Api 571 – Corrosion and Materials

Api 571 – Corrosion and Materials

Damage Mechanisms Affecting Fixed Equipment in the Refining Industry

Explanation of Failure Mechanisms

Creep and Stress Rupture

At high temperature, metal components can slowly and continuously deform under load below the yield stress. This time dependent deformation of stressed components is known as creep. The initial stages of creep damage can only be identified by scanning electron microscope metallography. Creep voids typically shows up at the grain boundaries and in later stages form fissure and than cracks. Threshold Temperature for Creep: 370˚C for C-Steel and 400˚C to 425˚C for C-Mo and Cr-Mo-Steels.

Thermal Fatigue

Thermal fatigue is the result of cyclic stresses caused by variations in temperature. The process starts on the surface in areas of high local stresses caused by notches (such as the toe of a weld) and sharp corners (such as the intersection of a nozzle with a vessel shell) and other stress concentrations may serve as initiation sites. The Process starts with locally movement of dislocations, blocking of dislocations on the grain boundaries, plastically deformation of local grains, creating of intrusions and extrusions, generating of corr. layers on the free surfaces of in- and extrusions in case of corrosive environment (Corrosion Fatigue). These are the initial stages of cracks. The cracks can be blocked through strengthening on tip of the cracks. Time of failure is a funktion of the magnitude of the stress and the number of cycles and decreases with increasing stress and increasing cycles.


In general, wear failures or Erosion may be defined as damage to a solid surface caused by the removal or displacement of material by the
mechanical action of a contacting solid, liquid, or gas. The term abrasive erosion is sometimes used to describe erosion in which the solid
particles move nearly parallel to the solid surface. The term impingement- or impact-erosion is used to desribe erosion in which the relative
motion of the solid particles is nearly normal to the solid surface. The collision at high speed of liquid droplets with a solid surface results in a
form of liquid erosion called liquid-impingement erosion. The high-velocity impact of a drop of liquid against a solid surface produces two effects
that results in damage to the surface: high pressure, which is generated in the area of the impact, and liquid flow along the surface at high speed
radially from the area of impact, which occurs as the initial pressure pulse subsides (water hammer pressure:liquid density x acoustic velocity of
the liquid x impact velocity). For example, for water impaction at 480 m/s =1728 km/h this pressure is about 1100 Mpa – considerably above
the yield strenght of many alloys. This value is somewhat reduced by the compressibility of the surface.


If gas-filled bubbles (or cavities) formed in a low-pressure region (suction side of a pump) pass into a region of higher pressure (pressure side of
a pump), their growth will be reversed, and they will collapse and disappear as the vapor condenses or the gas is resissolved in the liquid. A
vapor-filled cavity will implode, collapsing very rapidly. The collapse of cavities (bubbles) produces the damages to materials. Almost all of the
energy of the collapse will be used to compress the surrounding liquid. Only when the vapor pressure is high compared to ambient pressure or
when the dissolved-gas content is high.

Mechanical Fatigue

Fatigue cracking is a mechanical form of degradation that occurs when a component is exposed to cyclical stresses for an extended period,
often resulting in sudden, unexpected failure (Explanation: see Thermal Fatigue).

Reheat Cracking

Cracking of a metal due to stress relaxation during PWHT or in service at elevated temperatures. It is most often observed in heavy wall sections.
Reheat Cracking (or Stress- relief embrittlement) results in the loss of toughness within the HAZ and/or the weld metal as a result of stress
relieving of a welded structure. Reheat cracking is also thought to be caused by the same mechanisms and leads to intergranular cracking within
the weld zone upon stress relieving. Both phenomena (loss of toughness and intergranular cracking) have been observed only in those alloy
systems that undergo precipitation hardening. These system include low-alloy structural and pressure vessel steels, ferritic creep -resisting
steels, austenitic SS, and some nickel-base alloys. During welding, the HAZ is exposed to high temperatures, ranging up to the melting point
of the alloy. At these temperatures, existing precipitates in the base metal (in steels, carbides, and nitrides) are taken into solution, and grain
coarsening occurs. During cooling, some precipitation takes place at grain boundaries or within the grains, but the majority of the precipitates
remain in solution. Subsequent exposure at stress-relieving temperatures causes precipitation in the HAZ, leading to significant strengthening.
This results in the loss of toughness in the HAZ. Residual stresses in the structure are relieved through creep deformation. However, the
strengthening of precipitates of the grain interiors tends to concentrate creep strain at grain boundaries, leading to intergranular cracking.

Galvanic Corrosion

Galvanic Corrosion is a form of corrosion that can occur at the junction of dissimilar metals when they are joined together in a suitable
electrolyte, such as a moist or aqueous environment, or soils containing moisture. The less noble metal is more active and acts as Anode, i.e.
it dissolves. The more noble metal acts as Cathode, i.e. it remains intact. The Role of oxygen in Galvanic Corrosion: Oxygen is the most
corrosive gas in the presence of water. The maximum solubility of O2 in water is 8 ppm, i.e. sparingly soluble. The reduction of oxygen at the
cathode site, keeps the cyclic corrosion process on going. The corrosion rate depends on the rate of diffusion of O2 to the cathode site.
(H20 + 1/2O2 + e- = 2OH- ). The overall corrosion rate of O2 is about 72 times higher than CO2 and 200 times higher than H2S at low
concentrations (<2 ppm O2 and <200ppm H2S) and 400 times higher at high concentrations (8 ppm O2 and 800ppm H2S)

Atmospher Corrosion

A form of corrosion that occurs from moisture associated with atmospheric conditions. Marine environments are most severe. New C-steel
built in wet marine atmosphere a less protective layer Fe2O3, etc. With the time the thickness will be thicker and more brittleness.
Marine environments can be very corrosive 0.5 mm/year as specialy in the splash zone (riser), at coating failures (see Corrosion Under Insulation)
or in Crevices/Under Deposits.


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