module 6 General Damage Mech Flashcards by Wynne liu

Ammonium Bisulfide Corrosion (Alkaline Sour Water) VL*

NH4HS corrosion occurs from an ammonium salt, formed when ______ and _________ are absorbed in water.

Ammonia (NH3

/ Hydrogen Sulfide (H2S)

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water) VL*
affected material in increasing resistant

………..depending on (2)

Primarily Carbon Steel (CS)
• Low-Alloy Steels
• 300 Series SS, duplex SS, nickel-based alloys, and titanium are more resistant,

Ammonium Bisulfide (NH4HS) concentration and velocity.

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

critical factors? (7)

NH4HS Concentration
H2S partial pressure
Velocity (Turbulence)
pH
Temperature - It precipitates out of the gas phase below 120°F to 150°F .
Alloy Composition
Flow Distribution (Velocity / Turbulence)
O2 (is bad)

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water) VL*

Temp It precipitates out of the gas phase range?

120°F to 150°F .

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*
corrosion increase with _(2)___concentration? below what % is it not corrosive AND for what material ?

NH4HS salts may foul, but will not cause corrosion until _______

NH4HS / cyanides

2% - CS

they get wet (with water).

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

affected equipment?

• Primarily Hydroprocessing Units (Hydrotreaters & Hydrocrackers) at the inlet and outlet piping of reactor air coolers (below 150°F [66°C]) – Sour Water
✓ Sour Water Draw Piping – Sour Water
✓ Sour Water Strippers – Overhead & Piping – Sour Water
✓ Amine Units – Overhead Piping & Condensers – Sour Water

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

where is it less likely to be seen and why?

• FCCU & Delayed Coker – Fractionator Overhead Condensing Systems - (Sour Water) not as likely as Hydroprocessing units due to concentration

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

appearance at high velocity/ low velocity?

High Velocity – General loss with the potential for extremely high localized rates. Locating the site of the worst corrosion may be challenge.
Low Velocity – Localized under-deposit corrosion if insufficient water is available to dissolve the NH4HS salts

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

•Review Velocities especially when NH4HS concentrations exceed ____ % and approach ____ % or higher.

2 wt / 8 wt

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*
Use resistant materials of construction (e.g. duplex stainless steel, Alloy 825) at velocities above ________ depending on NH4HS concentration.

20 fps,

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

Provide sufficient ________ to ensure that an adequate amount of water remains as liquid to dilute the NH4HS salts

excess water

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

a. good design practice symmetrical and hydraulically balances in and out of air cooled exchangers.
b. use proper __(2)____

injection spray nozzles and metallurgy

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

inspection method? typical (2), for (CV -1) , for (air cooler /exchanger tube -5)

UT scanning and/or RT profile.
UT downstream of control valves due to increased velocity.
IRIS/MFL/NFT/RFT/ECT for air cooler and exchanger bundle tubes

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

a.Ammonium bisulfide corrosion can be________and difficult to locate

high localized

How well did you know this?

Not at all

Perfectly

Ammonium Bisulfide Corrosion (Alkaline Sour Water)VL*

b. Special attention to _____ location in area of expected water impact
c. Water injection facilities and flow meter to ________(2)

water injection

monitor and inspect spray nozzles.

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
One way salt can form is by molecules of\_\_\_\_\_\_ gas and \_\_\_\_\_\_\_\_\_\_

Ammonia (NH3)
Hydrogen Chloride (HCl)

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
source of CL?

A source of Cl is in the hydrogen that is produced by the Catalytic Reformer (they inject it
for catalyst activation).

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
NH3 originates with the small amount of nitrogen present in\_\_\_\_

oil.

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
critical factors (3)

Concentration (NH3, HCl, or amine salts)
Temperature –
Water Availability

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
wheat temp can ammonium chloride salt precipitate?

around 400°F

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
What is HYGROSCOPIC ?

NH3Cl - absorbs water when there is none

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
corrosion rate

> 100Mpy

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
Ammonium Chloride is very \_\_\_\_\_\_\_\_, highly corrosive, and form an acidic solution when mixed with water

water soluble

How well did you know this?

Not at all

Perfectly

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL*
\_\_\_\_\_\_\_\_amine can also react with HCL to form amine hydrochlride that can act in a similar fashion.

neutraalizing

How well did you know this?

Not at all

Perfectly

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* affected equipment? (2) ```

a. RX effluent streams and the H2 recycle system are subject ammonium chloride b. overheads where condensing occurs

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* appearance ? (color ) ```

whitish , greenish or brownish appears

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* may not be evident during ...(NDE)......due to water washing or steam out that remove deposits ```

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* prevention in crude units? ```

* Improve desalting – Operational item. * Add caustic to the desalted crude – (caustic reacts with the chlorides to prevent the formation of NH3Cl). * Add water wash to contact and dilute the NH3Cl salts. * Add neutralizer and filming amines (corrosion inhibitors) to deter the corrosive effects

Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* inspection? a. ammonium chloride corrosion may be highly _____and difficult to locate.

localized

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* (3) NDE ```

C. RT/UT | d. Guide wave tech(GWT)

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* when is deposit often detected? ```

when pressure drop increase

``` Ammonium Chloride (NH3Cl ) Corrosion and Amine Hydrochloride (HCL) Corrosion VL* related mech (4) ```

HCL acid corrosion Aqueous organic acid corrosion Concentration cell corrosion CL SCC

``` Hydrochloric Acid (HCl) Corrosion Very aggressive ___( 2) ____- ```

general and/or localized corrosion

``` Hydrochloric Acid (HCl) Corrosion Damage associated with______ corrosion of hydrochloric acid. ``` condense from the overhead stream of a distillation, fractionation or stripping tower.

dew point

``` Hydrochloric Acid (HCl) Corrosion must have ____? ```

WAter

``` Hydrochloric Acid (HCl) Corrosion The _____water (acid) droplets that condense can be highly acidic ```

FIRST

``` Hydrochloric Acid (HCl) Corrosion affected materials ```

all metals

``` Hydrochloric Acid (HCl) Corrosion __(2)____ are not usefully resistant to HCL at any concentration or temp ```

300/400 SS

``` Hydrochloric Acid (HCl) Corrosion _______(2) _____ have GOOD resistance to dilute HCI ```

alloy 400,Ti and other Nickle based alloy

``` Hydrochloric Acid (HCl) Corrosion ____presensce of what is bad ```

``` Hydrochloric Acid (HCl) Corrosion critical factor? (3) ```

HCL acid concentration, temp and alloy composition

``` Hydrochloric Acid (HCl) Corrosion HCL gas is not normally corrosive in ____process streams , but is in ______ ```

dry / wet

``` Hydrochloric Acid (HCl) Corrosion CS/low alloy are subject to excessive corrosion when pH ______ ```

< 4.5.

``` Hydrochloric Acid (HCl) Corrosion affected equipment (3) ```

• Crude Tower – Overhead condensers. It’s a problem in the Vacuum condensers too. • Hydroprocessing – Chlorides may enter the unit with the feed or with the hydrogen. • Catalytic Reforming – Chlorides are ADDED to the catalyst for activation. As time goes on, it is stripped off and sent downstream through the recovery section. A byproduct of the reforming reaction is hydrogen, which is collected and sent to Hydroprocessing Units. This hydrogen stream may also contain stripped chlorides.

``` Hydrochloric Acid (HCl) Corrosion 1. appearance for CS/ low alloy & SS ```

a. CS and Low-Alloy Steels - General Thinning, Localized Corrosion or Under-Deposit Attack. b. 300 Series SS and 400 Series SS - suffer pitting attack. 300 Series SS can also experience Cl SSC (depending on the temperature)

``` Hydrochloric Acid (HCl) Corrosion prevention at Crude Units: (5) ```

• Improve desalting – Operational item. • Add caustic to the desalted crude – (caustic reacts with the chlorides to prevent the formation of NH3Cl). • Add water wash to contact and dilute the NH3Cl salts. • Add neutralizer and filming amines (corrosion inhibitors) to deter the corrosive effects. -Monitor locations for chlorinee contents

``` Hydrochloric Acid (HCl) Corrosion prevention at hydro processing (2) ```

* Limit chlorides in the M/U H2. | * Use water wash in the reactor effluent to flush/neutralize

``` Hydrochloric Acid (HCl) Corrosion prevention at Catalytic Reforming, Crude Units, Hydro processing (2) ```

➢ Use a water wash in the reactor effluent to flush/neutralize. ➢ Add a filming amine and/or neutralizer

``` Hydrochloric Acid (HCl) Corrosion related mech ```

Amonnum chloride CL SCC Aqueous organic acid corrosion

Hydrofluoric (HF) Corrosion | appearance?

general/localized corrosion

Hydrofluoric (HF) Corrosion material generally not suitable? suitable?

300/400/low alloy / CS / Copper ni, alloy 400

Hydrofluoric (HF) Corrosion | material use in some application

nickle based alloy such as alloy c276 is used in some applications

Hydrofluoric (HF) Corrosion | critical factor

water content), temperature, alloy composition, presence of contaminants including oxygen and sulfur compounds are the controlling factors

Hydrofluoric (HF) Corrosion | __(2)____will remove the protective fluoride scale layer.

Water & velocity

Hydrofluoric (HF) Corrosion | Corrosion rates __increase_____as HF concentrations ____and temp ____

decrease / increase

Hydrofluoric (HF) Corrosion | Water content in HF, at >_____ % in acid increases corrosion rates.

1.5 wt

Hydrofluoric (HF) Corrosion | CS operating above________. Higher temperatures can increase corrosion rates.

150 °F (65 °C)

Hydrofluoric (HF) Corrosion | In CS, the Residual Element (RE) content should be ___%

0.20%

Hydrofluoric (HF) Corrosion | _____ hF have a very low general CR in CS

dry , fresh

Hydrofluoric (HF) Corrosion | ______phase change in azeotropic will cause localized corrosion of CS.

rich HF

Hydrofluoric (HF) Corrosion | Preheating rich HF > 160F require what material?

alloy 400

Hydrofluoric (HF) Corrosion | high CR on piping/equipment______F

>150F

Hydrofluoric (HF) Corrosion | appearance in CS

CS – Local or general corrosion

Hydrofluoric (HF) Corrosion | what material will scale? and not?

CS / Alloy 400

Hydrofluoric (HF) Corrosion | Alloy 400 appearance

may thin

Hydrofluoric (HF) Corrosion | Non PWHT Alloy 400 is susceptible to_____ when in the presence of wet HF & O2

SCC

Hydrofluoric (HF) Corrosion | ______reduce problems associated with SOHIC

PWHT reduce problems associated with SOHIC

Hydrofluoric (HF) Corrosion | _______can be use to eliminate problem associate with blistering, HIC or SOHIC

Alloy 400

Hydrofluoric (HF) Corrosion | What content can be monitor with AP751?

RE content of CS

Hydrofluoric (HF) Corrosion | related mech

• Hydrogen stress cracking in HF acid (3.41), hydrogen blistering and HIC/SOHIC, HF

Naphthenic Acid Corrosion | primarily temp range? and as low as?

425 - 750°F range. | 350F

Naphthenic Acid Corrosion | where nap comes from?

some crude oil

Naphthenic Acid Corrosion | affected materail?

CS, Low-Alloy Steels, 300 SS, 400 SS, and Nickel-Based alloys - almost all the same

Naphthenic Acid Corrosion | NAC is not reduced by ___(like sulfidation), but it is resistant to______and at what %?

Chrome / Molybdenum (2-2.5%)

Naphthenic Acid Corrosion | NAC appears in ____hydrocarbon steam that _____contain a free water phase.

hot dry / DO NOT

Naphthenic Acid Corrosion TAN may be misleading because this family of acids has a range of boiling points and tends to concentrate in various cuts. Therefore, NAC is determined by the _______ of the actual stream not the crude charge.

acidity

Naphthenic Acid Corrosion | critical factor? (5)

* Naphthenic Acid concentration * Temperature * Velocity * Sulfur Content (the more the better) * Alloy Composition

Naphthenic Acid Corrosion | critical factor? (5)

* Naphthenic Acid concentration * Temperature * Velocity * Sulfur Content (the more the better) * Alloy Composition

Naphthenic Acid Corrosion | affected equipment? (2)

* Crude and Vacuum heater tubes/transfer lines. | * Piping systems are susceptible in areas of high velocity

Naphthenic Acid Corrosion | appearance? general? high / low velocity?

* NAC is characterized by localized corrosion, pitting, or flow induced grooving in high-velocity areas. * In low-velocity condensing conditions, it shows up as uniform loss and/or pitting.

Naphthenic Acid Corrosion | At temperatures below ____, NAC can have a smoother, more uniform appearance than is typical at higher temperatures.

~450°F (230°C)

Naphthenic Acid Corrosion | what alloys to use and have resistance? and material #?

Use alloys with higher molybdenum content for improved resistance (317L is common).

Naphthenic Acid Corrosion | Use ___-chemical inhibit to reduce NAC ? and how else to reduce __2___?

Use NAC chemical inhibitors. | • NAC can be reduce by blending crude and/or increase sulfer content

Naphthenic Acid Corrosion | Use ___-chemical inhibit to reduce NAC ? and how else to reduce __2___?

Use NAC chemical inhibitors. | • NAC can be reduce by blending crude and/or increase sulfer content

Naphthenic Acid Corrosion | NDE?

UT/RT

Naphthenic Acid Corrosion | NDE?

UT/RT

Aqueous Organic Acid Corrosion | description?

Organic compounds present in some crude oils decompose in the crude furnace to form low molecular weight organic acids.

Aqueous Organic Acid Corrosion | affected material?

Carbon steel and low-alloy steels.

Aqueous Organic Acid Corrosion | What MW acid more corrosive ? name 4?

low MW acids | formic , acetic, propionic, butric

Aqueous Organic Acid Corrosion | one source of these acids is believe to be the _________ of nap acids .

thermal decomposition

``` Aqueous Organic Acid Corrosion affected equipment (2) ```

* Crude tower overhead system * Vacuum tower overhead system * Coker Fractionator overhead system

``` Aqueous Organic Acid Corrosion affected equipment (3) ```

* Crude tower overhead system * Vacuum tower overhead system * Coker Fractionator overhead system

Aqueous Organic Acid Corrosion | prevention (4)

• Neutralize • Use filming amine • Use corrosion resistant alloys -blend crude to not have Nap / Tan

Aqueous Organic Acid Corrosion | inspection? (3)

* UT & RT * Monitor process water for pH * Use corrosion probes and/or corrosion coupons

Aqueous Organic Acid Corrosion | • It is difficult to distinguish between these Organic acids and____

HCl acid

Aqueous Organic Acid Corrosion | • It is difficult to distinguish between these Organic acids and____

HCl acid

Phenol (Carbolic Acid) Corrosion **NL | when is phenol used?

used as a solvent to remove aromatic compounds from lubricating oil.

Phenol (Carbolic Acid) Corrosion **NL | increasing resistance material?

-CS , 304L, 316L and alloy C276

Phenol (Carbolic Acid) Corrosion **NL | critical factor? (4)

Temperature, Water Content, Alloy Chemistry and, Flow Velocity.

Phenol (Carbolic Acid) Corrosion **NL | dilute aqueous solutions______ can be very corrosive

(5-15% phenol

Phosphoric Acid Corrosion (NL) | Phosphoric acid is used as a _______in _______units.

catalyst / polymerization

Phosphoric Acid Corrosion (NL) | where does it most often occur?

Most corrosion probably occurs during water-washing operations at shutdowns

Phosphoric Acid Corrosion (NL) | Solid phosphoric acid catalysts are not corrosive to ____…unless _______ is present.

CS / unless free water is present

Sour Water Corrosion (Acidic) - | is to due to ___ and ___ in SW

H2s and NH3

Sour Water Corrosion (Acidic) - | pH between?

4.5 - 7

Sour Water Corrosion (Acidic) - | affected material

``` Sour Water Corrosion (Acidic) - critical factor (5) ```

* H2S Concentration - H2S Partial Pressure * Ph - pH between 4.5 and 7.0 * Temperature - H2S concentration in the sour water decreases as temperature increases. * Velocity - High velocity removes the protective iron sulfide layer. * Oxygen content – Bad!

Sour Water Corrosion (Acidic) - | as ____concentrations decrease pH to about _______

H2S / 4.5

Sour Water Corrosion (Acidic) - H2S concentration 1. in the sour water decreases as temperature _____. 2. increase h2s _____pH

increases / decrease

Sour Water Corrosion (Acidic) - | affected equipment?

• Acid sour water corrosion is a concern in overhead systems of FCCU and Coker gas fractionation plants with high levels of H2S and low NH3 level

Sour Water Corrosion (Acidic) - | prevention

process monitor and control.

Sulfuric Acid Corrosion * (VL) | In order of increasing resistance:

CS, 316L SS, Alloy 20, high-silicon cast iron, high-nickel cast iron, Alloy B-2, and Alloy C-276

``` Sulfuric Acid Corrosion * (VL) critical factor (5) ```

* Acid Concentration – Acid concentration below 65% causes extensive corrosion for CS. * Temperature – The colder the better. * Alloy Content * Velocity – CR increase if velocity >2-3 fps. * Contamination and presence of oxidizers (oxygen)

Sulfuric Acid Corrosion * (VL) | Acid concentration below ___%causes extensive corrosion for CS.

65%

Sulfuric Acid Corrosion * (VL) | temp that better

colder

Sulfuric Acid Corrosion * (VL) | velocity range when CR increase?

>2-3 fps increase significantly

``` Sulfuric Acid Corrosion * (VL) Affected equipment (2) ```

* Sulfuric acid units and waste water treatment plants are affected. * Water mix points cause heat and dilutes caustic so corrosion increases.

Sulfuric Acid Corrosion * (VL) | best (3) material?

a. Use alloys that are more resistant such as Alloy 20,904L, C276

Sulfuric Acid Corrosion * (VL) | NDE (2)

UT / RT suspected areas

Ammonia Chloride | not normal to be affected by ammonium chloride corrosion?

Sulfuric acid units caustic regenerators

Ammonia Chloride | Best effective NDE?

RT/UT

Hydrochloric Acid (HCl) Corrosion The presence of oxidizing agents (oxygen, ferric and cupric ions) will increase the corrosion rate, particularly for alloy 400 and Alloy B-2. Titanium performs well in oxidizing conditions but fails rapidly in ________HCl service.

dry

Hydrofluoric (HF) Corrosion Severe fouling due to iron ____corrosion product has been observed in the piping, heat exchangers and in the tops of the Isostripper and Depropanizer towers

fluoride

Hydrofluoric (HF) Corrosion Corrosion can be prevented by careful operation of the unit to minimize ___(3) ____and other contaminants in the feed

water, oxygen, sulfur

NAC A form of _____temperature corrosion that occurs primarily in ___(2)____, and downstream units that process certain fractions or cuts that contain naphthenic acids.

high crude and vacuum units

Phenol (Carbolic Acid) Corrosion **NL | Carbon steel and 304/304L stainless steel corrode rapidly in phenol service above ___°F (232o C).

450

Phenol (Carbolic Acid) Corrosion **NL | Corrosion will be in the form of______corrosion of carbon steel.

general or localized / erosion corrosion

Phosphoric Acid Corrosion (NL) It can cause both ______corrosion and _____corrosion of carbon steels depending on water content.

pitting / localized

Phosphoric Acid Corrosion (NL) Solid phosphoric acid catalysts are not corrosive to carbon steel unless ____ is present. When water is present, _____corrosion of carbon steel may occur.

free water / severe

Phosphoric Acid Corrosion (NL) | Corrosion rates increase with ____temperature.

increasing

Phosphoric Acid Corrosion (NL) | Corrosion can penetrate a 1/4-inch thick steel tube in ____hours

eight

Phosphoric Acid Corrosion (NL) Corrosion is usually found in ______velocity areas where there is little or no circulation such as piping manifolds, the bottom of kettle-type reboilers, partial penetration welds, and exchangers where there is sufficient residence time to permit the settling of acid droplets.

low

Sour Water Corrosion (Acidic) - | The presence of ____or oxidants may increase the corrosion and usually produces pitting or underdeposit attacks.

air

Sour Water Corrosion (Acidic) - 300 Series SS can be used at temperatures below about _____(60o C) where Chloride Stress Corrosion Cracking (CSCC) is not likely.

140F

Sour Water Corrosion (Acidic) - In some instances at a pH above 4.5, a thicker, porous sulfide film layer can form. This can promote ____under sulfide deposits.

pitting

Sulfuric Acid Corrosion * (VL) Sulfuric acid promotes ____and ____corrosion of carbon steel and other alloys. Carbon steel _______may experience severe corrosion.

general / localized HAZ

Sulfuric Acid Corrosion * (VL) | Mix points with water cause ______to be released and high corrosion rates can occur where the acid becomes diluted.

heat

Sulfuric Acid Corrosion * (VL) | Mostly general in nature, but attacks carbon steel _________ rapidly.

weld heat affected zones

Sulfuric Acid Corrosion * (VL) | Sulfuric acid attacks ____left from welding.

slag

Sulfuric Acid Corrosion * (VL) | Acidified product streams can be washed with caustic to ____the acid.

neutralize

Sulfuric Acid Corrosion * (VL) Corrosion of steel by dilute acid is usually in the form of overall metal loss or pitting and becomes more severe with increasing ___(2)___

temperature and velocity.