Гальваническая коррозия и выбор крепежа: предотвращение э...

What Is Galvanic Corrosion?

Galvanic corrosion (also called dissimilar metal corrosion) occurs when two different metals are in electrical contact in the presence of an electrolyte (water with dissolved salts). The more active (anodic) metal corrodes preferentially, while the more noble (cathodic) metal is protected. The driving force is the difference in electrochemical potential between the two metals — the greater the difference, the faster the corrosion of the anodic metal. In fastener applications, this means the fastener (the smaller component) often corrodes rapidly while the parent material (the larger component) remains intact. For example, a steel bolt on an aluminum plate in seawater can fail within weeks — the steel (more active) corrodes while the aluminum (more noble) is protected.

The Anodic Index: Understanding Metal Compatibility

The Anodic Index (AI) is a ranking of metals by their electrochemical potential in seawater. Metals with a larger difference in AI are more likely to experience galvanic corrosion when coupled. As a rule of thumb: AI difference < 0.15V — negligible galvanic risk; 0.15-0.25V — slight risk, monitor in harsh environments; 0.25-0.50V — moderate to high risk, use insulation; > 0.50V — severe risk, avoid direct coupling. For fasteners, the practical implication is that using a steel bolt on a copper pipe, or a stainless steel bolt on aluminum in a humid environment, can cause rapid fastener failure.

Fastener Compatibility with Common Parent Materials

When selecting fasteners for assemblies with different materials, the key principle is: the fastener should be no more than 0.15V different in anodic index from the parent material, OR the fastener should be the more active (anodic) metal (so the fastener corrodes rather than the parent material). Steel or HDG fasteners on structural steel — compatible, AI difference is minimal; Steel or HDG fasteners on stainless steel — may cause steel to corrode if stainless is cathodic; generally acceptable in dry environments, risky in wet. Stainless steel fasteners on aluminum — HIGH RISK in wet conditions; if unavoidable, use nylon isolation washers to separate the metals. HDG fasteners on aluminum — acceptable; the zinc is slightly more active and will sacrifice itself to protect the aluminum. Copper pipe with brass fittings — brass (copper alloy) is more noble; use brass or copper rivets, not steel.

Prevention Methods: Isolation and Material Selection

When dissimilar metals must be used together, several mitigation strategies exist: Electrical isolation — the most reliable method; separate the two metals with a non-conductive material (plastic washer, rubber gasket, bushing) at all contact points. The isolation must be complete — any metal-to-metal contact defeats the isolation. Organic coatings — paint or powder coating on one or both surfaces creates a barrier; but any coating holiday (pinhole) creates a localized galvanic cell and accelerates pitting. Greases and sealants — petroleum jelly or specialized dielectric grease reduces ion flow but is not a permanent solution; grease washes away over time. Cathodic protection — make the anodic material the sacrificial anode (more active metal that corrodes preferentially); used in buried pipelines and marine applications. Material selection — the simplest solution: select fasteners made of the same material as the parent, or use a material with a compatible anodic index.

African Environmental Considerations

African environments intensify galvanic corrosion risk: Coastal humid (West Africa, East Africa coast) — high salt content in air accelerates electrolytic conduction; all dissimilar metal combinations that would be safe inland become high risk; specify stainless steel 316 or polymer-coated fasteners for coastal work. Industrial pollution (mining areas, urban) — acid rain and industrial emissions lower pH of water, increasing electrolyte conductivity; copper and nickel alloys accelerate corrosion of steel in these conditions. Underground (groundwater with high mineral content) — buried dissimilar metal joints in African groundwater require special attention; use Denso tape or heat-shrink sleeving for corrosion protection. High altitude (Ethiopia, Kenya highlands, South African highveld) — lower oxygen levels at altitude affect corrosion mechanisms; stainless steel 304 performs better relative to HDG at altitude than at sea level.

Frequently Asked Questions