October 2010 Archives

Hydrogen embritlement occurs with hydrogen occlusion of metals. When considering a corrosion phenomena where steel dissolves with hydrogen out-gassing, the reaction is a steel redox process of oxidation by hydrogen ions.

When this reaction is viewed in detail, it can be divided into the following two reactions.

That is, the electrons (e) emitted by dissolution of steel combines with hydrogen ions (H+) within the solution, then hydrogen gas generation occurs. It is thought, in general, that such redox (Reduction-Oxidation) reaction occurs when metals corrode.

The mechanism of hydrogen gas emission from metal surfaces can be considered as follows. The hydrogen ions (H+) become of atomic hydrogen (H), then combine into molecular hydrogen gas (H2). The atomic state hydrogen will exist as adhered to the metal surface, and a portion of this hydrogen will penetrate into the metal.

The hydrogen entered into metal will diverge under stresses and congregate on lattice defects of the metal, then embrittles the metal. The stress causes the fractures. Especially, the high strength steel can generate hydrogen when corroding, and become embrittled by absorbing the hydrogen. When stresses are applied in this state, the steel will easily fracture.

There are two causes for hydrogen generation from metal surfaces. The first is when steel dissolves in acids, as explained above. When dissolving in solutions, steel generates hydrogen. It typically occurs during pickling and welding processes.

Another is when the subject metal becomes a negative in polarity and an electrical current flows. Typical cases applicable for this are cathodic electrolysis and passivation processes for surface treatments such as electro-plating and electro-cleaning.

The embrittlement fracture occurs only with metals highly susceptible to hydrogen occlusion such as high tensile strength steel. Steels with tensile strength less than 40kg/mm² are hardly affected.

It is said that when compounds such as hydrogen sulfide and arsenic exist in the corrosive environment, atomic state hydrogen bonding into becoming molecular state hydrogen gas. Therefore, the hydrogen concentration at metal surface will increase and causes embrittlement of even lower tensile steels. This phenomenon is also called Sulfide Corrosion Embrittlement, and often causes corrosion caused accidents on hydrogen sulfide containing natural gas/oil drilling, transportation, refining, and storages.

High tensile strength bolts are used for construction of giant structures such as bridges. These bolts, even in neutral air environments, may be subject to fractures due to minute amount of hydrogen in the air. This phenomenon is called "Delayed Fracture of High Tensile Bolts". Also, fractures occurring after a period of time when hydrogen occluded bolts are tightened at less than the rated tensile stress is called "Delayed Fracture".

As seen in metal tensile tests, a metal specimen pulled at two ends will eventually result in a fracture. This is called Tensile Strength. In certain environments, metals can fracture at less than rated tensile loads. The phenomenon is called Stress Corrosion Fracture. The stress that cause Stress Corrosion cracking is in tensile, including stresses during the usage and internal stress applied during machining.

[Fig.1] is a schematic illustration of Stress Corrosion cracking. Corrosion points occurring on the surface gradually progress. Either internal or external tensile stress is applied on this material and the corrosion and fracture tips will become sharp, and the direction of the advancing fractures will zigzag. The fractures may be crystal grain boundary type or intra crystal type.

Stress Corrosion cracking occurs in some specific combination of metals and environments, as shown in [Table 1]. As shown, the corrosion occurs only when the specific corrosion environment exists, even when tensile stress exists. It can be said that this is a type corrosion in rather limited environments.
A classical example of Stress Corrosion cracking known is the corrosion of steam locomotive boiler rivet areas. This was caused by a combination of carbon steel material and hot water with NaOH additive used as a corrosion inhibitor.
Also, well known is the seasonal fracture of brass material used for ammunition cartridges where numerous cartridge brass fracture was seen during the Monsoon season. The cartridge brass cold-process manufactured with a combination of brass and NH3 experienced Stress Corrosion cracking affected by humidity, oxygen, sulfur dioxide gas, and ammonia in the air.
Austenitic stainless steel is likely to experience Stress Corrosion cracking in chloride solutions and sea water, therefore, post forming stress relieved by heat treating is applied, as well as substituted with duplex grade material with austenite and ferrite characteristics.

[Table 1] Stress Corrosion cracking Causing Metals and Environments

Metals are generally composed of many crystals. The contacting surfaces of the crystals are called Grain Boundary. Within each crystal, the atoms are in an orderly alignment, but the alignments of the atoms in adjacent crystals are different. The atoms within the grain boundary areas must integrate with atoms of both crystals in dissimilar alignments, therefore, they are in mixed orientations. This means that the energy levels are in high state.

This high energy state of grain boundaries is evident from microscope observations of corrosive solution etched specimen where the grain boundaries are dissolved and the crystals becoming clearly visible.

This type of chemical corrosion remains on the surface and does not advance further, but if heated under certain conditions the crystal grain boundaries will forego chemical composition changes and some selective corrosions will occur. This phenomenon is evident on austenitic stainless steel such as SUS304, as explained below.

The reason for stainless steel's corrosion resistance is the passive surface layer, and existence of chromium is imperative for this. Stainless steel generally contain 12~13% chromium, but the element in question here is the amount of carbon content in steel.

Carbon tends to bind with chromium and easily form chromium carbide. The chromium carbide is formed when stainless steel is heated at 500~800 degrees C for a certain period of time, though the time duration varies depending on the carbon content. At 750 degrees C and 0.06% carbon content, the duration is shortest at less than one minute, and as long as several hundred hours at 500 degrees C.

The chromium carbides form at grain boundary areas. In a carbide form, chromium does not contribute in creation of passive layer. Therefore, exposed stainless steel surface will be lacking the passive layer along the grain boundaries. When stainless steel in this state is exposed to corrosive environments, corrosion along the grain boundaries will progress. This is called Intergranuler Corrosion. A sectional photo of SUS304 stainless steel with Intergranuler Corrosion is shown in [Fig.1]. Principle of this corrosion is based on formation of a battery where the corrosion progresses as the passive layer portion being the positive pole, and the portion without passive layer is the negative pole.

As countermeasures to prevent Intergranuler Corrosion, the following process is applied. A stainless steel material in the final form factor is heated to over 1000 degrees C to dissolve the chromium carbide content, and to promote the divergence of chromium toward the grain boundaries from the other areas, then rapid cooled with water. This is called Solution Heat Treating. Use of stainless steel processed to prevent Intergranuler Corrosion in high temperature applications such as welding is not recommended since Intergranuler Corrosion will occur. For such applications, lowered carbon content SUS304L and SUS316L (0.03%C) are used.
Intergranuler Corrosion does not occur in carbon steel, but occurs with high tensile aluminum alloy (with a few % copper alloyed).