Views: 147 Author: Site Editor Publish Time: 2026-05-17 Origin: Site
Trucks and trailers, as core equipment in road logistics transportation, endure complex service loads on their structural components and fastening systems over extended periods, including the combined effects of stone impacts, dust erosion, temperature fluctuations, mechanical vibration, and chemical corrosive media. Under these harsh conditions, corrosion failure of high-strength steel components, especially hydrogen-induced stress corrosion cracking (hydrogen embrittlement), has become a key technical bottleneck restricting the safe and reliable operation of commercial vehicles.
Hydrogen embrittlement (HE) is a highly dangerous delayed fracture failure mode (sudden brittle fracture of components without any warning signals).
For truck and trailer operations, sudden failure of structural components or fasteners means downtime, delivery delays, and high maintenance and indirect losses. Therefore, how to achieve efficient corrosion protection while mitigating the risk of hydrogen embrittlement is a core issue in the surface treatment technology of commercial vehicle components.
Zinc-aluminum coating, as a non-electrolytic deposition thin-film corrosion protection system, has become the preferred solution for corrosion protection of high-strength steel components due to its inherent safety characteristics of no hydrogen evolution during the process and its excellent cathodic corrosion resistance. We will systematically explain the application value of this solution in the truck/trailer industry from the perspectives of hydrogen embrittlement mechanism, influencing factors, technical principles, and performance advantages of zinc-aluminum coating.
Hydrogen Embrittlement Problem in High-Strength Steel Components: Mechanism and Influencing Factors
1. The Nature of Hydrogen Embrittlement and High-Risk Components
Hydrogen embrittlement risk mainly exists in high-strength steels with tensile strength greater than 1000 N/mm² (or hardness greater than 320 HV). These materials are widely used in critical parts of trucks/trailers, such as high-strength bolts (grade 10.9 and above), high-strength nuts (grade 10 and above), springs, and structural connectors. The fundamental reason is that atomic hydrogen diffuses into the crystal structure of the steel, accumulates in internal stress concentration areas, leading to a decrease in metallic bonding force and ultimately causing delayed brittle fracture.
According to DIN 50969-1, hydrogen-induced stress corrosion cracking is the result of the synergistic effect of multiple factors, which can be summarized into three levels:
2. Three Major Influencing Factors
2.1 Material Level
Structural defects, non-metallic inclusions, microscopic impurities, or residual stresses that may occur in steel during smelting and rolling constitute the material basis for hydrogen embrittlement. High-strength martensitic structures are particularly sensitive to hydrogen.
2.2 Manufacturing Level
Defects introduced during the forming, hardening, or heat treatment processes of components, such as microcracks, deformation bands, or inhomogeneous structures, further increase the material's sensitivity to hydrogen.
2.3 Coating/Surface Treatment Level
This is the most frequently discussed aspect of hydrogen embrittlement. In traditional electroplating pretreatment processes, pickling (using sulfuric acid or hydrochloric acid) generates atomic hydrogen in the treatment tank; the electroplating deposition process itself also releases a large amount of hydrogen gas at the cathode (i.e., the surface of the workpiece). This atomic hydrogen readily diffuses into the interior of the steel substrate. In actual operating conditions, the combined effect of multiple factors mentioned above often leads to component failure, typically without any visible damage before the failure.
3. The Progressive Microscopic Process of Hydrogen Embrittlement Fracture
Hydrogen atoms diffusing into the steel migrate to regions of high tensile stress, such as grain boundaries, internal and external notches, perforation edges, or burrs. Hydrogen atoms gradually accumulate at these locations, continuously weakening the metallic bonds until microcracks form. Although the formation of microcracks temporarily alleviates the stress state in the local area, a new stress concentration immediately arises at the crack tip, attracting more hydrogen atoms to accumulate, weaken, and propagate. This process iterates repeatedly until the remaining effective cross-section can no longer withstand the external tensile load, resulting in sudden delayed brittle fracture. The entire process can last from hours to months, exhibiting extremely high concealment and unpredictability.
Hydrogen Embrittlement Risks and Limitations of Traditional Anti-corrosion Coatings
1. Inherent Defects of Electroplating Zinc Process
Electroplating zinc is one of the most widely used anti-corrosion processes in the fastener industry, but its process chain contains two key hydrogen sources. First, the pickling process in the pretreatment stage uses sulfuric acid or hydrochloric acid to remove oxide scale and rust; the atomic hydrogen generated in this process can penetrate into the microstructure of the steel. Second, the cathodic reaction during electrodeposition releases a large amount of hydrogen gas, also posing a hydrogen permeation risk. For high-strength steel with a tensile strength exceeding 1000 N/mm², although electroplating zinc coatings can provide corrosion resistance for approximately 96-200 hours of neutral salt spray testing (according to ISO 9227 standard), the risk of hydrogen embrittlement makes it unsuitable for critical safety components.
2. Limitations of Dehydrogenation Treatment
According to ISO 9587 (Pretreatment Standard for Reducing the Risk of Hydrogen Embrittlement in Steel) and ASTM F1941, hydrogen can be diffused outwards by tempering after electroplating (typically held at 190–230°C for 4–8 hours). However, the effectiveness of dehydrogenation is highly dependent on the coating structure; a dense electroplated layer may obstruct the escape path of hydrogen. Furthermore, dehydrogenation treatment must be performed within a very short time after electroplating (usually within 4 hours), resulting in a narrow process window. This process not only increases production costs and cycle time but also cannot completely guarantee the removal of hydrogen, leaving residual risks.
3. Limitations of Hot-Dip Galvanizing Applicability
While hot-dip galvanizing provides good corrosion resistance, its typical coating thickness is generally >65μm, requiring secondary tapping or re-threading of threaded parts to ensure assembly accuracy. In addition, the immersion temperature of around 450°C may adversely affect the mechanical properties of high-strength steel that has already undergone quenching and tempering. Therefore, the applicability of hot-dip galvanizing in high-precision fasteners and high-strength structural components is significantly limited.
Zinc-Aluminum Coating: An Intrinsically Safe Corrosion Protection Solution
1. Technical Principles and Coating Structure
Zinc-aluminum coating is a non-electrolytic deposition inorganic metal coating system, consisting of numerous tiny zinc and aluminum flakes connected by an inorganic matrix (usually a silicate binder) to form a layered structure. Its technical specifications are defined by international standards ISO 10683 (for threaded fasteners) and EN 13858 (for non-threaded fasteners and other components).
The coating typically consists of a modular system of two layers: a primer and a topcoat. The primer, containing zinc flakes, forms the main corrosion-resistant layer, determining the coating's cathodic protection capability. The topcoat provides additional functions on top of the primer, such as chemical resistance, friction coefficient adjustment, color marking, and electrical insulation/conductivity. The total coating thickness is typically controlled within the range of 8–20 μm, far less than that of hot-dip galvanizing, yet it achieves corrosion resistance of over 240–1500 hours in neutral salt spray tests (depending on coating specifications and thickness grade).
2. Dual Anti-corrosion Mechanism
Barrier Protection: Layered zinc and aluminum sheets form a dense physical barrier layer, effectively preventing the penetration of moisture, oxygen, and corrosive media (such as chloride salts in road de-icing agents) into the steel substrate.
Sacrificial Cathodic Protection: Zinc, being a more reactive metal than iron (with a more negative potential), preferentially undergoes oxidation and dissolution in corrosive environments, essentially "sacrificing" itself to protect the steel substrate. Even if the coating is locally damaged (e.g., by stone impact or scratches), the exposed steel substrate still receives electrochemical protection from the surrounding zinc layer. This is the key reason why zinc-aluminum coatings are superior to ordinary organic coatings (which only provide barrier protection).
3. Core Advantages in Avoiding Hydrogen Embrittlement
The fundamental reason why zinc-aluminum coating has become the preferred solution for corrosion protection of high-strength steel components lies in the "hydrogen-free" nature of its entire process chain:
3.1 No Acid Pickling in Pretreatment
The pretreatment of zinc-aluminum coating uses a combination of alkaline aqueous solution degreasing and cleaning with shot blasting for mechanical rust removal, replacing the traditional acid pickling process. Alkaline cleaning removes grease and dirt, while shot blasting removes oxide scale and rust through the mechanical impact of steel shot. Neither process generates atomic hydrogen, eliminating the risk of hydrogen permeation in the pretreatment stage at its source.
3.2 Electrolysis-Free Coating Process
The zinc-aluminum coating is applied to the workpiece surface through physical methods such as dipping or spraying, and then cured and cross-linked at a temperature of 180–320°C. The entire coating process does not involve electrochemical deposition reactions, therefore no hydrogen gas is released from the workpiece surface.
3.3 Curing Process Promotes Hydrogen Escape
ISO 10683 clearly states that zinc-aluminum coatings have high hydrogen permeability. Even if trace amounts of hydrogen are absorbed during pretreatment stages (such as permissible phosphating), the subsequent high-temperature curing process promotes hydrogen diffusion and escape, further reducing residual risk.
Process Flow and Coating Method
1. Standard Process Flow
The typical application process for zinc-aluminum coatings includes: alkaline degreasing and cleaning → mechanical sandblasting for rust removal → zinc foil primer application → curing (typically 180–320°C) → topcoat application (if required) → secondary curing → quality inspection and shipment. The interval between cleaning and coating should be minimized to prevent secondary oxidation.
2. Coating Method Selection
Based on the size, shape, and batch requirements of the components, zinc-aluminum coatings are mainly applied in two ways:
Comprehensive Performance Advantages of Zinc-Aluminum Coatings
Besides mitigating hydrogen embrittlement, zinc-aluminum coatings demonstrate suitability for commercial vehicle components in multiple dimensions:
Furthermore, zinc-aluminum coatings possess excellent electrical conductivity (when the topcoat does not contain an organic insulating layer), allowing for flexible adjustment of the friction coefficient through the topcoat formulation to meet fastener tightening torque control requirements. They are also less prone to warm-loosening, which is particularly important for components in trucks/trailers subjected to alternating thermal loads.
Typical Applications in the Truck/Trailer Sector
In the truck and trailer sector, zinc-aluminum coatings are used on a wide range of key components:
1. High-Strength Fasteners
These include 10.9 and 12.9 grade high-strength bolts, 10 grade and above nuts, wheel bolts, chassis connection bolts, etc. These are high-risk components for hydrogen embrittlement and represent the core application scenario for zinc-aluminum coatings.
2. Springs and Elastic Elements
Suspension springs, leaf spring clamps, spring washers, and other elastic components subjected to high stress are particularly sensitive to hydrogen embrittlement. Zinc-aluminum coatings provide a safe corrosion protection solution for them.
3. Chassis Structural Components
Structural load-bearing components such as frame longitudinal beam connectors, crossbeam supports, and saddle assemblies are exposed to de-icing agents and mud and water erosion in road environments for extended periods, requiring reliable and long-term corrosion protection.
4. Trailer Connection and Locking Mechanisms
Functional components such as towing pins, locking mechanisms, and outrigger adjusting screws bear both high mechanical loads and face harsh corrosive environments. The thin coating properties of zinc-aluminum coatings do not affect their motion accuracy and assembly fit.
Technical Standards and Industry Specifications
The application of zinc-aluminum coatings in the truck/trailer sector is regulated by the following major international standards:
It is worth noting that since June 2017, zinc-aluminum coatings containing hexavalent chromium (Cr(VI)) have been subject to strict restrictions and increasingly stringent regulations in Europe. Currently, mainstream products in the industry all use hexavalent chromium-free formulations, complying with the environmental requirements of EU REACH regulations and RoHS directives. This trend has been widely recognized and implemented in the global commercial vehicle supply chain.
Economic Analysis and Value Assessment
From the perspective of the total life-cycle cost of commercial vehicles, the economic value of zinc-aluminum coatings far exceeds their initial coating cost. The consequences of failure of high-stress structural components or fastening systems in trucks and trailers due to corrosion or hydrogen embrittlement include: operational losses due to unplanned vehicle downtime, default costs due to delayed deliveries, high costs of emergency repairs, spare parts replacement and labor costs, and potential liability for safety accidents. These indirect losses are often orders of magnitude greater than the cost of the coating system.
Zinc-aluminum coating systems provide reliable protection and long-term value retention for high-value capital goods (trucks/trailers) by offering high-performance cathodic corrosion protection, eliminating process-related hydrogen embrittlement risks, and extending the corrosion resistance life of components. For commercial vehicle operators and OEMs seeking high reliability and low maintenance costs, zinc-aluminum coatings are a truly cost-effective option.
In summary,
hydrogen embrittlement is a significant systemic technical risk in the field of corrosion protection for high-strength components of trucks and trailers. Zinc-aluminum coatings, with their inherently safe non-electrolytic coating process, excellent cathodic protection, extremely thin coating thickness, and flexible functional customization capabilities, have become the benchmark solution for corrosion protection of high-strength steel components in the commercial vehicle industry.
With the global trend towards lightweighting and electrification in commercial vehicles, and the increasing demands for component reliability and environmental compliance, zinc-aluminum coating technology will play an even more crucial role in the field of commercial vehicle corrosion protection.
