110 close book 2 : 45
60 open book 3:45
MCQ 50
Multiple response 1-2
Drag and drop 3-4
No mark 30
Api 510 65-85 (78)
Api 572 10-16 (12)
Api 576 8-10 (10)
Api 578 1-2 (2)
Api 571 20-25 (22)
Api 577 5-8 (6)
Asme sec viii 12-20 (18)
Asme sec V 12-18 (16)
Asme sec ix 4-6 (6)
AMINE STRESS CORROSION CRACKING
Definition
Caused by action of tensile stress in amine environment mostly use to remove H2S and CO2 from gases
or liquid hydrocarbons.
Cause
Non PWHT welds
Prevention
PWHT and use of SS cladding or better alloy.
Affected Materials
Carbon Steel and low alloy steels
Safe Materials / Safe Conditions
Hight temperature increases amine stress corrosion cracking but found in low amine solutions like MEA
at room temperatures as well.
Mostly associated with lean amine services as rich amine services mostly form iron sulfide layer that
protects. But concentration of amine doesn’t effect on crack propagation.
Effective & Non-Effective NDE
WFMT, ACFM, angle beam UT (SWUT & PAUT), ET, Acoustic emission testing (AET) are effective
PT can be used but shouldn’t be only mean because it is not effective for tight cracks as are filled with
oxide.
RT is also not very effective finding fine tight crack.
Morphology
Intergranular and oxide filled with some branching mostly on welds or heat affected zones
Related Mechanisms
Caustic SCC, Alkaline SCC, wet H2S, ammonia SCC, carbonate SCC,
ATMOSPHERIC CORROSION
Definition
Mostly occurs because of moisture in environment.
Cause
Unpainted surfaces, Coating damage, Downwind from towers, Marine environments (20 mpy), industrial
environments containing acid fumes (5 -10 mpy), inland with moderate moisture (1-3 mpy), dry rural (<1
mpy)
Prevention
Proper painting/coating
Affected Materials
CS, low alloy steel and Copper Alloyed Aluminum
Safe Materials / Safe Conditions
Proper painting/coating
Effective & Non-Effective NDE
VT, UT, RT (profile), ECT
BRITTLE FRACTURE
Definition
Sudden rapid fracture under stress with no evidence of ductility
Cause
Low material fracture toughness
Size shape and stress concentration
Residual stress application
Prevention
Controlled pressure, temp environments
Affected Materials
Carbon steel, low alloy steel and 400 series SS
Safe Materials / Safe Conditions
Fully killed, fine grain austenitic steel with grain structure finer than 6
Austenitic SS (300 series)
Controlled pressure, temp environments
Effective & Non-Effective NDE
Routine inspection isn’t very effective.
PT, MT and UT for preexisting cracks or cracking in service.
Morphology
Straight non branching cracks showing no evidence of plastic deformation.
Fracture surface if undamaged may show chevron markings.
Cleavage with limited intergranular cracking
Related Mechanism
Temper embrittlement, strain age embrittlement, Titanium hydriding, sigma embrittlement
CAUSTIC CORROSION
Definition
Localized corrosion due to the concentration of caustic (alkaline) solutions such as NaOH and KOH,
and/or corrosive salts from those solutions, that usually occurs under evaporative or high heat transfer
conditions
Cause

Caustic (KOH, NaOH)
Prevention
In steam generating equipment by proper design minimizing caustic concentration by proper flooding of
water. Proper burner management to minimize hot spots.
Caustic injection facilities should be designed to allow proper mixing and adequate dilution of caustic.
Affected Materials
CS, Low alloy steel and 400 series SS
300 series usually resistant to caustic until passivity is damaged which can occurs at 160 F – 210 F
depending upon concentration.
In high-solution-strength caustic, temperatures above about 170 °F (75 °C), with the temperature limit
depending on the caustic concentration, will cause general corrosion of carbon steel.
Morphology
Caustic gouging is typically characterized by localized metal loss that may appear as grooves in a boiler
tube or locally thinned areas under insulating deposits
Effective & Non-Effective NDE
UT, Angle Beam (SWUT, PAUT) , RT within limits of technique
CAUSTIC STRESS CORROSION CRACKING
Definition
Caustic SCC is characterized by surface-initiated cracks that occur in piping and equipment exposed to
caustic (alkaline hydroxide solutions) at elevated temperature, primarily adjacent to non-PWHT’d welds
Cause
Caustic Solution in non PWHT areas
Prevention
PWHT A heat treatment at 1150 °F (620 °C) minimum with a minimum holding time of 1 hr is
considered an effective stress relieving heat treatment for carbon steel.
300 series SS offer little advantage in resistance to cracking over carbon steel.
Nickel-based alloys are more resistant to cracking and may be required at higher temperatures and/or
caustic concentrations. However, caustic SCC of these alloys has been observed at high temperatures that
promote the formation of molten caustic in the absence of free water [604 °F (318 °C), at atmospheric
pressure]. This damage has sometimes been referred to as molten caustic cracking.
Steam out of non-PWHT’d carbon steel piping and equipment should be avoided
Affected Materials
Carbon steel, low-alloy steels, and 300 series SS are susceptible. Duplex stainless steels are also
susceptible but have shown improved resistance compared to the 300 series SS. Nickel-based alloys are
more resistant.
Effective & Non-Effective NDE
WMFT, ACFM, ECT, Angle beam UT, AET
PT or MT can be effective. (PT may not useful for oxide filled cracks)
Morphology
The pattern of cracking observed on the steel surface is sometimes described as a spider web of small
cracks
Cracking in 300 series SS is most often transgranular but can be intergranular even in non-sensitized
material. When transgranular, it can be difficult to distinguish from Cl− SCC.
Caustic SCC of nickel-based alloys has been observed as either intergranular (Figure 3-15-9) or
transgranular
CHLORIDE STRESS CORROSION CRACKING
Definition
Surface initiated cracking of 300 series SS and some nickel-based alloys under the combined action of
tensile stress, temperature and aqueous chloride environment.
Cause
Chloride content, temperature, PH, tensile stresses, presence of oxygen
Mostly inorganic chloride CL- ion cause Cl-SCC
No practical lower limit exists for prevention.
Temperature above 140 F-400 F most concern.
Affected Materials
Mostly 300 series SS (welds in SS mostly contain ferrite producing duplex structure which is more
resistant)
Duplex SS are more resistant but still susceptible.
Nickle based alloys are highly resistant but still not immune.
Safe Materials / Safe Conditions
Stainless steel with nickel 8-12 % highly susceptible.
Alloys with Nickel content above 35 % are highly resistant
And above 45 % nearly immune.
Appearance
Spider web/ craze cracking, typically branched trans granular, 300 SS maybe intergranular, typically have
brittle appearance
Effective & Non-Effective NDE
VT for advance cracks
PT, ECT, Angle beam UT (PAUT, SWUT) is effective
RT is not very effective except in advance stage
COOLING WATER CORROSION
Definition
General or localized corrosion of carbon steels and other metals caused by dissolved salts, gases, organic
compounds, or microbiological activity in cooling water systems
Cooling water corrosion, fouling, and MIC are closely related and should be considered together.
Affected Materials
Carbon steel, all grades of stainless steel, copper and copper alloys, aluminum and aluminum alloys,
titanium, and nickel alloys.
Safe Materials / Safe Conditions
Low velocities can promote increased corrosion. Velocities should be high enough to minimize fouling
and dropout of deposits but not so high as to cause erosion.
Generally, velocities below about 3 fps (1 m/s) are likely to result in fouling
Process-side inlet temperatures of water-cooled exchangers should be maintained below 140 °F (60 °C).
Effective & Non-Effective NDE
Remote field testing is commonly used for inspection of ferrous (carbon steel) tubes. RFT has an equal
sensitivity to ID and OD indications and can detect and size corrosion and pitting as well as baffle cuts.
ECT is the preferred method for non-ferromagnetic materials as it has a higher probability of detecting
all types of damage than ultrasonic methods. Internal rotating inspection system IRIS is used when a
higher flaw detection and sizing capability is needed (compared to the other methods), but it is slower,
and thorough tube cleaning is required prior to inspection. IRIS can be used on both ferrous and nonferrous materials. IRIS is most used on carbon steel tubes.
CUI
Definition
Corrosion of piping, pressure vessels, and structural components resulting from water trapped under
insulation or fireproofing
Cause
Corrosion rates increase with increasing metal temperature up to the point where the water evaporates
quickly.
Affected Materials
Carbon steel, low-alloy steels, 300 series SS, 400 series SS, and duplex stainless steels
Safe Materials / Safe Conditions
Coating
Flame Sprayed aluminum coatings
Effective & Non-Effective NDE
Profile RT, Infrared thermography, VT, UT, Pit gauge, neutron backscatter (for wet insulation)
ERROSION / ERROSION-CORROSION
Definition
Erosion is the mechanical removal of surface material as the result of relative movement between, or
impact from, solids, liquids, vapor, or any combination thereof.
Prevention
Increase diameter to decrease velocity
Affected Materials
All metals, but mostly carbon steel and copper alloys in refining
Effective & Non-Effective NDE
UT, RT
HIGH TEMP HYDROGEN ATTACK
Definition
HTHA results from exposure of steels to hydrogen gas at elevated temp and pressure. Dissociated
hydrogen atom reacts with carbon and carbides to form CH4.
Affected Materials
In order of resistance: welded CS, non-welded CS, CS with PWHT, C-0.5Mo, Mn-0.5Mo, 1Cr-0.5Mo,
1.25 Cr- 0.5 Mo, 2.25Cr-1Mo, 2.25Cr-1Mo-V, 3Cr-1Mo, 5Cr-0.5Mo
Safe Materials / Safe Conditions
300 series SS, 5Cr, 9Cr, 12Cr alloys are not susceptible to HTHA
Use steels with high chromium and molybdenum content
Common design practice is 25 F - 50 F and 25-50 psia
Effective & Non-Effective NDE
The period where no HTHA detected internally or externally even by NDE is called incubation period.
VT (for blisters), TOFD, PAUT, AUBT (AUTOMATED ULTRASONIC BACKSCATTER TESTING)
is effective.
MT, PT, WMFT not effective at all unless crack has reached the surface. AET is also not proven method.
Effected Units
Hydrotreaters, hydrocrackers, catalytic reformers, boiler tubes in very high-pressure steam service
Appearance
Blisters, internal fissuring and cracking
HYDROCHOLORIC ACID CORROSION
Definition
HCL cause both localized and general corrosion. Mostly associated with dew point corrosion in which
vapors containing water and hydrogen chloride condenses.
Cause
HCL, vapors containing water and hydrogen chloride.
Prevention
Upgrading carbon steels to nickel based alloys or titanium will reduce the impact. Water wash & Caustic
injection is effective. Ammonia, neutralizing amines and filming amines can be injected.
Affected Materials / Units
All common materials used in refineries. Carbon steel and low alloy steel when exposed to HCL with ph
below 4.5. 300 SS & 400 SS also get affected. Alloy 400, titanium and nickel-based alloys show some
resistance to dilute HCL. Oxidizing agent increase corrosion for 400 series and alloy B-2. Titanium
performs well in oxidizing conditions but fails in dry HCL conditions.
Safe Materials / Safe Conditions
20 ppm chloride. Note: don’t confuse for hydrotest chloride requirement which is 50 ppm
Effective & Non-Effective NDE
Effective NDE includes: VT (look for orange yellow discoloration). UT mapping, AUT (automated UT),
RT
MECHANICAL FATIGUE
Definition
Mechanical form of degradation when component exposed to cyclic stresses for extended period.
Cause
Vibrations, water hammer, unstable fluid flow, vibrational load synchronize with natural or harmonic
frequency. Usually initiates on key holes on shafts, mechanical notches, tool markings, grinding marking,
lips on drilled hole, thread root notches and corrosion.
Prevention
Good design, stiffeners and dampers
Affected Materials
All engineering alloys (number of cycles may vary metal to metal)
300, 400 series SS aluminum and most non-ferrous metals don’t have endurance limit so eventually fails
after 106 𝑡𝑜 107 cycles.
Safe Materials / Safe Conditions
Titanium, carbon steels and low alloys steels has endurance limit if stress amp is below endurance limit
fatigue will not occur regardless of cycles. Ratio of endurance limit to Ultimate tensile Strength (UTS) is
between 0.4 and 0.5. Heat treatment is effective.
Effective & Non-Effective NDE
Effective: UT, MT, Angle beam (PAUT & SWUT: shear wave), compression wave UT for cracking in
bolts, vibration monitoring.
SOUR WATER CORROSION
Definition
Corrosion of steel due to acidic sour water containing H2S at a PH between 4.5 to 7.0.
Cause
acidic sour water (dangerous at lower ph)
Prevention
Controlled H2S, PH, Chloride, Cyanide, temp, velocity and oxygen concentration.
Affected Materials
CS
Safe Materials / Safe Conditions
SS, copper alloys and nickel-based alloys.
Effective & Non-Effective NDE
UT and RT.
SULFIDATION
Definition
Corrosion of carbon steels and other alloys resulting from their reaction with sulfur compounds in high
temp environments.
Cause
Sulfur compounds with service temp above 450 F should be concern but practical concern is above 500
F.
Prevention
Higher chromium steel.
Above 1193 F select nickel alloy with lower nickel content.
Affected Materials
All iron-based materials including CS, low alloy Steel, 400 series, 300 series (in order of susceptibility).
Above 1193 F alloy containing high nickel can suffer. Susceptibility increase with increase of nickel
content. Copper based alloys form sulfide corrosion at lower temp than CS.
Safe Materials / Safe Conditions
Increasing chromium content increase resistance. 300 series are better resistant at high temps. CS with
silicon content above 0.10% good resistant.
Effective & Non-Effective NDE
Effective: UT, RT, internal VT followed by UT,
Appearance / Morphology
Mostly uniform but can be localized or high velocity erosion corrosion damage
WET H2S DAMAGE
Definition
4 types of damage:
a) Hydrogen blistering:
Hydrogen blisters form bulges primarily on the ID surface of pressure vessels
b) Hydrogen induced cracking
in this case, when the hydrogen atoms diffuse into the steel, rather than forming blisters, internal
separations parallel to the surface of the steel eventually forming a thru-wall leak path.
c) Stress oriented hydrogen induced cracking
SOHIC results from an array of HIC (separations or cracks) stacked on top of each other. When acted
upon by a high stress level (residual or applied), the stacked HIC will connect and create a thru-thickness
crack that is perpendicular to the surface.
d) Sulfide stress cracking (SSC)
SSC is the cracking of a susceptible metal under the combined action of tensile stress and corrosion in the
presence of water and H2S
Cause
A value of 50 ppmw of H2S is often stated as minimum concentration where wet H2S damage can occur.
But damage has been found at even 1 ppmw
H2S partial pressure above 0.05 psia can cause SSC in steels with tensile strength above 90 ksia and with
hardness above 237 HB
Found minimum at PH 7 and increase with both increase/decrease of PH from 7.
At PH < 4 only small amount of H2S required for damage.
Increasing level of ammonia or ammine solution where PH > 7 also susceptible to damage.
at pH > 7.6 with 20 ppmw dissolved HCN in the water, as little as 1 ppmw total sulfide content in the
water can cause SSC
Blistering, HIC, and SOHIC have been found to occur between ambient temperature and 300 °F (150 °C)
or higher
SSC potential is greatest at about 70 °F (20 °C) and decreases with increasing or decreasing temperature
SSC is generally a concern below about 200 °F (95 °C); however, the limiting temperature above which
SSC is no longer a concern will depend on the situation, i.e. on the hardness of the steel involved and the
severity of other environmental factors such as pH.
Hardness is primarily an issue with SSC. Blistering, HIC, and SOHIC damage are not related to steel
hardness.
Welds in carbon steel should be controlled to produce weld hardness < 200 HB, and they will typically
achieve this without any special precautions. Carbon steel welds are not susceptible to SSC unless
localized zones of hardness above 237 HB are present
In hydroprocessing units, an ammonium bisulfide concentration above 2 % increases the potential for
blistering, HIC, and SOHIC
Affected Materials
Carbon Steel, in case of SSC low alloys steels and martensitic stainless steels (410 series)
Effective & Non-Effective NDE
Cracks from SOHIC or surface-breaking HIC may be seen visually. However, to enable crack detection at
an early stage, WFMT, ECT, or ACFM techniques can be used
Angle beam UT techniques including external SWUT and PAUT can be used. These techniques are
especially useful for volumetric inspection and crack sizing.
Appearance:
HB: blisters
HIC: Staircase