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Corrosion Forms II

Pitting and Crevice Corrosion

© Dr. Zuhair M. GasemME, KFUPM

Dr. Z. Gasem ME472-062

KFUPM2Forms of Electrochemical Corrosion

Four categories of corrosion forms:

General corrosion (uniform corrosion over the whole anode) Localized Corrosion (corrosion at isolated areas in the anode)

Galvanic corrosionpitting corrosion Crevice corrosion

Metallurgically Influenced Corrosion (corrosion is related to microstructure)

intergranular corrosion of stainless steelMechanically Assisted Corrosion (corrosion is accelerated due to

mechanical factors)Erosion corrosionCorrosion Fatigue

Environmentally Assisted Cracking (corrosion results in cracking) Stress corrosion crackingHydrogen damage


KFUPM3Forms of Corrosion


KFUPM4Pitting Corrosion

Metals and alloys can be active (no protective passive film) or passive (passive film 10-30 nm in thickness) Pitting corrosion : rapid corrosion penetration at small discrete areas where passive film breaks down and appears as small diameter holes (0.1-5 mm). The remaining surface is not attacked. Pitting corrosion is common in passive metals above pitting potential (Ep)Weight loss is negligible

Pits

passivefilm



Examples:Pitting of stainless steel, Ni alloys, Al alloys in aerated seawater or acids containing Cl-ionsSteel buried in the soil corrodes with the formation of pits. Pitting of steels and aluminum in alkaline solutions containing Cl-Pitting can occur in any alloy that form a protective film and exposed to stagnant solution containing Cl-Example: 316 SS centrifuge head exposed to CaCl2 solution showing deep pits



Right: pitting in carbon steel pipe carrying strong acid. Below: deep pits on a carbon steel in HCl.



Top: pits in stainless steel valve plate (stagnant solution).Bottom: internal pits in steel pipeline carrying water with dissolved CO2



Conditions favoring pitting:Stagnant solutions

pitting occurs during shutdown period more than during normal operationDuring shutdowns, solutions in pipes, pumps, and tubes are stagnant and allow pits formation.

Acidic solutionsPresence of Cl־ and Br ־ (halides)High temperature rough surface finishCommonly occur in passive metals (such as stainless steels)Presence of surface deposits (deposits form localized concentrated environments around them) Rupture of protective coatingsPits form in gravity direction



Why pitting corrosion is dangerous?Pitting corrosion is not easily detected and evaluated and thus more dangerous than general corrosion or galvanic corrosion.Pits can grow and become like cracksCan lead to failure by perforation with minimum weight lossPits formation may take place after a long period of initiation (tricky) Aluminum parts in the space shuttle Apollo were rejected because they contained pits having a diameter of a human hair.



Difficulty of detectionDifferent misleading pit (hole) shapesSmall weight lossCan’t use thickness measurementSmall pits are hard to observe visuallyCorrosion products may mask pits


KFUPM11Stages in Pitting Corrosion

Two stages in pitting: initiation and propagation Pits initiation at film break downs due to

Mechanical scratchLocalized High Cl- concentrationLocalized failure of coatings or paintsAt metal microstructural inhomogenity (inclusions in SS)

Pits initiation at surface deposits or scratches of paints Most initiated pits become unstable and do not grow if the passive film reforms or the highly concentrated environments are washed off If passive film reformation is not possible (high concentration of Cl-), then a pit will grow at faster rate due to autocatalytic effect.


KFUPM12Autocatalytic Nature of Pitting

In the propagation stage of pits in SS in aerated salt water (seawater):

Oxygen is depleted inside the pit and oxygen reduction takes place around the pitAnodic dissolution of the metal produces M+ ions inside the pitCl־ ions diffuse toward the pit to neutralize cations charge. Cl־ and M+ inside the pit combine and produce more H+ by hydrolysis

(M++Cl־)+H2O→MOH+H++Cl־The electrolyte becomes more acidic inside the pit and more corrosive because higher H+ and Cl־Reach autocatalytic stage where pit

growth follows d = ktn.

k is a constant and n ranges from 0.3 to 0.8

time

d (Pit depth)

initiation Propagation

O2 + 2H2O + 4e- →4 OH-

Parabolic growth


KFUPM13Effect of Temperature

Pitting is strongly influenced by the service Temperature In stainless steels

there is a critical pitting temperature below which pitting will not initiate for each stainless steel alloy 304SS (critical pitting temperature = -2.5 C°)317SS (critical pitting temperature = -0 C°)Pitting resistance is improved by increasing Cr (>18%) and Mo (2-4%).


KFUPM14Temperature effect

Three alloys are tested for pitting resistance by immersion for 24 hr in very aggressive solution.

All alloys have high resistance to pitting at 25 ̊C Alloy G: Ni-20Fe-22Cr-6Mo

Shows pitting at 70 ̊ CAlloy 625: Ni-22Cr-9Mo

Shows pitting at 102 ̊ CAlloy C-276: Ni-16Cr-16Mo-4W

No pitting until 102 C


KFUPM15Pitting Evaluation and Control

Use of standard charts to estimate pitting density (number of pits per cm2

of metal surface)Pitting evaluation is given according to standard charts

A-4 pit density=100,000/m2

B-3 pit surface size = 8 mmC-2 Pit depth = 0.8 mm

Use pit opening diameter Average pit depth


KFUPM16Control and Prevention of Pitting Corrosion

Reduce Cl- content in the electrolyte Lower acidity of solution, lower O2

Shot peen the surface. Passivate stainless steels (SS) by washing with 20% HNO3 to give it a strong passive film. Replace 304 SS with 316 which contains higher Mo Avoid stagnant solutions in tanks, tubes, and pipesRedesign to ensure proper drainage


KFUPM17Concentration Cells

Variation in the electrolyte in:dissolved ion concentrationdissolved O2

pHtemperature

create a concentration cell in the electrolyte where some region of the electrolyte will favor cathodic reactions and others favor anodic reactions. Corrosion of storage tanks: corrosion will be localized at the boundary between two electrolytes having different ion or O2 concentrations.

The electrode potential becomes more +ve as the metal ions concentration increases (see Nernst’sequation)


KFUPM18Oxygen Concentration Cell

In oxygen concentration cell, for example, cathodic reactions are favored in region having high concentration of Oxygen. Example 1 : a drop of water on a metal surface. The cathodic reaction is:

O2 + 2H2O + 4e- → 4 OH-

In neutral solution (pH=7)e=0.82+0.059/4*log PO2e is more +ve ∝ PO2

The drop surface dissolves O2 from the air and will have high concentration of O2. At the center of the drop, the O2 concentration is low. The metal at the periphery of the drop (high O2) will act as the cathode (more +ve e). The metal at the center of the drop (low O2) will act as the anode. This creates an oxygen concentration cell or air differential cell which localizes corrosion. Oxygen concentration cell is responsible for many corrosion problems.Example 2: oxygen concentration cells on the legs of offshore steel structure


KFUPM19Differential aeration cell

Cathodic reaction:

O2 + 2H2O + 4e- →4 OH-Corrosion is frequently caused by concentration difference in O2 from air. Oxygen solubility in water at RT is very small (8 ppm) and decreases with temp.

Cathode: at high conc of O2 near the top surface of tank or sea surface

Anode at low conc of O2below the surface


KFUPM20Differential aeration cell

corrosion in stainless steel thermometer pocket in sea water cooled steam condenser. A stagnant layer of sea water formed along the stem on the downstream side Differential aeration cell


KFUPM21Crevice Corrosion

Crevice: narrow opening or small gap between two contacting surfaces.Crevice corrosion: localized corrosion resulting from a concentration difference b/w the electrolyte within the crevice and the electrolyte outside the crevice due to stagnation of electrolyte.Possible concentration difference in:

O2

H+ (acidity) Cl-

Low PO2 , high [Cl-] and H+

High PO2 ,low [Cl-] and H+

(0.025-0.1 mm)



Inside the crevice: low concentration of O2, and high concentration of H+, and ions such as Cl-(more –ve Ecorr ⇒ anode) Outside the crevice: high concentration of O2, and low concentration of H+, and ions such as Cl-(more +ve Ecorr ⇒ cathode)A concentration or differential cell develops between the electrolyte inside and outside the crevice.Accelerated corrosion occurs inside the crevice while the outside is protected.



Crevice corrosion commonly occurs in three different ways:

Geometric crevices: metal-to-metal tight contact where the solution can get between the two contacting metals

Threaded partsBolted and lap joints

Metal-to-nonmetal absorbent: the nonmetal absorbs the solution and continually moisten the metal

Wood/metalsPaper gasket/metalsSponge rubber gaskets in metallic pipe joints

Deposit corrosion: deposits that are porous and absorbent of the solution such as sediment, lime, and rust on metal surfaces


KFUPM241. Geometric crevices

Example of a crevice corrosion of a bolted joint. The threads in the nut and bolt will experience crevice corrosion. Examples of crevices created in pipe connections:

a. Incomplete penetration of a butt weld

b. A threaded jointc. A socket connectiond. A flange connection with

foamed rubber gasket


KFUPM252. Metal-to-nonmetal absorbent

Crevices are created by design such as gaskets in flanges, O-rings, washers, and insulations.

Crevice corrosion at the flange/gasket contact


KFUPM263. Deposit corrosion

Thick calcium carbonate (CaCO3) deposits on a condenser tubes

Thick greasy deposits on steel tank. Corrosion attack is due to crevice effect.


KFUPM27Alloys susceptible to crevice corrosion

Crevice corrosion can attack active and active-passive metals and alloys but active-passive metals are more susceptible as stainless steels and aluminum. Example:

stainless steel universal joint showing crevice corrosion beneath the rubber grommet Crevice corrosion at the washer-bolt contact in SS fastener in seawater



Materials resistance to crevice corrosion is tested using Teflon blocks in very corrosive electrolyte:



316 SS is more resistant to crevice and pitting corrosion than 304 SS.

304 SS is pitted badly in very corrosive electrolyte (acid condensate) which prevented crevice corrosion (figure a)316 SS is not pitted in the same environment but crevice corrosion is evident (figure b)304 SS in mild aggressive environment showing Crevice corrosion (figure c)



Cast iron and steel show excessive crevice corrosion beneath rubber o-rings Often times crevice corrosion is masked by uniform corrosion in carbon steels


KFUPM31Development of Crevice Corrosion


KFUPM32Initiation and Propagation in C.C.

Propagation of crevice corrosion (SS in neutral aerated NaCl solution):Anodic (M→Mn++ne-) and cathodic reactions (O2 + 2H2O + 4e- →4 OH-)occur inside and outside the crevice. Oxygen is consumed inside the creviceOxygen can be supplied at the outside of crevice.A differential aeration cell develops. Oxygen reduction occurs only outside the creviceAnodic reaction inside the crevice produces more +ve ions by (M→Mn++ne-)Cl- migrate to the crevice for charge neutrality Mn+ and Cl- react and produce high concentration of H+ inside the crevice by hydrolysis (M++Cl־)+H2O→MOH+H++Cl־The pH inside the crevice ≈2 (acidic)The pH outside the crevice ≈7More anodic reaction inside the crevice


KFUPM33Pitting and Crevice Corrosion

Pitting and crevice corrosion are Different in initiation Similar in propagation (autocatalytic or self sustained corrosion)


KFUPM34Crevice Corrosion Control

Crevice corrosion is controlled by the same methods used for pitting and

Reduce crevices in design Use welding if possible instead of bolting two surfaces in contact. Use continuous welds instead of skip welds. Avoid electrolyte stagnation Use none-absorbent gaskets such as Teflon instead of absorbent gaskets such as fiber and wood gaskets. Cleaning to reduce local concentrations of acids, chlorides, ext


KFUPM35Condensate corrosion

Hot gases containing H2O vapor and other vapors such as SO2, CO2, or H2S may condensate in droplets on cooler metal surfaces producing localized corrosion which appear as severe pitting. Prevention

maintain the metal temp above the dew point of the hot gasses to prevent condensation. Apply insulation on pipes carrying hot gasesAvoid attaching bare metal to hot pipes which may act as cooling fin.

Insulated hot pipe

Connection pipe (no insulation)

Flow of hot gases


KFUPM36Filiform corrosion

A crevice can be created due to breach in protective paints. This type of crevice corrosion is very common and called filiform corrosion.