ENT 487 ENVIRONMENTALLY ASSISTED CRACKING IN METALS DR. HAFTIRMAN LECTURE 12 WED, 8 OCTOBER 2008

TYPES OF CORROSION Uniform attack Uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or over a large area. Galvanic (two-metal) corrosion A potential difference usually exists between two dissimilar metals when they are immersed in a corrosive or conductive solution. If these metals are placed in contact, this potential difference produces electron flow between them.

TYPES OF CORROSION

TYPES OF CORROSION

TYPES OF CORROSION

TYPES OF CORROSION

TYPES OF CORROSION Crevice corrosion Intensive localized corrosion frequently occurs within crevice and other shielded areas on metal surfaces exposed to corrosives.

TYPES OF CORROSION

TYPES OF CORROSION Pitting Pitting is a form of extremely localized attack that results in holes in the metal.

TYPES OF CORROSION

Intergranular corrosion. TYPES OF CORROSION Intergranular corrosion. Grain boundary effects are of little or no consequence in most applications or uses of metals. If metal corrodes, uniform attack results since grain boundaries are usually only slightly more reactive than the matrix.

TYPES OF CORROSION

TYPES OF CORROSION Erosion corrosion Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface. Generally this movement is quite rapid, and mechanical wear effects or abrasion are involved. Metal is removed from the surface as dissolved ions, or it forms solid corrosion products that are mechanically swept from the metal surface.

TYPES OF CORROSION

TYPES OF CORROSION Environmental cracking Environment cracking that are not specific to one particular mechanism. Some of these more general phenomena, such as occluded chemistry, threshold stress intensity, fluctuating vs static loads, and crack morphology.

CLASSIFICATION OF CRACKING MECHANISMS The term environmentally assisted cracking (EAC) is meant to be generic, as it refers to all cracking in metals that is aided by a chemical environment. There are four recognized types of EAC. Stress corrosion cracking (SCC) Hydrogen embrittlement (HE) Corrosion Fatigue (CF) Liquid metal embritllement (LME)

STRESS CORROSION CRACKING (SCC) SCC refers to crack propagation that is driven by an anodic corrosion reaction at the crack tip. The crack propagates because the material at the crack tip is consumed by the corrosion reaction. In many cases, SCC occurs when there is little visible evidence of general corrosion on the metal surface, and is commonly associated with metals that exhibit substantial passivity.

STRESS CORROSION CRACKING (SCC) Figure is a simple illustration of SCC. In order for the crack to propagate by this mechanism, the corrosion rate at the crack tip must be much greater than the corrosion rate at the walls of the crack.

STRESS CORROSION CRACKING (SCC) If the crack faces and crack tip corrode at similar rates, the crack will blunt. Under conditions that are favorable to SCC, a passive film (usually an oxide) forms on the crack walls. This protective layer suppresses the corrosion reactions on the crack faces. High stresses at the crack tip cause the protective film to rupture locally, which exposed the metal surface to the elctrolyte, resulting in crack propagation due to anodic dissolution.

HYDROGEN EMBRITTLEMENT (HE) When atomic hydrogen is introduced into an alloy, the toughness and ductility can be reduced dramatically, and subcritical crack growth can occur. HE involves the loss of a metal’s bond strength due to the presence of atomic hydrogen at grain boundaries and interstitial sites in the crystal lattice. O particular interest in the present context are situations where the presence of atomic hydrogen leads to crack propagation. In many of these instances, an electrochemical corrosion reaction acts as a hydrogen source at the crack tip. Hydrogen embrittlemen is a very common mechanism for EAC.

HYDROGEN EMBRITTLEMENT (HE) Hydrogen atoms are small compared to most metallic atoms such as iron, aluminum, and titanium. As a result, hydrogen atoms can fir within interstitial sites in a metallic crystal, as well as at grain boundaries. Moreover, atomic hydrogen readily diffuses through metals, even at room temperature.

HYDROGEN EMBRITTLEMENT (HE) Hydrogen embrittlement should be divided into two categories: Hydrogen-environment assisted cracking (HEAC). Internal-hydrogen-assisted cracking (IHAC)

HYDROGEN EMBRITTLEMENT (HE) In both HEAC and IHAC, hydrogen is concentrated at the fracture process zone near the crack tip. The high degree of stress triaxiality near the crack tip causes the crystal lattice to expand, which increases the hydrogen solubility locally. The high local concentration of hydrogen causes the process zone to be embrittled (rapuh). This embrittlement, along with the high local stresses, results in microcracking in the process zone. The microcracks that form in the process zone link up with the main crack, resulting in cracking extension. The main crack propagates over time, as the local crack-tip processes of hydrogen uptake and microcracking occur continuously.

HYDROGEN EMBRITTLEMENT (HE) HEAC involve hydrogen entering the material at the crack tip. An example of HEAC is when a material is exposed to H2 gas. Atomic hydrogen is produced at the crack tip when H2 molecules disassociate. In the absence of a crack under stress, the amount of atomic hydrogen absorbed into the material is negligible at ambient temperature. However, the triaxial stress at the tip of a crack under an applied load affects the equilibrium between H2 and atomic hydrogen.

HYDROGEN EMBRITTLEMENT (HE)

HYDROGEN EMBRITTLEMENT (HE) Figure shows lists two other sources of hydrogen that can drive HEAC: water vapor and electrolyte. In the later case, a corrosion reactions occurs inside the hydrogen are absorbed on the surface through such electrochemical processes. IHAC occurs when there is dissolved hydrogen in the material. The solubility of atomic hydrogen in most materials is very low at ambient temperature but is significant at elevated temperature.

HYDROGEN EMBRITTLEMENT (HE) A material can become hydrogen charged at an elevated temperature when it is exposed to H2 gas or other compounds that contain hydrogen, such as H2S. Upon cooling to ambient temperature, atomic hydrogen difuses out of the material because it is supersaturated. The concentration of hydrogen in the fracture process zone can be one or more orders of magnitude greater than the bulk concentration.