Zinc Plating Specifications

Understanding Zinc Plating: A Critical Surface Finishing Process

In the demanding maritime environment of Atlantic Canada, where salt air and harsh weather conditions constantly challenge metal components, zinc plating stands as one of the most effective and economical corrosion protection methods available to manufacturers and engineers. This electrochemical process deposits a thin layer of zinc onto steel or iron substrates, creating a sacrificial barrier that protects the underlying metal from oxidation and environmental degradation.

For manufacturing operations throughout Nova Scotia and the broader Maritime region, understanding zinc plating specifications is essential for ensuring product longevity, meeting quality standards, and optimising production costs. Whether you're producing components for the shipbuilding industry in Halifax, agricultural equipment for the Annapolis Valley, or industrial machinery for the energy sector, proper zinc plating specifications can mean the difference between premature failure and decades of reliable service.

Key Zinc Plating Standards and Classifications

The zinc plating industry operates under several internationally recognised standards that govern thickness, adhesion, corrosion resistance, and appearance. Canadian manufacturers typically reference the following specifications when designing and producing zinc-plated components:

ASTM B633 - Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel

ASTM B633 is the most widely referenced standard for zinc electroplating in North America. This specification classifies zinc coatings into four service condition categories based on the severity of the operating environment:

• SC1 (Mild): Indoor environments with minimal exposure to moisture. Minimum thickness of 5 µm (0.2 mils). Suitable for interior hardware and components protected from weather.

• SC2 (Moderate): Moderate exposure conditions including some humidity. Minimum thickness of 8 µm (0.3 mils). Appropriate for sheltered outdoor applications.

• SC3 (Severe): Harsh environments with regular exposure to moisture, salt, and temperature fluctuations. Minimum thickness of 12 µm (0.5 mils). Common requirement for Maritime industrial applications.

• SC4 (Very Severe): Extremely corrosive conditions including marine atmospheres and chemical exposure. Minimum thickness of 25 µm (1.0 mil). Essential for coastal Nova Scotia applications.

ISO 2081 - Metallic and Other Inorganic Coatings

For manufacturers exporting products internationally or working with European partners, ISO 2081 provides comparable zinc plating specifications. This standard uses a designation system that specifies the substrate, coating type, thickness, and post-treatment. A typical designation might read Fe/Zn 12c, indicating iron or steel substrate with 12 µm zinc coating and chromate conversion treatment.

CSA Standards for Canadian Applications

The Canadian Standards Association (CSA) maintains specifications relevant to zinc-plated components used in construction, electrical, and transportation applications. CSA G164, for instance, covers hot-dip galvanising but often intersects with electroplating specifications when engineers select appropriate coating methods.

Zinc Plating Thickness Requirements and Measurement

Coating thickness is the primary determinant of corrosion protection performance. The relationship between zinc thickness and service life is well-documented, with each additional micrometre of zinc providing proportionally extended protection in most environments.

Thickness Measurement Methods

Quality control in zinc plating operations relies on several measurement techniques, each with specific advantages:

• Magnetic Induction: Non-destructive method using instruments that measure the magnetic attraction between a probe and the steel substrate through the non-magnetic zinc layer. Accuracy typically within ±5% for coatings above 10 µm.

• X-Ray Fluorescence (XRF): Provides rapid, non-destructive measurement with high accuracy. Particularly useful for complex geometries and quality assurance sampling programmes.

• Coulometric Method: Destructive technique that electrochemically strips the zinc coating while measuring the electrical charge required. Offers excellent accuracy but requires sacrificing the tested component.

• Microscopic Cross-Section: Involves cutting and polishing a sample to measure coating thickness directly under magnification. Considered the referee method when disputes arise.

Practical Thickness Considerations for Maritime Applications

In the Atlantic Canadian context, where facilities may be located within kilometres of the ocean, specifying adequate zinc thickness is critical. Salt-laden air can accelerate zinc consumption rates significantly compared to inland environments. Engineering analysis suggests that coastal Nova Scotia locations may experience zinc corrosion rates of 4-8 µm per year, compared to 1-2 µm per year in protected inland environments.

For a component requiring a 20-year service life in Amherst—which, while not directly coastal, still experiences Maritime climate influences—engineers might specify SC3 or SC4 service conditions with correspondingly thicker zinc deposits to ensure adequate protection throughout the intended service period.

Chromate Conversion Coatings and Passivation

Bare zinc plating, while providing excellent galvanic protection, is susceptible to the formation of white corrosion products (zinc oxide and zinc hydroxide) that can affect appearance and, in some cases, functionality. Chromate conversion coatings address this limitation by creating a protective film that enhances corrosion resistance and improves appearance.

Traditional Hexavalent Chromate Treatments

Historically, hexavalent chromium compounds provided superior corrosion protection and were available in several colours:

• Clear/Blue: Minimal chromate film providing basic protection and bright appearance. Salt spray resistance of 12-24 hours to white corrosion.

• Yellow/Iridescent: Heavier chromate conversion offering 72-96 hours salt spray resistance. Distinctive golden colour readily identified.

• Olive Drab: Heavy chromate treatment for military and severe service applications. May exceed 150 hours salt spray resistance.

• Black: Specialised treatment providing aesthetic black finish with moderate corrosion protection.

Trivalent Chromium and RoHS Compliance

Environmental regulations, particularly the European Union's Restriction of Hazardous Substances (RoHS) directive, have driven the industry toward trivalent chromium alternatives. While Canadian federal regulations do not directly mandate elimination of hexavalent chromium in all applications, many manufacturers—particularly those exporting to Europe or supplying automotive and electronics industries—have transitioned to trivalent processes.

Modern trivalent chromate conversion coatings have achieved performance levels approaching traditional hexavalent treatments, with clear and blue finishes regularly achieving 72+ hours salt spray resistance to white corrosion. Black trivalent coatings remain an area of ongoing development, with performance continuing to improve.

Sealers and Topcoats

For applications requiring maximum corrosion protection, supplementary sealers and topcoats can be applied over chromate conversion coatings. These organic or inorganic treatments can extend salt spray resistance to 500+ hours and provide additional benefits such as improved torque-tension characteristics for fasteners, enhanced lubricity, and colour coding capabilities.

Zinc Plating Process Types and Their Applications

Not all zinc plating processes are identical. Different bath chemistries and deposition methods produce coatings with varying characteristics, and specification engineers should understand these differences when selecting appropriate treatments.

Acid Zinc Plating

Acid chloride and acid sulphate zinc baths operate at pH levels below 6 and produce bright, lustrous deposits with excellent levelling characteristics. These processes offer high current efficiency (95-98%) and fast plating rates, making them economical for high-volume production. The resulting deposits are typically harder and more brittle than alkaline zinc, with hardness values ranging from 100-200 HV.

Acid zinc plating excels in applications where appearance is important and where the substrate geometry is relatively simple. The process may struggle with complex shapes due to poor throwing power, potentially leaving recessed areas with inadequate coating thickness.

Alkaline Non-Cyanide Zinc Plating

Alkaline zinc processes operate at pH levels above 12 and provide superior throwing power, ensuring more uniform coating distribution on complex geometries. The deposits are softer and more ductile than acid zinc, with hardness values typically in the 50-70 HV range.

For manufacturers in Nova Scotia producing intricate components—such as the complex castings and fabrications common in mining equipment, marine hardware, and agricultural machinery—alkaline zinc plating often provides more consistent results across the entire part surface.

Zinc Alloy Plating

For enhanced corrosion protection beyond what pure zinc provides, zinc alloy plating offers compelling advantages:

• Zinc-Nickel: Contains 12-15% nickel and provides 5-8 times the corrosion resistance of pure zinc at equivalent thickness. Widely specified in automotive and aerospace applications. Salt spray resistance can exceed 1,000 hours to red rust.

• Zinc-Iron: Contains 0.3-0.8% iron and offers improved weldability compared to pure zinc while maintaining good corrosion protection. Popular in automotive body panel applications.

• Zinc-Cobalt: Provides performance intermediate between pure zinc and zinc-nickel, with easier processability than zinc-nickel systems.

Hydrogen Embrittlement Considerations

One of the most critical aspects of zinc plating specifications for high-strength steel components is hydrogen embrittlement (HE) relief. During the electroplating process, hydrogen atoms can be absorbed into the steel substrate, potentially causing delayed brittle fracture under sustained tensile loading.

Susceptible Materials and Applications

Components with hardness values exceeding 31 HRC (approximately 300 HV or 1,000 MPa tensile strength) are generally considered susceptible to hydrogen embrittlement. This includes:

• Heat-treated alloy steel fasteners (Grade 8, 10.9, 12.9)

• Spring steel components

• Case-hardened parts

• Tool steels

• High-strength structural bolts

Baking Requirements

ASTM B850 provides guidance on hydrogen embrittlement relief baking procedures. For most susceptible materials, baking at 190-220°C for a minimum of 4 hours within 4 hours of plating completion is required. Higher-strength materials (above 39 HRC) may require extended baking times of 12-24 hours.

Specifications should clearly state baking requirements, including temperature, duration, and maximum time permitted between plating completion and baking initiation. For critical applications—such as structural fasteners in bridges, mining equipment, or marine installations common throughout Atlantic Canada—verification testing may be warranted.

Quality Assurance and Testing Requirements

Comprehensive zinc plating specifications must address quality assurance requirements to ensure consistent coating performance. Testing protocols should be specified based on the criticality of the application and production volumes.

Standard Quality Tests

• Thickness Measurement: Per ASTM B487 or B568, with sampling frequency and acceptance criteria defined.

• Adhesion Testing: Bend tests, thermal shock tests, or tape tests per ASTM B571 to verify coating integrity.

• Salt Spray Testing: Per ASTM B117, with duration specified based on service condition requirements. Typical requirements range from 96 hours (SC2) to 500+ hours (SC4 with sealers).

• Coating Composition: For zinc alloys, XRF or other analytical methods to verify alloy content within specification.

Documentation and Traceability

For critical applications, specifications should require certificates of conformance, test reports, and lot traceability. This documentation proves invaluable when investigating field failures or verifying compliance with customer requirements.

Partner with Sangster Engineering Ltd. for Your Zinc Plating Specification Needs

Developing comprehensive zinc plating specifications requires balancing corrosion protection requirements, manufacturing constraints, environmental compliance, and economic considerations. At Sangster Engineering Ltd. in Amherst, Nova Scotia, our team brings decades of experience in materials selection and surface finishing specification to every project.

Whether you're developing new products for the Maritime market, optimising existing manufacturing processes, or troubleshooting coating performance issues, we provide the technical expertise to ensure your zinc plating specifications deliver reliable, cost-effective corrosion protection. Our understanding of Atlantic Canadian operating conditions—from the salt air of the Bay of Fundy to the industrial environments of the region's manufacturing centres—enables us to develop specifications precisely matched to your application requirements.

Contact Sangster Engineering Ltd. today to discuss your zinc plating specification requirements. Our professional engineering team is ready to analyse your application, recommend appropriate coating systems, and develop specifications that ensure your products perform reliably throughout their intended service life.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.