Selection of zinc coatings for bolts
There are four common processes for zinc coating of bolts. These are hot dip galvanizing, electroplating, sherardising, and mechanical plating.
The zinc coating process selected is decided primarily by the duration of protection desired which should ideally be like the life of the protective coating selected for the structure itself. With thicker and more durable zinc coatings, such as created with the hot dip galvanizing process, allowances in the thread dimensions must be made to accommodate for the additional thickness of the coating.
Hot dip galvanizing
The hot dip galvanizing of fasteners produces a heavy coating of zinc ideally suitable for long-term outdoor exposure. The coating is applied by the immersion of clean, prepared steel items in molten zinc. The resulting zinc coating is metallurgically bonded to the base steel and consists of a succession of zinc-iron alloy layers and an outer zinc layer.
Due to their small relative size, threaded fasteners, nuts, and washers are usually hot dip galvanized via the centrifuge process. Like other moving parts, each part needs to be galvanized separately.
Australian Standard AS/NZS 1214 Hot-dip galvanized coatings on threaded fasteners (ISO metric coarse thread series) provides for a standard minimum coating thickness regardless of fastener dimensions. This Standard, revised in 2016, is a modified adoption of the International Standard ISO 10684, Fasteners — Hot dip galvanized coatings, and includes the option to also galvanize washers. The revision to AS/NZS 1214 was aligned with the release of the new structural bolt Standard, AS/NZS 1252.1, High-strength steel fastener assemblies for structural engineering – Bolts, nuts and washers, Part 1: Technical requirements.
The HDG process develops a coating with a minimum average thickness of 50µm on threads, as defined in AS/NZS 1214. This Standard also provides information on the recommended maximum coating thickness to avoid galling. It is required under AS/NZS 1252.1 for nuts to be galvanized blank and the threaded portion cut after galvanizing. In this case the galvanized coating on the thread of the stud or bolt will provide corrosion protection for the uncoated internal thread of the nut.
All galvanized nuts must be provided with a lubricant that is clean and dry to the touch to prevent seizure on assembly. This lubricant should also act to prevent corrosion on the ungalvanized internal thread prior to assembly. It is not necessary to also lubricate the galvanized bolts.
Economics of hot dip galvanized coatings on bolts
Corrosion protection on bolts should preferably match the rest of the structure and in most circumstances, economics favours the use of galvanized bolts rather than painting after erection. Where the durability of the galvanized bolts is lower than the rest of the structure, inspection and replacement may be a more economical solution than maintenance by painting. Due to their significantly lower coating thickness, electroplated bolts do not generally provide significant exterior durability and are therefore not recommended for external applications.
Use of galvanized bolts, nuts and washers will be effective in providing all-round protection that will generally withstand erection procedures. If bolts are painted prior to installation, the paint will be damaged during tightening and the corrosion protection compromised. Bolts, nuts, and washers can also be further protected using LDPE caps designed and engineered to fit snugly over the fixings.
Table 20: Indicative cost-in-place relationships for unpainted, painted, and galvanized M20 bolts in structural applications
Durability of HDG Bolts
The life to first maintenance of HDG bolts in Australia can be estimated from
AS/NZS 2312.2 which in turn uses the information supplied in AS 4312.
The corrosivity category can be determined from local site data, the performance of existing structures over time, or by consulting a durability expert. The GAA provides a free online durability estimator to assist with the initial assessment and a guide to the service life of HDG coatings. The estimated range for the years to first maintenance (Table 21) is based on the minimum coating thickness required to be achieved in
AS/NZS 1214. Thicker coatings are often achieved in practice and the estimates shown below are usually conservative.
Table 21: Nominal minimum life to first maintenance for HDG bolts
Oversize tapping allowances for hot dip galvanized nuts
To accommodate the relatively thick galvanized coating on external threads and to avoid the issue of galling during tightening, galvanized bolts are manufactured to standard thread dimensions and nuts are tapped oversize after galvanizing. AS/NZS 1252.1 uses AZ/g tolerances for structural bolts (Table 22) and all fasteners galvanized to AS/NZS 1214 will be suitable.
For bolts with a galvanized coating, the increase in the effective diameter due to the galvanized coating is 4 times the coating thickness so the minimum clearance between bolt and nut thread must be at least four times the coating thickness of the bolt thread (Figure 23). This is shown as a recommended maximum coating thickness in Table 22, although this only needs to be considered if a thicker coating than the Standard is desired and able to be supplied by the galvanizer.
Table 22: Fundamental deviations and upper limits of coating thicknesses for assemblies with nuts tapped oversize
The relationship between the coating thickness and the increase in effective diameter of an external thread (Figure 23) is shown by the triangle ABC were AB is the coating thickness, t, and BC is half the increase in the effective diameter. The effective diameter of an internal thread will be reduced by the same amount.
Zinc electroplating on fasteners produces relatively light, uniform coatings of excellent appearance which are generally unsuitable for outdoor exposure without additional protection.
There is, in general, an economic upper limit to the coating mass which can be applied by electroplating, although certain specialised roofing fasteners are provided with zinc plated coatings up to 35 μm to 40 µm thick. Where heavy coatings are required, galvanizing is usually a more economical alternative.
Zinc plated bolts having a tensile strength above 1000 MPa must be baked for the relief of hydrogen embrittlement and more on this subject can be found in Fundamentals of Hydrogen Embrittlement in Steel Fasteners.
The threads on electroplated nuts must meet tolerance class 6H in accordance with AS 1897 after coating. Electroplated nuts conforming to AS/NZS 1252.1 must be supplied with a lubricant coating which is clean and dry to the touch to provide for satisfactory assembly.
Sherardising produces a matte grey zinc-iron alloy coating. The process impregnates steel surfaces with zinc by rumbling small components and zinc powder in drums heated to a temperature of about 370°C. The process is best characterised by its ability to produce a very uniform coating on small articles and avoids hydrogen embrittlement. It provides a good solution for fine threaded fasteners although these are not commonly used in structural applications.
The thickness of sherardised coatings is generally about 20 µm but can vary depending on cycle time up to about 80 µm. Sherardised coatings usually fall between zinc plated and galvanized coatings in thickness and life. The cost of sherardised bolts is usually more than galvanized bolts and thicker coatings require a longer time in the process, hence the cost of production is more than the normal sherardised bolts.
AS/NZS 1252.1 does not offer instruction on the tolerances for sherardised bolts and nuts and the user will need to decide whether the nut thread is formed before or after sherardising, however they must be supplied with a supplementary lubricant coating which is clean and dry to the touch to provide for satisfactory assembly.
Mechanical (peen) plating
Mechanical or peen plating offers advantages in the zinc coating of fasteners. Coatings are uniform, and because the process is electroless there is no possibility of hydrogen embrittlement. High strength fasteners are not susceptible to embrittlement and need not be baked after coating.
The mechanical plating process often results in zinc wastes contaminated with processing fluids that are difficult or impossible to economically recycle and for this reason the process is considered less sustainable than galvanizing.
AS/NZS 1252.1 does not offer instruction on the tolerances for mechanically plated bolts and nuts and the user will need to decide whether the nut thread is formed before or after plating, however mechanically plated nuts must be supplied with a supplementary lubricant coating which is clean and dry to the touch to provide for satisfactory assembly.