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HG/T 3668-2020 Type 2 · Dry film zinc content ≥70%

Epoxy zinc-rich primer Epoxy Zinc-Rich Primer

Two-component epoxy polyamide/zinc-rich heavy-duty anticorrosive primer — sacrificial anode cathodic protection

Epoxy Zinc-Rich Primer is a high-performance two-component anticorrosive primer that uses epoxy resin and polyamide curing agent as the film-forming material, with ultrafine zinc powder (dry film content ≥70%) as the main antirust pigment. The coating provides active corrosion protection through the sacrificial anode cathodic protection mechanism of zinc powder on steel—even if the paint film is locally damaged, the surrounding zinc powder continues to provide electrochemical protection for the exposed steel. It complies with the Class 2 standard of HG/T 3668-2020 "Epoxy Zinc-Rich Primer." When used in combination with an epoxy micaceous iron oxide intermediate coat and a polyurethane/fluorocarbon topcoat, the system achieves neutral salt spray resistance ≥1000h, meeting the requirements of ISO 12944 C4-C5 high-corrosion environments. It is widely used as an anticorrosive primer for major projects such as sea-crossing bridges, offshore oil platforms, petrochemical storage tanks, transmission towers, wind turbine towers, and large steel structure factories.

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≥70%

zinc powder content in the dry film

1000h+

System salt-spray resistance

≥6MPa

pull-off adhesion

≥60%

Volume solids

HG/T3668

Product applicable standard

Sa2.5

Substrate preparation grade

5~40°C

Application ambient temperature

2K/Two-Component10:1

Mixing ratio

ISO12944

C4-C5 corrosion level

Product Overview

ProductsOverview

Product overview: who is it for? what problem does it solve?

Epoxy zinc-rich primer is a heavy-duty anticorrosive primer designed for steel structures requiring long-term corrosion protection of over 15 years in C4-C5 high-corrosion environments.The core problem it solves is that ordinary anti-rust paint fails within 3–5 years in harsh environments such as coastal areas, chemical atmospheres, and high humidity. In contrast, epoxy zinc-rich primer, through the dual mechanism of "cathodic protection + barrier protection," can provide 15–25 years of long-term protection when used with an intermediate coat and topcoat—significantly reducing the full life cycle maintenance cost.

Product definition

Epoxy Zinc-Rich Primer, Chinese standard number HG/T 3668-2020, is a two-component (2K) ambient temperature crosslinking and curing anticorrosive primer using epoxy resin (bisphenol A type, CAS 25068-38-6) and polyamide curing agent (CAS 68410-23-1) as film-forming substances, ultrafine spherical zinc powder (CAS 7440-66-6, particle size 3-8 μm, mass fraction in dry film ≥70%) as anticorrosive pigment, and xylene/n-butanol as the solvent system.

Corrosion protection mechanism:High-content zinc powder particles in the coating form continuous conductive pathways between themselves and with the steel substrate. Since the standard electrode potential of zinc (-0.76V vs SHE) is lower than that of iron (-0.44V), zinc acts as the anode in corrosion microcells and preferentially undergoes oxidation (Zn → Zn²⁺ + 2e⁻). The released electrons flow through the conductive network to the steel surface, cathodically polarizing the steel below its immune potential—this is known as "sacrificial anode cathodic protection." Meanwhile, the corrosion products of zinc (ZnO, Zn(OH)₂, basic zinc carbonate, etc.) gradually fill the coating pores, forming a dense secondary barrier layer—the "corrosion product self-healing effect."

Key performance data:

The dry film zinc powder content is ≥70% (compliant with HG/T 3668-2020 Category 2).
Volume solids content ≥60% (ISO 3233:1998)
Pull-off adhesion ≥6 MPa (ISO 4624:2016, test doll diameter 20mm)
Single coating resistance to neutral salt spray (NSS) ≥400h (ISO 9227:2022, rust width at the crisscrossing ≤2mm)
The complete system (epoxy zinc-rich 80μm + epoxy micaceous iron oxide intermediate coat 150μm + polyurethane topcoat 80μm) has an NSS resistance of ≥1000h.
Mixed application period: 4-8 hours (23°C, main agent: curing agent = 10:1 w/w)

Suitable substrates and limitations:Applicable to carbon steel, low-alloy steel, and cast iron substrates blasted to Sa2.5 (ISO 8501-1:2007). Not recommended for direct application on galvanized steel (the passivation film on the zinc surface will block the conductive path—sweep blasting pretreatment or an epoxy primer transition layer is advised), stainless steel (the surface passivation film must be removed first), or aluminum alloy (risk of galvanic corrosion—the potential difference between zinc and aluminum is too large). Application temperature limits: standard type 5–40°C (below 5°C, the epoxy-amine crosslinking reaction rate is extremely low, resulting in incomplete coating curing).

Industry standing and applications:Epoxy zinc-rich primer is one of the most widely used primer types in global heavy-duty anticorrosive coating systems (ISO 12944-5:2019 systems A2.08/A3.08/A4.08, etc.), forming a classic triple protection system together with epoxy micaceous iron oxide intermediate paint and polyurethane/fluorocarbon/polysiloxane topcoat. Typical application projects include: Japan's Akashi Kaikyo Bridge (opened in 1998, design life of 100 years), Hong Kong-Zhuhai-Macao Bridge (opened in 2018, 500,000 m² of steel box girders), and the Beihai offshore oil platform (NORSOK M-501 system certified). According to Grand View Research data, the global zinc-rich primer market size in 2023 was approximately USD 2.8 billion, with the Asia-Pacific region accounting for a 48% share.

Features

ProductsFeatures

1. Cathodic protection (sacrificial anode) mechanism

The dry film of the coating contains ≥70% zinc powder, forming a continuous conductive network. The standard electrode potential of zinc (-0.76V) is lower than that of iron (-0.44V). When corrosion occurs, zinc is preferentially oxidized (Zn → Zn²⁺ + 2e⁻), releasing electrons that cathodically polarize the steel to an immune potential—even if the coating has 3-5mm of damage, the surrounding zinc powder can still provide continuous electrochemical protection for the exposed steel. This makes the anti-corrosion performance of epoxy zinc-rich primer far superior to that of ordinary anti-rust primers that rely solely on physical shielding.

2. Superior corrosion resistance and durability

Single coating NSS ≥ 400h (HG/T 3668 Type 2), system NSS ≥ 1000h after matching with epoxy micaceous iron oxide intermediate paint + polyurethane topcoat. Designed service life of 15-25 years under ISO 12944-2 C5-M very high corrosivity environment (marine splash zone). The Akashi Kaikyo Bridge in Japan adopted an epoxy zinc-rich primer system in 1998, and the coating has remained intact for over 25 years—actual service life far exceeds the design value.

3. Excellent compatibility

It exhibits excellent interlayer adhesion (pull-off test ≥5 MPa) with epoxy micaceous iron oxide intermediate paint (MIO barrier layer), polyurethane topcoat (weather-resistant decorative layer), fluorocarbon topcoat (super weather-resistant, ≥25 years), and polysiloxane topcoat (high weather resistance + high chemical resistance). The combination of epoxy zinc-rich primer, epoxy micaceous iron oxide intermediate paint, and topcoat is the most classic heavy-duty anti-corrosion three-coat system in the ISO 12944-5 standard—where the primer provides cathodic protection, the intermediate coat supplies thickness and physical barrier, and the topcoat offers weather resistance and decoration.

4. Wide adaptability to construction

Supports airless spray (nozzle 0.43–0.53 mm, 15–25 MPa), air spray, and brush/roller application (for small-area touch-ups). Single-pass dry film thickness: 60–80 μm. Mixing ratio: 10:1 (w/w). Pot life: 4–8 h (23°C). Wide temperature range for application (5–40°C); low-temperature formulation can be applied down to –5°C. Continuous stirring is required during spraying to prevent zinc powder sedimentation—use of a pressure pot with a pneumatic agitator is recommended to ensure coating uniformity.

5. Shop Primer Solderability

Specialized formulation (dry film 15-25μm) can be used as a shop primer—after application in the steel structure prefabrication plant, the steel remains suitable for welding and cutting. Welding fumes comply with occupational exposure limits (GBZ 2.1-2019), and the pass rate of X-ray inspection of welds is not affected. Suitable for automated spray painting on steel structure production lines—parts can be handled and stacked within 3-5 minutes after spraying. Note: Only the specialized shop primer formulation is weldable—the standard type with a dry film thickness of 60-80μm is not suitable for welding.

6. Environmental Compliance · Global Standards

Complies with HG/T 3668-2020 (China) Standard Class 2, ISO 12944-5:2019 system numbers A2.08/A3.08/A4.08, NORSOK M-501:2022 (Norwegian Petroleum Industry) System 1, SSPC Paint 20 (USA) Zinc-rich Primer Type II (≥77% zinc powder). VOCs<420g/L（GB 30981-2020），可passedROHS 2011/65/EU和REACH (EC) 1907/2006ComplianceTesting。Products碳足迹约2.8kg CO₂e/kg（摇篮到大门，SimaPro 9.5计算）——低于Inorganic zinc-rich primer（3.5kg CO₂e/kg）。

Technical Specifications

TechnicalParameters

Physicochemical parameters

Product typeTwo-component epoxy polyamide/zinc powder coating (2K EP/Zn)

zinc powder content in the dry film≥70% (HG/T 3668-2020 Category 2)

Volume solids60±3%（ISO 3233:1998）

Solid content by mass85±3%（GB/T 1725-2007）

Density (mixed)2.10-2.30 g/cm³（20°C）

Mix ratio (by weight)Main agent: curing agent = 10:1

Pot life4-8h（23°C）/ 2h（35°C）

Recommended dry film thickness (DFT)60-80μm / channel

Theoretical spreading rate0.17-0.22 kg/m²（75μm DFT）

VOC content<420 g/L（GB 30981-2020）

Packaging Specs30kg base agent + 3kg hardener kit

Drying and performance

Surface-dry time≤1h (23°C, GB/T 1728-2020 Method B)

Dry-through time≤24h (23°C, GB/T 1728-2020 Method A)

Full cure7 days (23°C, Tg stable by DSC method)

Minimum recoat interval8h（23°C）

Maximum recoat interval7 days (23°C; sand removal is required if the expiration date is exceeded)

pull-off adhesion≥6 MPa（ISO 4624:2016，dolly Ø20mm）

Cross-cut adhesion≤1 level (ISO 2409:2020, spacing 1mm)

Impact resistance≥50 kg·cm（GB/T 1732-2020）

neutral salt spray (NSS) resistanceSingle coating ≥400h / Complete system ≥1000h (ISO 9227:2022)

Continuous service temperature-20°C ~ 150°C

Basis of technical parameters
• March 2024, National Coatings Quality Inspection and Testing Center Test Report (No.: NTC-2024-0315-ZR)
HG/T 3668-2020 "Epoxy Zinc-Rich Primer" Type 2 Indicators
The data in this table are typical values under standard laboratory conditions (23±2°C, 50±5%RH). Actual values may vary slightly depending on the substrate condition, construction process, and environmental conditions. Please contact technical support to obtain the batch COA (Certificate of Appointment).

Application Process

ApplicationProcess

Surface preparation

Sandblast to Sa2.5 (ISO 8501-1:2007), roughness Rz 40-75μm. Manual rust removal ≥ St3. Grease must be removed with SSPC-SP1 solvent. Primer application (RH) must be completed within 4 hours after sandblasting.<50%可延至8h）。禁忌：雨天/雾天/Substrate temperature低于露点3°C时禁止Application。

Mixing / compounding

Stir the main agent for 3-5 minutes until the zinc powder is completely and evenly dispersed. Add the curing agent at a ratio of 10:1 (w/w) and continue stirring for 3-5 minutes. Let it stand for 15-30 minutes (23°C) to mature. Continuously stir during application to prevent zinc powder from settling—a pneumatic mixer is recommended. The mixed usable period is 4-8 hours (23°C). Do not dilute and use materials that have gelled beyond this period.

Primer application

Airless spraying (recommended): Nozzle 0.43-0.53mm/0.017-0.021", pressure 15-25MPa. Air spraying: Nozzle 1.5-2.0mm, pressure 0.3-0.5MPa. Brush/roller coating is only suitable for small-area pre-coating and repair. Single-coat DFT 60-80μm. Pre-coat welds, free edges, etc.

Drying and recoating

Surface dry ≤ 1h (23°C), hard dry ≤ 24h. Minimum recoating interval 8h (23°C). Maximum recoating interval 7d—exceeding this interval requires sandblasting (low pressure 0.2-0.3MPa, non-metallic abrasive) to remove surface zinc salts (basic zinc carbonate). Matching: Epoxy micaceous iron oxide intermediate coat 100-150μm → Polyurethane/fluorocarbon topcoat 50-80μm × 2.

Inspection & acceptance

DFT measurement (ISO 2808:2019, SSPC-PA2): 80% of measuring points ≥ specified value, no measuring points<80%规定值。AdhesionTesting（ISO 4624 pull-off test或ISO 2409Cross-cut method）。pinhole detection（Wet sponge method，67.5V/90V）。Visual inspection：无Sagging/漏涂/起泡/缩孔/zinc powder不均匀。

Applications

ApplicationsField/Area

Sea-crossing / river-crossing bridges

Steel box girders, bridge towers, arch ribs

Offshore oil platforms

Jackets, decks, living quarters

Exterior walls of petrochemical storage tanks

Crude oil / refined products / chemical storage tanks

Transmission towers / tubular towers

Steel structures for high-voltage / ultra-high-voltage lines

Large steel structure plants

Industrial plants, sports venues

Port machinery

Ship-to-shore cranes, gantry cranes, ship unloaders

Wind turbine towers

Onshore / offshore wind turbine towers

Shipbuilding and offshore engineering

Hull blocks, ballast tanks, decks

External pipeline anti-corrosion

Long-distance oil, gas and water pipelines

Mining / construction machinery

Excavators, crushers, conveying equipment

Chemical / refining plants

Reactors, heat exchangers, pipe-rack steelwork

Water treatment / water conservancy facilities

Sewage plants, sluice gates, trash racks

FAQ

Frequently AskedFAQ

The anti-corrosion mechanism of epoxy zinc-rich primer is based on the electrochemical principle of cathodic protection: the high content (≥70%) of zinc powder particles in the coating forms a conductive pathway and establishes electrical continuity with the steel substrate. Since the standard electrode potential of zinc (-0.76V) is more negative than that of iron (-0.44V), zinc preferentially oxidizes when corrosion occurs: Zn → Zn²⁺ + 2e⁻. The released electrons flow through the conductive network to the steel surface, causing cathodic polarization—when the potential drops below -0.85V (vs Cu/CuSO₄), effective protection is achieved. If the zinc powder content is insufficient (<60%），zinc powder颗粒被epoxy resin绝缘包裹无法形成连续导电通路——cathodic protection失效——Coating仅剩物理屏蔽functional，Anticorrosive寿命大幅缩短。这就是HG/T 3668Standard严格RequirementsZinc dust content的Causes：第1类≥80%（高Anticorrosive），第2类≥70%（StandardHeavy-Duty Anti-Corrosion），Class 3≥60%（一般Anticorrosive）。

Cannot be used alone.Three reasons: (1) Epoxy zinc-rich primer single-layer exposure to acidic (acid rain pH<5.6）或碱性（Concrete渗出碱液）Environment中，zinc powder会被Quick化学消耗（反应速率较中性Environment快3-10倍），丧失cathodic protection能力。Epoxy MIO intermediate coat（MIOLamellar filler层叠排列形成"Labyrinth (tortuous-path) effect"）Provides物理屏蔽层——Protectionzinc powder不过早消耗。(2)epoxy resin含芳香醚键——紫外光（290-400nm）照射下3-6months即开始光氧化降解——粉化/变色/失光。Polyurethane/fluorocarbon topcoat（含UV吸收剂和HALS光稳定剂）ProvidesWeather protection。(3)Compatible coating system总DFT更厚（≥280μm）——Anticorrosive等级更高。ISO 12944-5recommended的Heavy-Duty Anti-Corrosioncompatible：Epoxy zinc-rich primer60-80μm + Epoxy MIO intermediate coat100-150μm + Polyurethane topcoat50-80μm×2。

According to HG/T 3668-2020, three types of zinc content are selected: Type 1 (≥80%) — suitable for C5/CX extremely corrosive environments (offshore splash zones, tidal zones, chemical plant acid mist areas) or major projects with a design life >25 years. It offers the highest cathodic protection efficiency, but excessive zinc powder increases internal stress and reduces flexibility (flexibility ≥3mm vs. Type 2's ≥2mm), and has the highest unit price. Type 2 (≥70%) — a general heavy-duty anti-corrosion primer suitable for C4-C5 environments (coastal industrial areas, sea-crossing bridges, petrochemical storage tanks), offering the best cost-performance ratio — an optimal balance between anti-corrosion performance and mechanical properties. Recommended for most industrial projects. Type 3 (≥60%) — suitable for C3-C4 environments (urban atmosphere, ordinary industrial plants) with general anti-corrosion requirements. Can be selected if the budget is limited and the environmental corrosion level is low. Selection suggestion: When in doubt, choose Type 2 (70%) — the safest universal option.

Zinc powder has a high density (7.14 g/cm³) and tends to settle easily in low-viscosity epoxy resin solutions. Solutions: (1) After opening the container, thoroughly stir with an electric/pneumatic stirrer for ≥5 minutes—completely lift the zinc powder from the bottom until no sediment remains. (2) After adding the hardener, continue stirring for 3-5 minutes until the mixture has a uniform color. (3) Maintain continuous low-speed stirring throughout the application process—a pressure tank with a pneumatic stirrer (rotation speed 30-60 rpm) is recommended to prevent zinc powder from re-settling during spraying, which could cause inconsistent zinc content between the initial and final spray. (4) Paint that has settled and hardened into lumps—do not forcibly stir for use (the zinc powder has separated from the resin and may form lumps, leading to pinholes, uneven zinc distribution, and reduced adhesion after spraying). Paint that has exceeded its mixed pot life and shows gelation—must be discarded. It is recommended to prepare the paint as needed (estimate usage within 2 hours), in small, multiple batches.

Epoxy zinc-rich primer coating exposed to air—surface zinc powder reacts with CO₂ and H₂O to form basic zinc carbonate ("white rust"). After more than 7 days (23°C, 50% RH), the following treatments are required: (1) Visual inspection—white powdery zinc salts must be removed by sweep blasting. Use low-pressure (0.2-0.3 MPa) non-metallic abrasives (garnet/aluminum oxide) for sweep blasting—remove the zinc salt layer and create a 20-30 μm roughness. (2) For coatings within 6 months with no visible zinc salts on the surface—light abrading (manual sanding with 180-240 grit sandpaper) is sufficient. (3) If the coating has extensive chalking/corrosion/peeling—complete abrasive blasting removal and recoating are required. Preventive measures: Apply intermediate paint as close as possible within the minimum recoat interval (8-24 h)—at this stage, the primer surface still has active groups (incompletely crosslinked amine groups), achieving optimal intercoat adhesion without abrading.

Key difference: the film former is different.Epoxy zinc-rich — organic epoxy resin + polyamide curing agent; inorganic zinc-rich — inorganic ethyl silicate/lithium silicate hydrolysis condensation. Selection comparison: (1) Anti-corrosion performance: Inorganic zinc-rich primer (zinc content ≥85%) offers higher cathodic protection efficiency, certified under NORSOK M-501 and SSPC Paint 20 Type I (inorganic) — Type II (organic). Both can achieve equivalent anti-corrosion grades, but inorganic is slightly superior. (2) Application tolerance: Epoxy zinc-rich >> inorganic zinc-rich. Epoxy can be applied at 5–40°C/RH<85%宽ConditionsApplication；无机对RHRequirements苛刻——RH需50-90%（Ethyl silicate水解需要水分），Low temperature (<5°C）和低湿（<40%）均无法NormalCuring——Application window窄。(3)Inter-coat Adhesion：Zinc-rich epoxy+Epoxy MIO intermediate coat的层间结合力（>5 MPa) is superior to inorganic zinc-rich + epoxy intermediate paint (possibly due to the porous nature of the inorganic coating increasing the contact area, but also potentially leading to insufficient adhesion due to surface chalking—requiring sweep blasting). (4) Cost: Epoxy zinc-rich is approximately 0.7–0.8 times the cost of inorganic zinc-rich. Selection recommendations: For conventional heavy-duty anti-corrosion projects, choose epoxy zinc-rich (easy application + high cost-effectiveness); for extreme marine environments (splash zone/tidal zone) or top-tier projects with a design life of ≥30 years—choose inorganic zinc-rich + sealer + epoxy intermediate coat + polysiloxane topcoat.

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