Bolts, nuts, screws and other fasteners are susceptible to corrosion degradation after long-term service, which impairs the stability and safety of mechanical connections. During the product design phase, engineers must carefully select materials, surface platings and protective coatings according to actual service environments to mitigate corrosion, prevent performance deterioration and service life loss, and guarantee the overall operational reliability of mechanical equipment. This paper systematically elaborates the common corrosion modes, internal mechanisms and protective principles of surface coatings for fasteners, providing technical support for material selection and anti-corrosion optimization of fastener products.

1. Basic Concept of Corrosion

Metal corrosion refers to the destructive degradation of metal substrates caused by chemical or electrochemical interactions with ambient media, and it is one of the most prevalent failure modes of mechanical fasteners.

Pure chemical corrosion occurs when fasteners directly contact corrosive chemical substances without the participation of electric current. For example, leaked battery electrolyte can directly erode fastener surfaces and damage base materials. In actual industrial service, most corrosion failures of fasteners result from indirect electrochemical reactions, typically including steel rusting and galvanic corrosion.

Corrosion propagation in fasteners resembles the decay of teeth. It originates from tiny invisible defects and spreads rapidly, gradually undermining the structural integrity of threaded joints. Progressive corrosion consumes metal substrates, reduces mechanical strength, and ultimately leads to loosening or fracture of fasteners. In addition, galvanic reactions can also induce corrosion damage to adjacent connected components.

Apart from conventional strength attenuation, corrosion can trigger two special failure types. The first is stress-induced corrosion failures such as hydrogen embrittlement under the coupling effect of corrosion and tensile stress. The second is corrosion fatigue failure, where microcracks generated at corroded regions expand continuously under alternating loads and eventually cause fatigue fracture of fasteners.

Improper material selection is a key cause of chemical corrosion. Chemical corrosion takes place when fastener materials are soluble in corrosive media. For instance, ordinary carbon steel bolts will be rapidly dissolved and corroded when exposed to hydrochloric acid. For severe and predictable corrosive environments, corrosion-resistant alloys such as stainless steel and nickel-based alloys are preferred. Meanwhile, dense and impermeable protective coatings can be adopted to fundamentally isolate corrosive media and avoid chemical corrosion.

In industrial applications, fastener corrosion is dominated by electrochemical corrosion driven by micro spontaneous currents, which features faster propagation speed and wider damage range. Electrochemical corrosion relies on four indispensable conditions: anode and cathode regions, conductive electrolyte media, potential difference, and closed conductive loops. Continuous electrochemical corrosion will proceed once all conditions are satisfied.

1.1 Corrosion Mechanism of Carbon Steel

Rusting is the most typical and fundamental form of electrochemical corrosion for steel materials. When water droplets adhere to steel surfaces, a potential difference forms at the interface between the steel substrate and the aqueous electrolyte, generating micro electric currents and initiating the rusting reaction. Oxidation reactions occur at anode regions, where iron atoms ionize and dissolve into the electrolyte. Correspondingly, reduction reactions take place at cathode regions, where atmospheric oxygen reacts with water to produce hydroxide ions.

Iron ions combine with hydroxide ions in the electrolyte, forming iron oxide deposits, namely rust, on fastener surfaces. Long-term exposure to humid environments sustains the cyclic electrochemical reaction, resulting in continuous substrate erosion and progressive corrosion deterioration.

1.2 Galvanic Corrosion

Galvanic corrosion, also defined as dissimilar metal corrosion, is a typical electrochemical corrosion mode. Consistent with general electrochemical corrosion, it requires four essential conditions: anode, cathode, electrolyte and potential difference. A closed corrosion loop is formed when two dissimilar metals with different electrode potentials are in direct contact.

When two contacted dissimilar metals are exposed to electrolyte, the metal with higher electrode potential acts as the cathode, while the one with lower potential serves as the anode. In the closed loop, atoms of the anodic metal are continuously ionized and consumed, leading to progressive corrosion failure. A larger potential difference between the two metals will accelerate and aggravate galvanic corrosion.

The galvanic series table is a critical guideline for anti-corrosion design of fasteners. The position of each metal in the table represents its electrode potential level. The farther the two metals are separated in the series, the greater their potential difference and the higher the risk of galvanic corrosion. For example, magnesium alloy and platinum lie at the two extreme ends of the series, making them an incompatible assembly combination. In contrast, metals with similar potential values produce negligible electrochemical reactions and possess low corrosion risks.

The severity of galvanic corrosion is determined by three core factors:

(1) Potential difference: The spacing of two metals in the galvanic series directly determines the corrosion degree. The combination of aluminum parts and 316 stainless steel components suffers more severe galvanic corrosion than the matching of carbon steel and tin parts.

(2) Electrolyte activity: Higher ion concentration improves the conductivity of electrolyte and accelerates corrosion reactions. Brine contains abundant ions and serves as a far more active electrolyte than deionized water, leading to faster galvanic corrosion in salt environments.

(3) Cathode-anode area ratio: A larger cathode area relative to the anode significantly intensifies anodic corrosion. For example, small aluminum fasteners assembled on large stainless steel plates act as tiny anodes and corrode rapidly in electrolyte environments. On the contrary, small stainless steel fasteners matched with large aluminum plates form a small cathode area, restricting corrosion to the contact edge and alleviating overall damage.

1.3 Fretting Corrosion

Fretting corrosion is a special non-chemical and non-electrochemical corrosion mode, commonly occurring under high-load friction conditions. Relative sliding and continuous compression between mating surfaces wear off the native protective oxide films of fasteners. The newly exposed fresh metal substrate directly contacts the external environment and undergoes rapid corrosion.

Fasteners made of stainless steel, aluminum alloy and titanium alloy are highly sensitive to fretting corrosion. Their bearing surfaces and thread mating areas depend entirely on protective oxide films for corrosion resistance. Once the oxide layers are worn away, the substrate will suffer continuous and progressive corrosion damage.

1.4 Crevice Corrosion

Crevice corrosion is a concealed localized electrochemical corrosion mode. It occurs in narrow metal gaps, chamfers, arc transitions and areas prone to dust accumulation and water retention. Concentration differences of electrolyte inside and outside the narrow gaps trigger localized electrochemical reactions and cause selective corrosion damage to internal metal substrates.

Crevice corrosion is localized and difficult to detect in the early stage, which may lead to severe structural damage after long-term accumulation. Meanwhile, a large number of atomic hydrogen generated during crevice corrosion will be absorbed by the metal matrix, easily inducing hydrogen embrittlement and increasing the fracture risk of fasteners.

1.5 Pitting Corrosion

Pitting corrosion is a highly localized corrosion form that generates tiny pits on metal surfaces. These initial micro defects gradually deepen and expand into obvious macroscopic corrosion pits with sustained corrosion reactions.

Compared with other corrosion modes, pitting corrosion has limited influence on the overall structural integrity and mechanical performance of fasteners without causing rapid failure. Its core mechanism lies in local oxygen deficiency on the metal surface, which forms independent anodic regions, triggers localized electrochemical reactions, and eventually develops into deep corrosion pits.

2. Protection Mechanisms of Fasteners

Based on the above corrosion mechanisms, targeted structural optimization and process protection measures can be adopted to cut off corrosion conditions and suppress electrochemical reactions. There are four core anti-corrosion protection mechanisms for fasteners, which can be applied independently or in combination to achieve optimal anti-corrosion performance.

2.1 Barrier Protection Mechanism

Barrier protection is the most basic anti-corrosion method. A dense and continuous protective coating is covered on the metal surface to isolate the substrate from corrosive media and block corrosion propagation paths. Its protective effect depends entirely on the integrity of the coating. Intact coatings provide long-term stable anti-corrosion protection, while scratches, peeling or wear will expose the substrate and immediately induce corrosion. Typical applications include paint films and other non-metallic protective coatings.

2.2 Sacrificial Anode Protection Mechanism

The sacrificial anode protection mechanism adopts a more active surface coating that corrodes preferentially to protect the metal substrate. This protective effect lasts only when the sacrificial coating remains complete. Once the coating is completely consumed, the substrate will be exposed and corroded. Electro-galvanized coating is a typical application of this mechanism, where zinc with higher chemical activity corrodes preferentially to protect carbon steel substrates.

2.3 Passivation Layer Protection Mechanism

Passivation protection relies on inert, dense oxide films spontaneously formed on metal surfaces. The chemically stable passivation layer isolates external corrosive media and provides long-term anti-corrosion performance. Fasteners made of stainless steel, titanium alloy and aluminum alloy achieve durable corrosion resistance through their native passivation films.

2.4 Self-healing Protection Mechanism

Self-healing protection is an advanced and high-reliability anti-corrosion technology. Slight damage to protective coatings or passivation films can be spontaneously repaired in natural environments to maintain continuous protective performance. Although its application scope is limited, it delivers excellent anti-corrosion stability. The passivation film of austenitic stainless steel possesses outstanding self-healing capability, which can regenerate rapidly after minor damage and restore corrosion resistance.

3. Conclusion

Fasteners are exposed to multiple corrosion risks including chemical corrosion, electrochemical corrosion, fretting corrosion, crevice corrosion and pitting corrosion, all of which damage structural integrity and reduce connection reliability. Therefore, engineers must fully evaluate potential corrosion types and failure risks according to service conditions and environmental characteristics in engineering design and practical application. Reasonable material selection, optimized plating and coating matching, and structural improvement can effectively prevent or delay corrosion failure, ensuring the long-term stability of threaded connections and the safe operation of mechanical equipment.