Due to their effective corrosion resistance, electrodeposited zinc and zinc-alloy coatings have been extensively used in a wide variety of applications, from transport, automotive, marine, to aerospace. Since electrodeposited zinc coatings form a protective barrier against moisture, they have become the industry choice for many applications to protect carbon and low alloy steels (that form materials like nuts, bolts, screws, and fasteners) against degradation. ASTM B633-23: Standard Specification For Electrodeposited Coatings Of Zinc On Iron And Steel provides material and process specifications for zinc coatings to prevent their degradation upon exposure in corrosive atmospheres, such as industrial, marine, and coastal environments.
The ASTM B633-23 Standard for Zinc Electrodeposited Coatings
ASTM B633-23 covers material and process requirements for electrodeposited zinc coatings applied to iron or steel articles to protect them from corrosion and consequently extending their service life. The standard specifies that coatings shall be non-alloyed zinc produced by electrodeposition. Additionally, the coatings are provided in four standard thickness classes (i.e., 25 μm, 12 μm, 8 μm, and 5 μm) and one of the following six types of supplementary finishes:
- As-plated without supplementary treatments
- With colored chromate coatings
- With colorless chromate conversion coatings
- With phosphate conversion coatings
- With colorless passivate
- With colored passivate
It is crucial to note that, high strength metals, including high strength steels having a tensile strength greater than 1700 MPa (247 ksi, 46 HRC) should not be zinc electroplated in accordance with ASTM B633-23. This is because the pretreatment and plating process can introduce hydrogen that can cause internal hydrogen embrittlement in high strength steels, leading to loss of strength and ductility. The severity of hydrogen embrittlement is a function of temperature—most metals are relatively immune to hydrogen embrittlement, above approximately 150°C. Steels below 1200 MPa (39 HRC) are not susceptible to such embrittlement. In other words, those steels can tolerate the presence of higher concentrations of hydrogen without any delayed degradation of their mechanical strength.
What Is Hydrogen Embrittlement?
When atomic hydrogen enters steel and certain other metals and alloys, it can cause overtime loss of ductility or strength, or both. This can lead to cracking (usually microcracks) and eventually to catastrophic brittle failures at applied stresses below the normal strength of the material. As defined in with ASTM B633-23, hydrogen embrittlement is “a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with stress, either externally applied or internal residual stress.” Basically, it is the study of how a stressed material performs in the absence and then in the presence of absorbed hydrogen. Certain metals (e.g., iron, titanium, and nickel) are more susceptible to hydrogen embrittlement than others (e.g., copper, aluminum, and austenitic stainless steel).
Service Life of Zinc
The service life of zinc coating is a function of its thickness and the type of environment to which it is exposed. The data, based on worldwide testing, in ASTM B633-23 can be used to compare the mean corrosion rate of electrodeposited coatings of zinc per year in various atmospheres:
- Industrial Atmosphere – 5.6 µm/year
- Urban nonindustrial or marine Atmosphere – 1.5 µm/year
- Suburban Atmosphere – 1.3 µm/year
- Rural Atmosphere – 0.8 µm/year
- Indoors Atmosphere – considerably less than 0.5 µm/yea
What Service Conditions Cause Corrosion?
ASTM B633-23 specifies that various conditions drive appropriate requirements for plating, thickness, stress relief baking, in-application protection, etc. Here are those conditions ranging in severity:
- Very Severe: Exposure to harsh conditions, or subject to frequent exposure to moisture, cleaners, and saline solutions, plus likely damage by denting, scratching, or abrasive wear (E.g., plumbing fixtures and pole line hardware).
- Severe: Exposure to condensation, perspiration, infrequent wetting by rain, and cleaners (E.g., tubular furniture, insect screens, window fittings, builder’s hardware, military hardware, washing machine parts, and bicycle parts).
- Moderate: Exposure mostly to dry indoor atmospheres but subject to occasional condensation, wear, or abrasion (E.g., tools, zippers, pull shelves, and machine parts).
- Mild: Exposure to indoor atmospheres with rare condensation and subject to minimum wear or abrasion (E.g., buttons, wire goods, and fasteners).
ASTM B633-23: Standard Specification For Electrodeposited Coatings Of Zinc On Iron And Steel is available on the ANSI Webstore.