Fasteners and Galvanic Corrosion

Match the fastener to the assembly and its service access

Common mechanical joining components

Component	Purpose	Design check
Bolt and nut	Provide a removable threaded connection that can clamp parts together.	Provide tool access, suitable thread engagement and a compatible strength and material specification.
Washer	Provides a bearing surface or distributes load where the design requires it.	A plain washer does not by itself prevent loosening or electrically isolate an entire joint.
Rivet	Forms a retaining head through permanent deformation.	Suitable for repeated sheet assembly; removal normally destroys the rivet. Blind rivets permit installation from one side.
Threaded insert	Provides an internal thread in a host part.	Select an insert designed for the host material and thickness. Check installation access, pull-out and resistance to rotation.

Standard fasteners often reduce dedicated tooling for short runs, but every fastener adds parts and assembly time. Riveting or automated fastening may suit long runs. Removability, inspection and repair can justify a threaded joint even when another method is faster.

A thread size alone does not specify a complete fastener

The mating threads must agree in diameter, spacing and thread system. Similar-looking inch and metric parts are not interchangeable. Pitch describes axial spacing between adjacent thread forms; threads per inch describes a count over a unit length.

A purchase or drawing specification also needs the relevant length, head style, material or strength grade, coating and thread-fit requirements. The wrench size is not the thread diameter. Use the applicable thread reference and supplier drawing to decode a designation before choosing mating parts.

Read diameter and thread spacing separately

Example	Diameter	Spacing and system
1/4-20 UNC	Nominal thread diameter: 1/4 inch.	20 threads per inch; UNC is the Unified coarse series. The pitch is 1/20 inch, not 20 inches.
M6 × 1	Nominal metric thread diameter: 6 mm.	Pitch: 1 mm between corresponding points on adjacent threads. The 1 is not a count of threads per inch.

These are thread-size examples, not complete fastener specifications. Nominal diameter identifies the size; tolerances govern the actual mating dimensions. A 1/4-20 thread and an M6 × 1 thread differ in both diameter and spacing.

Sources for thread names and size examples

McMaster-Carr explains inch diameter/TPI and metric diameter/pitch, including M6 × 1. Its UNC catalog identifies 1/4-20 as UNC. NASA's Fastener Design Manual (PDF), printed pages 12–13 / PDF pages 16–17, explains Unified coarse/fine series and thread classes. The manual is background on thread terminology, not a current procurement specification.

Practice decoding a thread designation

For dimensional fit concepts, see Tolerances and Fits. For a permanently joined alternative, see Joining Metal.

Galvanic corrosion needs a metal pair, electrical contact and an electrolyte

Dissimilar metals can form a corrosion cell when they are electrically connected and share a conductive liquid, called an electrolyte. Moisture containing salts can provide that liquid. The more active member of the pair acts as the anode and may corrode faster; the other acts as the cathode.

A galvanic series ranks metals by their behavior in a specified environment. It helps screen combinations; it does not predict a service life. Alloy condition, surface films, wetting and exposed areas matter. A dry pair is not automatically a working galvanic cell.

Why the wetted areas matter

A small active area coupled to a large noble area can suffer concentrated attack when both areas contact the electrolyte. That is why coating only the active member is not automatically a cure: a small defect in that coating can leave a small wetted anode coupled to a large cathode. The relevant areas are those participating in the wet corrosion cell, which can change as a joint dries. A compatibility decision must consider the complete joint and environment.

Sources for galvanic contact, coatings and area effects

ASSDA Technical FAQ 1 (PDF), pages 1–2, explains electrical contact, conductive liquid, coating defects and relative wetted areas. Hilti's Corrosion Handbook (PDF), printed pages 12 and 26–27, illustrates the cell, separation measures and environment-specific material pairing.