Environmental Engineering Reference
In-Depth Information
strength steels working in ''sour'' oil fields as a result of hydrogen damage.
Stainless steels of all types, aluminum, nickel, copper and their alloys, titanium
and zirconium alloys, and refractory metals like tungsten, niobium, vanadium,
and tantalum suffer from hydrogen damage.
Hydrogen-induced subcritical crack growth has been suggested as the domi-
nant stress corrosion cracking (SCC) mechanism for ferritic steels in particular,
and for metastable stainless steels, nickel-base alloys, titanium alloys, and alumi-
num alloys.
8.2 SOURCES OF HYDROGEN
Hydrogen may enter the metal from various sources. Atomic hydrogen, rather
than molecular hydrogen, is considered to be the detrimental species in metals
which may, however, be absorbed from a molecular hydrogen gas atmosphere.
The hydrogen is readily available in environments such as water, water vapor,
moist air, hydrocarbons, acids, hydrogen sulfide, and in various liquids and gases
involved in chemical process operations.
A major source of hydrogen in liquid metals is from water contained in the
scrap used as charge material in the furnace, from the slag ingredients, or from
the refractory materials of the furnace linings. The solubility of hydrogen de-
creases with decreasing temperatures in metals such as iron, nickel, cobalt, cop-
per, chromium, and aluminum, and hydrogen gets entrapped in these metals dur-
ing solidification after melting or welding operations. The entrapped hydrogen
is responsible for solidification porosity and the formation of hydrogen ''flakes''
or ''fish-eyes'' (Section 8.3.3) in rolled or forged steel products. High-strength
steels are more prone to hydrogen pickup when melted under high-humidity con-
ditions. As a result, their vacuum degassing becomes necessary before pouring.
Liquid steels, in general, show rapid absorption of hydrogen after deoxidation.
As such, the killed steels become more susceptible to flaking or blistering than the
semikilled steels. Moisture contamination is a particular problem with electroslag
refining (ESR) process, and special care must be taken to avoid contamination
of slag materials.
Hydrogen may be introduced during several stages of equipment or component
manufacture, even before they are put in service. Welding, heat treatment in
hydrogen-containing furnace atmospheres, acid pickling, or electroplating opera-
tions can each introduce hydrogen into the lattice of the metal.
Underbead cracking is an embrittlement phenomenon in weldments that is
associated with hydrogen pickup during welding operations. Moisture in elec-
trode coating, high humidity in the atmosphere, and organic contaminants on the
surface of the prepared joints are responsible for hydrogen entry in the metal.
Upon rapid cooling of the weld, entrapped hydrogen can produce internal fissur-
ing and other damages.
