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Corrosion Fundamentals

Hydrogen Damage

Hydrogen can cause a number of corrosion problems. Hydrogen embrittlement is a problem with high-strength steels, titanium, and some other metals. Control is by eliminating hydrogen from the environment or by the use of resistant alloys.

Hydrogen blistering can occur when hydrogen enters steel as a result of the reduction reaction on a metal cathode. Single-atom nacent hydrogen atoms then diffuse through the metal until they meet with another atom, usually at inclusions or defects in the metal. The resultant diatomic hydrogen molecules are then too big to migrate and become trapped. Eventually a gas blister builds up and may split the metal as shown in the picture below.

Gas Blister which has Split Metal

Hydrogen blistering is controlled by minimizing corrosion in acidic environments. It is not a problem in neutral or caustic environments or with high-quality steels that have low impurity and inclusion levels.

SEM Micrograph of Hydrogen Embrittlement

SEM of Intergranular Cleavage

The broken spring above on the left was brought to the KSC Materials Laboratory for failure analysis. Examination at high magnification in the scanning electron microscope (above right) revealed intergranular cleavage characteristic of hydrogen assisted cracking (hydrogen embrittlement). The part was zinc plated during refurbishment, and the hydrogen which entered the metal during the plating process had not been baked out. A postplating bakeout procedure should be standard for high strength steels.

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