Exhaust U Bolt

Introduction to Surface Treatment

The surface treatment process of the exhaust U bolt needs to focus on addressing three major challenges: high temperature, high humidity, and salt spray corrosion. The following is a detailed technical introduction and selection suggestions for the surface treatment.

（1）Dacromet Coating - Preferred Choice

Core Strengths:

Excellent high-temperature resistance: Capable of withstanding temperatures up to 300°C for an extended period.

Superior corrosion resistance: The salt spray test can last for over 1,000 hours, which is 10 to 20 times longer than that of electro-galvanizing.

No hydrogen embrittlement risk: The process does not produce hydrogen ions.

Self-repairing feature: The zinc particles in the coating can still provide cathodic protection at the scratch site, preventing the substrate from rusting.

Limitations:

The cost of purchasing the Dacromet exhaust U bolt is relatively high (approximately 2 to 3 times the cost of electro-galvanizing).

It is a single color (silver-gray) and has no decorative elements.

（2）Hot-Dip Galvanizing, HDG

Core Strengths:

Thick coating, strong corrosion resistance: The outdoor environment service life can reach 15-20 years.

Extremely strong adhesion: The alloy layer is firmly bonded to the substrate, and it is not prone to peeling off.

Limitations:

High temperature makes it prone to softening.

Thicker coating: May affect the accuracy of thread fit, so a tolerance margin needs to be reserved.

High-temperature resistance performance is average: Under continuous high-temperature conditions of the exhaust system, the zinc layer will accelerate oxidation and turn gray, and it will still fail over time.

（3）Electro-galvanized

Core Strengths:

The cost is extremely low.

The appearance is shiny and suitable for display or retail packaging.

Limitations:

Not resistant to high temperatures: The zinc layer begins to accelerate oxidation at temperatures above 100°C.

The coating is thin and has poor resistance to salt spray: red rust usually appears within 48-72 hours.

（4）Zinc-aluminum diffusion coating.

Core Strengths:

High bonding strength: It will not peel off.

Good resistance to high and low temperatures: The working temperature range covers -200°C to 400°C.

High hardness and excellent wear resistance: The surface hardness of the exhaust U bolt can reach 300 to 400 Vickers hardness units.

Limitations:

The investment in process equipment is considerable.

The cost is relatively high (close to that of Dacromet).

（5）Black Oxide / Phosphating

Core Strengths:

The cost is extremely low.

It has an extremely minor impact on the size.

The adsorbable oil film can serve as a temporary anti-rust measure.

Limitations:

It has almost no anti-corrosion ability. The salt spray test only lasts for about 12 to 24 hours before it starts to rust.

Not resistant to high temperatures and not wear-resistant.

Only suitable for extremely dry indoor environments.

How to choose?

（1）Core principle: In the face of the high temperatures of the exhaust system, the Dacromet coating is the preferred choice. The heat resistance and corrosion resistance of the surface treatment form the basis for ensuring the long-term stable operation of the exhaust U bolt.

（2）Alternative solution: If cost is the primary consideration and the exhaust system does not have strict requirements for corrosion prevention, then multi-alloy co-plating or high-quality coating treatment can be considered.

（3）Key considerations: When making a choice, it is essential to comprehensively evaluate the working temperature (ordinary galvanized materials will lose their effectiveness at around 100℃), the usage environment (in coastal areas or regions where salt is spread in winter, the corrosiveness will be stronger), the cost budget, and whether the product complies with international standards (such as DIN 3570, ASME B18.31.5, etc.).