold temperatures. The absence of chloride compounds confirms the corrosion mechanism at Mohe is fundamentally sulfate-driven. Not chloride-driven. The aluminum oxides develop more slowly here than the LDH compounds seen in the marine case.
5.5 Electrochemical Analysis. Mohe
EIS Nyquist impedance magnitudes at Mohe are much smaller than at Wanning, reflecting lower overall corrosion activity in the cold environment. At 6 months, the skyward side shows a steep diagonal (diffusion-controlled, protective initial layer), while the field-ward side shows a small semicircle. At 12 months, the skyward arc grows substantially. The protective layer consolidating. By 24 months, both sides show a drastic reduction in impedance, consistent with the paper’s finding that after 24 months at Mohe, the two sides exhibit similar corrosion rates. Unlike the persistent divergence seen at Wanning.
6. Corrosion Mechanism
The paper’s mechanism schematic shows three stages:
Stage 1. Pristine coating. Zn-rich phase (large dendrites), binary Zn-MgZn₂ eutectic (lamellar), and ternary Zn-Al-MgZn₂ eutectic (fine) on SPCC steel substrate. No corrosion product.
Stage 2. Thin liquid film. O₂ dissolves into the water film. MgZn₂ corrodes preferentially (least noble phase), releasing Mg²⁺ and Zn²⁺ ions. Primary products ZnO and Zn(OH)₂ nucleate on the surface.
Stage 3. Protective product formation. Atmospheric pollutants enter the film and react. Three key reactions produce the barrier layer:
- Zn(OH)₂ + 4Zn²⁺ + 6OH⁻ + 2Cl⁻ + H₂O → Zn₅(OH)₈Cl₂·H₂O (simonkolleite). The primary barrier in marine environments
- Zn(OH)₂ + 4Zn²⁺ + 4OH⁻ + 2CO₃²⁻ → Zn₅(OH)₆(CO₃)₂ (hydrozincite). Forms in both environments
- 3ZnO + Zn²⁺ + 7H₂O + SO₄²⁻ → Zn₄(SO₄)(OH)₆·4H₂O (zinc hydroxysulfate). Dominant in cold and inland environments
In the marine environment (Wanning), Cl⁻ drives reaction 1, producing dense simonkolleite that fills pits and cracks. In the cold environment (Mohe), CO₂ and SO₄²⁻ drive reactions 2 and 3, producing a similar barrier without the chloride reinforcement. Explaining why the pit shape factor at Mohe shows sharp β > 1 pits at 12 months before broadening to β < 1 at 24 months.
7. Full Comparison
| Parameter | Marine. Wanning | Cold. Mohe |
|---|---|---|
| 24-month mass loss | ~24 g/m² | ~10 g/m² |
| Peak corrosion rate | ~17 g/m²/a (at 12m) | ~6.5 g/m²/a (at 24m) |
| Primary corrosion mode | Eutectic network dissolution | Localized pitting in eutectic |
| Pit shape at 12m field-ward | Inward propagation along eutectic | Sharp deep pits (β = 1.7) |
| Pit shape at 24m | Dense product layer forms | Pits broaden (β → 0.65) |
| Sky vs. field-ward rate | Sky < field-ward; diverges over time | Sky < field-ward; converges at 24m |
| Dominant corrosion products | Simonkolleite (Cl-driven) | Zinc hydroxysulfate (SO₄-driven) |
| Skyward Rp at 6m | ~97 kΩ·cm² | High (diffusion-controlled) |
| Field-ward Rp at 6m | ~16 kΩ·cm² | Low |
8. What This Means for Solar Mounting Specification
Coating weight matters most in marine environments. At Wanning, 24-month mass loss is ~24 g/m² from a 275 g/m² coating. About 8.7% consumed in two years. In a 25-year asset life, higher initial coating weight (ZM310, nominally 310 g/m²) provides more sacrificial reserve. The accelerating rate at Wanning (peaking at 17 g/m²/a at 12 months) shows the first two years are the critical period before the protective layer stabilizes.
Field-facing surfaces are the weak point. The field-ward side consistently shows 3. 6× lower polarization resistance than the skyward side at Wanning, and consistently higher early corrosion rates at Mohe. For ground mount piles, the pile wall exposed to soil and splash is effectively the “field-ward” surface. The Rp data quantifies why drainage design and avoiding moisture-trapping section geometries matter for long-term durability.
Cold environments produce a different failure mode. At Mohe, the sharp pit morphology (β = 1.7) at 12 months is the highest substrate-penetration risk in the study. More localized and concentrated than the distributed eutectic dissolution at Wanning. For installations in sub-arctic or high-altitude conditions with freeze-thaw cycling, ZAM’s 141 HV hardness (vs. 64 HV for standard hot-dip galvanized) directly addresses coating delamination at stress concentration points.
The self-healing mechanism works in both environments. Both sites show pit broadening (β declining toward <1) between 12 and 24 months, confirming that the simonkolleite and zinc hydroxysulfate product layers form and redirect corrosion laterally. The mechanism functions in cold conditions. Just more slowly. The 24-month data confirms it works.
Source
Liu Y., Gu T., Liu M., Wang Z., Cao G., Guo Q., Wang C. (2024). Corrosion mechanism of a high corrosion-resistance Zn. Al. Mg coating in typical extremely harsh marine and cold environments. Journal of Materials Research and Technology, Vol. 33, pp. 4290. 4302. DOI: 10.1016/j.jmrt.2024.10.080. Published under Creative Commons CC BY-NC-ND 4.0. Full paper available without subscription.
Figures reproduced under CC BY-NC-ND 4.0 with attribution to the original authors.
Solaracks specifies ZAM alloy coating as the standard for ground mount racking, pile foundations, and cable tray systems in ISO corrosivity categories C3 and above. Contact our engineering team for coating grade recommendations by project location and exposure class.