5.1 Corrosion
Corrosion can be defined as the deterioration of a substance (usually a metal) or its properties because of a reaction with its environment. In the case of corrosion failure of pressure vessel, heat exchangers and boilers also depend on the environmental and metallurgical factors. Corrosion failure can be classified into many forms. The uniform loss of the metal is defined as a general corrosion or uniform corrosion . However, some of cases, metal loss is frequently localized in the form of discrete pits or larger localized areas.
5.2 Pitting
Pitting occurs when the metal undergoing corrosion suffers metal loss at localized areas rather than over the deepest metal penetration at the local areas to be average metal penetration..
Mechanism of Corrosion
Corrosion problems related to pressure vessels, boilers and heat exchangers are due to presence of water. The chemical reaction of the metal dissolution at the anode site (iron dissolution):-
2+ -
Fe ---------> Fe + 2e
Whereas the cathode site :-
2H + 2e -------------> H2
or if the presence of oxygen, two other reactions are also possible :
+ -
02 + 4H + 4e ------------> 2H2 0
- -
02 + 2H2 + 4e ------------> 4OH
However in in-service condition for the oil and gas production, with the presence of CO2
and H2S , the rust or scale formation is complex.
5.3 Erosion and Erosion - Corrosion
Erosion is a strictly mechanical phenomenon, while erosion-corrosion is a combination of mechanical action and chemical or electro – chemical reaction. Pure erosion seldom occurs in aqueous systems. Erosion – corrosion is characterized by grooves, gullies, waves, rounded holes and valleys, and usually exhibits a directional pattern. In copper alloy heat exchanger tubes, the attack frequently results in the formation of horseshoe – shaped depressions. Erosion – corrosion is the acceleration of metal loss because of the relative movement between a fluid and a metal surface. Generally the movement is rapid, and the effects of mechanical wear are involved. Metal is removed as dissolved ions or as solid corrosion products that are swept from the surfaces. Most metals are susceptible to erosion – corrosion. Soft ones, such as copper, aluminum, and lead, are particularly susceptible because they are readily worn mechanically. Many metals depend upon a passive film for their corrosion resistance, and when erosive forces remove these films, erosion – corrosion proceeds. Particles or gas bubbles carried in suspension can increase metal loss due to erosion – corrosion. The presence of dissolved oxygen in waters can cause a great increase in the erosion – corrosion of copper. Erosion – corrosion normally occur at the lower bottom of internal side of pressure vessels. In table 5.1, different phenomena governing erosion – corrosion is listed. The flow characteristics and shear stress between metal and medium will determine which of the phenomena is predominant. Table 5.1 also indicates increasing and decreasing influence of corrosion and mechanical factors. Materials, design and flow characteristic are important features of erosion – corrosion. This phenomena must be taken into account at the design stage and when selecting critical areas for inspection.
Table 5.1
EROSION – CORROSION – PHENOMENA
MECHANICAL WEAR
CAVITATION DEFORMATION
CAVITATION FATIGUE
CAVITATION CORROSION
IMPINGEMENT
TURBULENCE CORROSION
CORROSION IN FLOWING
ELECTROLYTE
5.4 Microbial Corrosion
- Separation Equipment:
Mineral scales often form in separation equipment due to pressure drop and temperature increase. Corrosion is often associated with the scale, especially in heater – theaters and in wet gas streams downstream of regulators. The water “leg” of separation vessels is the area with constant water contact, and must usually be protected by coatings, cathodic protection, or inhibition programs.
- Storage :
Tanks and vessels can be thought of “fat sports in the pipeline” where flow rate is slow or ever stagnant. Suspended solids tend to settle and form deposits, and even small amounts of water can settle to form a water “leg”. Stagnant areas are also excellent breeding grounds for bacterial colonies, so it is not surprising that tank bottoms are vulnerable to corrosion. The undersides of the roof of stock tanks and water tanks are subject to corrosion, due to condensation of water droplets in the presence of acid gases and oxygen. This present a constant safety hazard for persons walking on a tank roof, since the corrosion may be very extensive and is not visible. Above – coatings usually protect ground exterior surfaces of tankage. Cathodic protection is used to protect exterior surface in contact whit soil or moist concrete.
5.5 Defects in weld
Defect may mean discontinuity, imperfection, flow or in homogeneity. Some weld defects or in homogeneities are unlikely to result in a services failure. Others can be critical under specific services conditions or environments, and have caused or contributed to failures. The effects of defects in weld and base materials on the services life of pressure vessels, boilers and drums are largely misunderstood by inspectors in inspection codes establishing acceptability limits. The evaluating defect conditions with actual service experience often meet with difficulty. Many weldments that have provided completely satisfactory services under operating conditions considered to be extremely critical have contained defects of a magnitude claimed to be extremely dangerous in specific laboratory tests.
Weld defects have been studied for more extensively than base metal defects.
Arc Strikes
Arc strikes represent any localized heat – affected zone or change.
- Services Experience:
Only rarely have are strikes produced cracking leading to service failure.
Arc strikes have also occasionally resulted in under bead cracking. In the majority of instances involving a 1 ½Cr – ½Mo and 2 ¼Cr – 1Mo alloy steels, crack propagation during service has not occurred.
The arc strikes were sufficiently closed to the butt weld to be included in the stress relief heat treatments involving between 1 and 3 hours at temperatures between 1250 and 1375 F.
This has removed the effects of localized hardening and residual stresses, and has improved toughness and ductility.
Crack
- Definition :
Cracks represent linear ruptures of metal stress. Although cracks are often very narrow separations within the weld or adjacent base metal.
Hot cracking
Hot cracking occurs at elevated temperatures during cooling after the weld metal has been deposited and started to solidify from the molten state. The principal metallurgical factors are the presence of segregations or liquid phases along the grain boundaries. The latter tend to remain liquid to lower temperatures than weld metal does normally by itself.
* Ferritic Steels:
The most harmful elements causing hot cracking in ferrite steel weld deposits are described below in decreasing order of potency weld deposits are described below in decreasing order of potency
S, B, P, As, Cb, Sn, Sr, Ta, and Cu.
Silicon and nickel may also increase the susceptibility to hot cracking. The following elements are most generally responsible for hot cracking in decreasing order of potency :
B, S, Cb, As, Sn, Pb, Zr, AND Cu.
They tend to reduce the cracking tendency of some elements such as Cb or Ta by forming carbides, or contribute to cracking of other elements by reducing the presence of ferrite. Si enhances the cracking tendency in the presence of P, Ni reduces the cracking tendency of Cu. Ta adds to the crack susceptibility of Cb. Since many trace element are presents in base and weld metals, their interrelations are often difficult to evaluate. The welding filler metal after dilution with the base shall not contain on excess of embrittling phases such as Mg, Si, Cu, A12, A1FeSi, MnA16, ect.
Cold cracking
Cold cracking in steels refers to cracking, which occurs below 400 F, usually near or at room temperature. In general, cold cracking starts in the heat – affected zone unless the weld metal exhibits a higher harden ability than the base metal. In steels, cold cracking is primarily associated with the combined effects of hydrogen and martensite formation. Increasing the carbon content in the base metal and the manganese content in the weld also tends to promote cold cracking. To minimize the tendency of hydrogen to cause cold cracking in ferritic steels, particularly alloy steels, electrodes with special so-called law hydrogen coatings are extensively used for welding pressure.
Micro fissuring
Micro fissures or micro cracks are very small fissures not detectable at magnifications less than ten diameters. Often, they are visible at a magnification. The tendency toward cold cracking varies with different welding conditions such as welding process, procedure, filler metal composition, restraint, weld length, preheat, post heat, etc. Although specific welding variables make predictions difficult, the tendency toward cold cracking is influenced by the following factors.
Factors reducing cracking tendency:
· Preheat
· Post heat immediately after welding
· Increasing arc energy
· Skip welding
Factors increasing cracking tendency :
· Increasing wall thickness
· Increasing restraint
· Increasing strength of base or filler metal
· Increasing length of weld
- Service Experience:
It is generally recognized that fully austenitic stainless steel weld deposits are susceptible to micro fissuring during cooling soon after solidification (hot cracking), serviced in 1% columbium-stabilized 18Cr – 12Ni – 1 1/2Mo weld deposits producing fully austenitic weld metal. This composition has been used extensively in high-temperature steam power plant services in Germany.
Crater Pits
- Definition :
A crater pit represents an approximately circular surface condition extending into the weld metal in an irregular manner. It is caused by volumetric construction of molten metal during solidification, usually the result of abrupt interruption of the welding arc. Cracking starts from these pits, they are generally not considered a cause for rejection. Occasionally however, cracking may start from craters.
- Services Experience:
Unless cracks or incomplete fusion were present in the original weld, crater pits have not been responsible as primary causes acting as crack starters in welds which failed in service.
High / Low
- Definition:
Hi-low represents mismatch or misalignment of the plate or pipe ends across the weld root. It may be caused by quality of the pressure vessel ends, by differences of inside diameters, by problems of fit-up, by non-uniform weld end preparations.
Service Experience:
Where the weld joints have exhibited proper penetration and fusion in the weld root, service failures due to mismatch have been difficult to prove. Exceptions have been environments involving severe mechanical or thermal fatigue fatigue or stress corrosion.
Incomplete Fusion
- Definition:
Incomplete fusion involves lack of complete melting and coalescence (fusion) of some portion of the metal in a weld joint. It may occur either between weld beads or between weld base metal.
- Service Experience
Only a few instances of service failures due to incomplete fusion on the inside of weld have been reported. Where failures did occur the lack of internal fusion normally was quite severe, amounting to over 10 to 30 % of the effective wall thickness. Failures have resulted due to mechanical or thermal fatigue or stress-corrosion cracking.
Lack of Penetration
- Definition
Lack of penetration involves non complete penetration of the weld through the thickness of the joint. It usually applies to the initial weld pass or passes made from one or both sides of a joint.
- Service Experience:
Lack of penetration is one of the most critical defects and has resulted in service failures in pressure vessel and tank. When it occurs at the inside surface of a vessel weld failures may result in service involving mechanical or thermal fatigue or stress corrosion.
However, in noncritical service applications, lack of penetration may not result in cracking and service failures.
Overlap
- Definition:
Overlap represents an excess (overflow) of weld metal which extends beyond the limits of fusion over the surface of the base metal.
- Services Experience:
In butt welds, minor overlap has resulted in service failure severe overlap in fillet welds has led failure because of insufficient joint strength.
Oxidation
- Definition:
Surface oxidation is the result of insufficient protection from the atmosphere of the weld and adjacent base-metal surface oxidation is concern generally only on the underside of weld joints made without backing rings.
- Services Experience:
There has been no evidence that surface oxidation of the type has resulted in cracking or service failures.
Porosity
- Definition:
Porosity is the presence of gas pockets or voids (usually spherical in shape) caused by the entrapment of gas evolved during weld metal solidification. Sometimes elongated tubular gas pockets are also described as worm holes or piping. Large isolated gas pockets may be referred to as blowholes. These may be partially filled with slag.
-Service Experience:
Very rarely has porosity been associated with actual service failures. No failures have been reported in welds where the porosity was within applicable Code requirements. Quite likely porosity limits two to four time those now established in Codes could be tolerated in the majority of critical service applications.
Sink or Concavity
- Definition:
Sink or suck-up in a root weld bead refers to concavity. It is produced by gravity sink of the molten metal or by surface tension of the weld bevel pulling the molten weld metal into the bevel. The occurrence of sink depends on various factors including weld joint preparation, welding procedure variables and materials. Too much heat in the second weld pass may also cause some suck-back in the first root pass weld. Although root concavity is most general associated with welds made by the inert-gas tungsten-arc welding process.
- Service Experience:
Excessive sink has been involved in a number of service failures involving severe mechanical or thermal fatigue. In several instances, cracking appeared to have occurred in the weld root where the welding was done originally. The amount of concavity which can be tolerated will depend on the service requirements, material and joint dimensions.
Slag inclusion
- Definition:
Slag represents nonmetallic solid material entrapped in the weld deposit or between weld metal and base metal. Slag inclusions may appear as individual particles as linear continuous or interrupted bands.
Slugging
- Definition :
Slugging, also known as stubbing refers to the addition of a separate piece or piece of material in a joint before or during welding. It may be the result of entrapped welding filler wires or electrodes.
- Service Experience:
Where strength welds are required, slugging can be very harmful.
Undercut
- Definition:
Undercut represents an intermittent or continuous groove or channel melted into the base metal adjacent to the toe or surface of a weld and left unfilled by weld metal.
- Service Experience:
In services involving severe mechanical or thermal fatigue or pipe movement, undercut representing a sharp notch condition may result in cracking and failure. Applicable Codes covering critical application normally do not permit undercut or limit its depth to 0.020 in. Actually, however, slight undercut is not uncommon, particularly on pipe and tank welds where the weld is made in the horizontal position. There is no evidence that such undercut has caused failures. Undercut shall probably be considered in the same light as some surface laps and slivers on steel plate and pipe which in fact are permissible to a depth of 5%.
Weld Reinforcement
- Definition:
The weld reinforcement represents the metal on the face of a weld in excess of the metal necessary for the specified weld size. Whereas on pressure vessels the weld reinforcement is generally removed, it is frequently left intact on pipe joints. The weld reinforcement shape and appearance differs with welding process and even the weld metal or electrode used. The reinforcement shape includes the height and which of the reinforcement, and the radius of the bead curvature at the junction between the reinforcement and the plate or pipe surface.
5.6 Hydrogen Blistering
Hydrogen blistering is a general term which refers to mechanical damage of metal caused by the presence of, or interaction with hydrogen. Hydrogen blistering results from the penetration of hydrogen into a metal. The results are local deformation and in extreme cases, complete destruction of the vessel wall. The interior contains an acid electrolyte, and the exterior is exposed to the atmosphere. Hydrogen evolution occurs on the inner surface as a result of a corrosion reaction on cathodic protection. At any time there is a fixed concentration of hydrogen atoms on the metal surface, and some of these diffuse into the metal rather than combining into molecules, as shown. Much of the hydrogen diffuses through the steel and combines to form hydrogen molecules on the exterior surface. If hydrogen atoms diffuse into a void a common defect in rimmed steels, they combine into molecular hydrogen. Since molecular hydrogen cannot diffuse, the concentration and pressure of hydrogen gas within the void increases. The equilibrium pressure of molecular hydrogen in contact with atomic hydrogen is several hundred thousand atmospheres, which is sufficient to rupture any known engineering material. Hydrogen blistering can be occurs in storage tanks and in refining processes. Schematic illustration showing the mechanism of hydrogen blistering
5.7 Brittle Fracture
Introduction
Unexpected and sudden failure in pressure vessels and other structure and failure is due to ductile to brittle transition of the steel at the low temperatures.
- Brittle fractures in pressure vessels are due to three factors: -
(i) Steel properties.
(ii) Design details.
(iii) Fabrication methods.
5.7.1 Characteristic of Ductile vs. Brittle Behavior
Steel is generally considered in the ductile manner. However the microstructure phase will changing when the steel subjected to the low temperatures. Hence, the brittle failure with the little plastic deformation causes the rupture of the steel.
3 factors control the behavior of the brittle fracture
i) High rate stress concentration eg. Notches, nicks, scratches, internal flow. etc.
ii) A high rate of straining
iii) A low environmental temperature
5.7.2 Evaluation of Brittle Behavior
Mechanical testing can carry out the evaluation of brittle behavior.
The mechanical test shall involved in the following categories:-
i) Impact energy
ii) Fracture appearance
iii) Notch ductility
The criteria are also referred to as energy transition, fracture transition and ductility transition.
5.7.3 Effect of Temperatures
The transition from ductile to brittle behavior of carbon and low alloy steel is shown in the Figure.
The liquefaction temperatures of the most frequently used industrial gases and of materials recommended for used at these temperatures, are given in table 5.2.
Table 5.2: liquefaction temperatures of gases and recommended types of material
Gas | Liquefaction Temperature C | Type of material recommended |
Ammonia | - 33.4 | Carbon steel fine grain Al |
Propane (LPG) | - 42.1 to – 45.5 | killed steel |
Propylene | - 47.7 | 2.25 % Ni steel |
Carbon disulphide | - 50 |
|
Hydrogen sulphide | - 59.5 | 3.5 % Ni steel |
Carbon dioxide | - 78.5 |
|
Acetylene | - 84 |
|
Ethane | - 88.4 |
|
Ethylene (LEG) | - 103.8 | 5.9 % Ni steel |
Krypton | - 151 |
|
Methane (LNG) | - 16.3 |
|
Oxygen | - 182.9 |
|
Argon | - 185.9 |
|
Fluorine | - 188.1 |
|
Nitrogen | - 195.8 | Austenitic Stainless steel |
Neon | - 246.1 |
|
Heavy hydrogen | - 249.6 | Al alloys |
Hydrogen | - 252.8 | 12% Ni fine grain steel |
Helium | - 268.9 |
|
Absolute zero | - 273.18 |
|
5.7.5 Effect of Notches and Notch Sensitivity
The sudden brittle failures that occur without measurable deformation are generally to the notch sensitivity of the steel at the operating temperature to which the steel was exposed. Notch sensitivity is usually associated with an inability of the steel to deform in a plastic manner (that is to flow) underneath the notch. This resistance to flow is increased by the tri-axial state of stress induced under a notch by tensile stress. Figure in shall the behavior of deferent notch on the transition temperature.
Note: The greater the sharpness of the notch, the greater the severe of restraint
Cause: The more severe of stresses.
Steel, which contains extremely severe notches, will fail in a brittle manner ‑ at higher ambient temperatures than if less severe notches were present.
5.7.6 Code and Standard Requirements
ASME Boiler and Pressure Vessel Code Section VIII ‑ require 3 charpy Keyhole specimens to have average impact value of 15ft 16 at the lowest operating temperatures.
5.7.7 Factors Determination Transition Temperatures
The fracture, which influences the transition temperature range metallurgical factors and mechanical factors.
‑ Metallurgical Factors:
i. Composition
ii. Homogeneity
iii. Grain Size
iv. Straining and Aging
v. Heat Treatment
‑ Mechanical Factors:
i. Stress System
ii. Section size
iii. Design
iv. Workmanship and welding.
5.8 The Effect of Stress on Corrosion
5.8.1 Environment Sensitive and Insensitive Behavior
The general and localized corrosion phenomena are largely affected by the stress state of the metal ie. They are stress ‑ insensitive. Thus for example the rate of atmospheric corrosion of steel, the rate of general corrosion of stainless steel in sulphuric acid and the rate of dezincification of brass in sea‑water are, to a very good approximation, independent of the presence of mechanical stresses in the corroding metals. Similarly, many of the mechanical properties of metals are, in engineering terms, environment‑ insensitive. Thus, for example, the yield strength, the fracture toughness and the ductility of most metals are not affected, within broad limits, by the corrosively of the environment in which they are measured.
There are, however, exceptions to these generalizations, when may be interactions between the stresses in a metal and its environment. There are two stress dependent corrosion phenomena which are particularly important in practice, and which will be dealt with in this section.
5.8.2 Corrosion ‑ Fatigue
Metals that are subject to alternating stresses may fail by fatigue. Basically this involves the initiation and relatively slow growth or propagation of a crack in the metal. Fatigue is a process that can occur in a completely inert environment such as pure, dry argon. However, it is frequently observed that when metals are simultaneously exposed to cyclic loads and a corrosion environment, there is a greater probability of fatigue failures. Typically, the number of cycles to failure is reduced and the crack growth rate is increased. This phenomenon which is known as corrosion fatigue, is therefore one of environmentally accelerated slow (ie. sub‑critical) crack growth.
5.8.3 Stress Corrosion Cracking (SCC)
When certain metals are simultaneously exposed to a tensile stress and a fairly specific corrosive environment they may be susceptible to stress corrosion. This also involves crack initiation and relatively slow crack propagation, but in contrast to the case of fatigue and corrosion fatigue, stress corrosion always requires the conjoint action of a tensile stress and a corrosive environment; in the absence of either, crack propagation cannot occur. The phenomenon of stress corrosion is therefore one of environmentally induced, sub‑critical crack growth.
5.8.4 Practical Importance of Stress Corrosion Cracking and Corrosion ‑ Fatigue.
Stress corrosion cracking and Corrosion fatigue are two extremely important forms of localized corrosion. Several surveys of corrosion failures in different companies and various industries have indicated that 20 ‑ 40% of all the failures experienced were due to stress corrosion or to a lesser extent to corrosion fatigue. Furthermore, since stresses are involved in both cases, the proportion of the failures due to stress corrosion or corrosion fatigue that were regarded as serious or potentially dangerous was higher than in the case of other forms of corrosion. Examples of service failure due to stress corrosion and corrosion fatigue are shown in Fig as shown. Although stress corrosion and corrosion fatigue both result in time‑ dependent, or delayed failures involving crack initiation and propagation they are basically different phenomena. Furthermore, although electrochemical factors are involved in both cases, the mechanisms of stress corrosion and corrosion fatigue crack propagation are different.
