DETERIORATION BY PROCESS OF CORROSION

CORROSION IS A NATURAL PROCESS

METAL OXIDE

REDUCTION

METAL

CORROSION

METAL OXIDE

MECHANISM OF CORROSIONWHEN PUT IN AQUEOUS SOLUTON

AT SOME LOCATIONS (KNOWN AS ANODE) METAL IONISES

ANODIC REACTION -- M = Mn+ + neELECTRON RELEASED HAS TO BE USED TO SUSTAIN

REACTION. THIS OCCURES AT ANOTHER POINT WHEREELECTRON IS CONSUMEDCATHODIC REACTION -- 2H+ + 2e = H2--

Cu2+ + 2e = CuO2 + 2H2O = 4(OH) -

RESULTS IN FLOW OF CURRENT FROM ANODE TO CATHODE ---- A MEASURE OF CORROSION RATECORROSION IS THEREFORE AN ELECTRO CHEMICAL

REACTION

DRIVING FORCE FOR FLOW OF CURRENT

* DIFFERENCE IN POTENTIAL BETWEEN ANODE AND CATHODE

* WHAT ARE THE FACTORS * ALL METALS DEVELOP POTENTIAL SPECIFIC

TO ENVIRONMENT VARYING FROM HIGH NEGATIVE TO HIGH POSITIVE

* VARIATIONS IN CHEMICAL (OXYGEN & SALT CONCENTRATION) & MECHANICAL (STRESS, TEMPERATURE) ON METAL SURFACE

MORE NEGATIVE THE POTENTIAL HIGHER THE CORROSION (ANODIC)

HIGHER THE POTENTIAL DIFFERENCE HIGHER IS CURRENT OR CORROSION

DRY BATTERY IS AN EXAMPLE OF CORROSION GENERATING ELECTRICITY WHEN ZINC CORRODES

PASSIVATION IS DRASTIC DECREASE IN CORROSION CURRENT IN CRITICAL POTENTIAL RANGE DUE TO FORMATION OF PROTECTIVE OXIDE SCALE

EXAMPLE : ALUMINUM, STAINLESS STEELS

FORMS OF CORROSION

UNIFORM CORROSIONGALVANC CORROSION

PITTING & CREVICE CORROSIONINTERGRANULAR CORROSION

IMPINGEMENT CORROSIONDE ALLOYING

STRESS CORROSION CRACKINGHYDROGEN DAMAGE

ATMOSPHERIC

CORROSION ---

AN EXAMPLE OF

UNIFORM

CORROSION

General Corrosion - This 40 year old sample of 8 in. schedule 80 pipe, while clearly containing deposits of iron oxide, shows very even wall loss and long remaining service life.

Iso-corrosion diagram for various stainless grades in

formic acid

BOILING POINT CURVE

SANICRO 28

904L (2RK65)

SAF 2507

SAF 2205

316L

304L

GALVANIC CORROSION

• TWO METALS WITH LARGE DIFFERENCE IN POTENTIAL IN ELECTRIC CONTACT ACTIVE METAL ACTS AS ANODE WITH RESPECT TO NOBLER METAL

* EXAMPLES : Zn / Fe ; Fe / Ad. BRASS ; NAVAL BRASS / SS ; MONEL / TiIN SUCH COMBINATIONS Zn , Fe , NAVAL BRASS & MONEL WILL BE ANODE AND CORRODE WHILE THE NOBELER METAL WILL BE PROTECTED

• DEGREE OF INCREASED CORROSION DEPENDS

ON POTENTIAL DIFFERENCE; CATHODE TO ANODE RATIO; RESISTIVITY OF MEDIA

Galvanic Corrosion - A common problem area where carbon steel pipe is connected directly to either brass valves or copper pipe in a highly humid atmosphere

Example of material mix-up. A tube of type 304 (left) was unintentional welded to a tube of type 316 (right). When exposed to the process solution 304 gradually dissolved and the tube wall was heavily reduced.

PITTING CORROSION

• LOCLISED ATTACK WITH NONE TO MILD GENERAL ATTACK

• DEPOLRISERS LIKE OXYGEN AND CHLORIDE MAINLY RESPONSIBLE FOR LOCAL CORROSION CELL

• OCCURS IN MOST METALS INCLUDING CARBON AND ALLOY STEELS

EXAMPLE : PITTING BELOW TUBERCULUS IN WATER PIPE

ATTACK UNDER DEPOSIT DUE TO OXYGEN RICH & OXYGEN DEFICIENT AREAS

PITTING STARTED AT POINTS OF FILM BREAK DOWN

PITTING OF CARBON STEEL IN WATER UNDER DEPOSITS

PITTING OF STAINLESS STEEL IN CHLORIDE ENVIRONMENT

-PITTING REISTANCE OF SS IS INCERASED BY ADDITION OF Cr, Mo AND N. THE RELATIONSHIP IS EMPERICALLY RELATED AS PITTING RESISTANCE EQUIVALENTS (PRE)

FERRITIC = %Cr + 3.3%Mo AUSTENETIC = %Cr + 3.3%Mo + 30%N DUPLEX = %Cr + 3.3%Mo + 16%N

--PITTING RATE INCREASES WITH TEMPERATURE; CRITICAL PITTING TEMPERATURE (CPT)

Comparison of CPT- and CCT-values for some stainless steels (obtained by the modified ASTM

G48 method).

CREVICE CORROSION

-- INTENSE LOCALISED CORROSION WITHIN MICRON DIMENSION CREVICE FORMED BY OVERLAPPING THIN LAYER OF WATER CORRODES AND THEN FOLLOWED BY MIGRATION OF CHLORIDE ION AS IN PITTING. LOW PH RESULTS IN LOCALISED ATTACK

-- SOLID DEPOSITS AND BIOFILM DEPOSITS ALSO LEAD TO CREVICE ATTACK

crevice

INTERGRANULAR CORROSION

-- CORROSION RESISTANCE OF STAINLESS STEELS IS DUE TO Cr-OXIDE FILM FORMED ON SURFACE.

-- DURING WELDING Cr-CARBIDE IS PRECIPITED AT GRAIN BOUNDARIES IN HAZ RESULTING IN Cr DEPLETED BAND ALONG GRAINS

-- CORROSION THEN PROCEEDS ALONG GRAIN BOUNDARY

-- PREVENTED BY USING LOW CARBON OR ADDING STABILISERS LIKE Ti OR Cb (Nb)

EDAX ANALYSIS OF GRAIN BOUNDARY CARBIDE

WELD

HAZ ATTACK

IMPINGEMENT, EROSION AND CAVITATION CORROSION

-- UNDER TURBULANCE OR EROSIVE CONDITIONS PROTECTIVE FILM IS DESTROYED RESULTING IN DEEP LOCALISED CORROSION

-- INTENSITY OF ATTACK IS DEPENDENT ON CORROSIVITY, DEGREE OF EROSIVE ACTION AND PROPERTY OF FILM

-- SHOWS TYPICAL DIRECTIONAL EFFECT

-- CONTROLLED BY VELOCITY, INHIBITION, DESIGN AND MATERIAL UPGRADATION

PIPE BEND EXCHANGER TUBE

U - BEND

FAILURE EXAMPLECorrosion of Steam Tracing Jump-Overs Due To Erosion

Corrosion

• Repeated failures were experienced in the jump-overs of low pressure steam tracing lines provided in piping conveying sour gas in a refinery.. The low radius carbon steel jump overs got punctured at the outer radius of jump-overs.

• Low pressure steam contains droplets of condensed water which normally have low pH. Impinging low pH water droplets resulted in erosion-corrosion.

• Use of SS 304 bends prevented this type of failure.

* STRESS CORROSION CRACKING OCCURS BY COMBINED)• CORROSIVE ENVIRONMENT – ALLOY SPECIFIC

EXAMPLES : Cu ALLOYS CRACK IN AMMONIA BUT NOT SS CARBON STEELS CRACK IN CAUSTIC BUT NOT Cu ALLOYSSS AND Al CRACK IN CHLORIDE BUT NOT CARBON STEEL

OR Cu ALLOYS

* CRACKING MODE IS INTER OR TRANS GRANULAR NORMALLY RELATED TO ALLOY AND ENVIRONMENT

* TRANSGRANULAR CRACK – SS IN CHLORIDE, CARBON STEEL IN CO+C02 INTERGRANULAR CRACK – CARBON STEEL IN CAUSTIC AND Cu ALLOYS IN NH3

Alloy Environment

Carbon Steel Carbonates and Bicarbonates / Caustic Nitrate, Cyanide, Anhydrous Ammonia,

CO/CO2/H2O Mixtures

Austenitic Stainless Steel Organic and Inorganic Chlorides Acidic Hydrogen Sulfide, Caustic Sulfurous and Polythionic Acids

Nickel-Base Alloys Caustic above 6000F (above 3150F) Hydrofluoric Acid

Copper-Base Alloys Amines, Dilute Ammonia, Ammonium Hydroxide, Sulfur Dioxide

SOME ALLOY / ENVIRONMENT SYSTEMS CAUSING SCC

STRESS VS. CRACKING TIME

INTER GRANULAR

CRACKING

TRANS GRANULAR

CRACKING

CSCC SUSCESTIBLE AREA IN SS EXCHANGERS

DEALLOYING

DE ZINCIFICATION OF ADMIRALITY BRASS TUBE. FATIGUE CRACKS IN DEZINCIFIED SPOTS

Denickelification of the cupronickel tube.

LIQUID METAL EMBRITTLEMENT

-- PREFERENTIAL PENETRATION OF A METAL IN LIQUID ALONG GRAIN BOUNDARIES

-- FORMATION OF BRITTLE INTERMETALLIC SURFACE COMPOUNDS

-- THIS TYPE OF ATTACKS LEAD TO BRITTLE FRACTURE UNDER TENSION

-- EFFECT IS DEPENDENT ON SPECIFIC SOLID/LIQUID METAL COMBINATION

EXAMPLES :-- SOLID COPPER / LIQUID

ZINC-- SOLID COPPER / LIQUID

MERCURY-- SOLID STAINLESS STEEL /

LIQUID ZINC

 MICROBIOLOGICALLY INFLUNCED CORROSION (MIC)

• BIOLOGICAL ORGANISMS AFFECT CORROSION BY DAMAGING FILM, INFLUNCING ANODE/CATHODE REACTION OR BY PRODUCING CORROSIVE SUBSTANCE DEPENDING ON NATURE OF ORGANISM

• AS MICROBS FORM MOUNDS LOCALISED CORROSION OCCURS

• SOIL, WATER AND HYDROCARBON ARE MAIN ENVIRONMENTS

Corrosion Under Insulation (CUI Corrosion) - Often identified only after the removal of pipe insulation for another purpose, water from rain and snow typically penetrates most exterior installations to cause wide ranging wall loss.

CUI Corrosion - Eexterior pipe corrosion due to moisture condensation can take the form of either deep and random pitting, or, as shown above - a generalized deterioration of the entire surface. Since iron oxide is approximately 25 times less dense that the original steel, it often suggests a much greater wall loss than actually exists. CUI is the greatest threat to smaller diameter piping..