The special corrosion form of titanium screws!

Titanium alloy screws (such as TC4) are known for their excellent corrosion resistance, but this does not mean that they are "never corroded" under any conditions. In certain specific and harsh environments, titanium screws may experience special forms of corrosion.
Understanding these special forms of corrosion is crucial for the proper selection and installation of titanium screws.

Core principle: "Passivation film" of titanium

The corrosion resistance of titanium comes from an extremely thin (about 3-5 nanometers) but abnormally dense and stable oxide film (mainly TiO ₂) on its surface. This "passivation film" can self repair in most environments and protect the metal inside. However, under the following special conditions, this film will be damaged, leading to corrosion.

Several special corrosion forms of titanium screws

1. Gap corrosion

This is the most common and alarming form of localized corrosion of titanium alloys in chlorine containing environments.

Occurrence conditions:

• There are narrow gaps (usually between 0.025mm and 0.1mm in width).
• There are stagnant corrosive media in the gaps (especially aqueous solutions containing chloride ions, such as seawater, saltwater, and humid salt spray air)

Why did it happen:

1. It is very difficult for oxygen to diffuse into the interior through gaps and cannot maintain the required oxygen concentration for the passivation film.
2. The concentration of chloride ions in the gap increases, and the pH value decreases (acidity becomes stronger)
3. When the environmental pH value is below a certain critical value (approximately 1-2 for titanium alloys), the passivation film will be damaged and undergo active dissolution.

Specific risks associated with screws:

• Between the screw cap and the contact surface of the component.
• At the gap of threaded engagement.
• The contact surface of the washer under the screw head.
• Any blind spot where electrolytes are prone to accumulate.

2. Galvanic corrosion

Also known as "contact corrosion" or "heterogeneous metal corrosion". This is the easiest place to make mistakes when installing titanium screws.

Occurrence conditions:

Titanium screws come into direct contact with a metal with a more negative (active) potential, such as aluminum or carbon steel, in the electrolyte.

Who will be damaged:

The key is that titanium itself will be protected, while the reactive metals in contact with it (such as aluminum frames and magnesium parts) will accelerate corrosion!

Why did it happen:

Titanium has a positive potential (more "inert") among common metals, equivalent to a "cathode". Aluminum, steel, and other materials are "anodes". When they form a circuit through the electrolyte, the anode will accelerate dissolution to protect the cathode.

Specific risks associated with screws:

• Directly screwing titanium screws into the threaded holes of aluminum alloy frames can cause rapid corrosion and decay of the aluminum threaded holes in humid environments, ultimately leading to thread failure and screw loosening.
• Contact between titanium screws and carbon steel washers or parts

3. Hydrogen embrittlement

This is a serious form of failure that may lead to sudden brittle fracture.

Occurrence conditions:

• Titanium atoms easily absorb hydrogen atoms.
• When titanium is in the following environment, hydrogen will infiltrate:
• Excessive cathodic protection: For example, in galvanic corrosion, titanium as the cathode will undergo hydrogen evolution reaction.
• In reducing acids such as hydrochloric acid and sulfuric acid.
• High temperature and high pressure hydrogen environment.
• Inappropriate coating or surface treatment (such as acid washing with hydrogen containing acid).

4. Stress corrosion cracking

Brittle fracture occurs under the combined action of specific corrosive environment and tensile stress.

Occurrence conditions:

Specific corrosive media: For titanium alloys, the main ones are fuming nitric acid, methanol, halogen elements, solid chlorides, etc. TC4 titanium alloy has excellent stress corrosion resistance in ordinary atmosphere or seawater.
Continuous tensile stress: It can be an external load or an internal residual stress generated during screw tightening.

Risk of screws:

It may occur in extremely special environments (such as chemical equipment), but for daily applications such as mountain bikes, the risk is extremely low.