corrosion fundamental

7/29/2019 Corrosion Fundamental

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Basic Inspection

& Corrosion

INSTITUT TEKNOLOGI PETROLEUM PETRONAS


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Basic Inspection & Corrosion @INSTEP 2

OutlineFundamentals of corrosion Corrosion cell Corrosion corrodant


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What is corrosion?

Definition of corrosion :

Deterioration of material (metal) or its properties because ofreactionwith environment.


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What is corrosion?

Unstable materials want to return to a more stable compound. For example,gold and silver, can be found in the earth in their natural, metallic state andhave little tendency to corrode.

Iron ores contains oxides of iron and rusting of steel by water and oxygen resultin hydrated iron oxide

Rusting- steel and iron corrosion, other metals from oxides when corrosionoccurs

Steel mill

Refining

Casting

Rolling

Shaping

Iron Ore

(iron oxide)Mine

sheet

pipe

Auto Body

(atmosphere)

Underground

pipeline

(soil and water)

Rust

(hydrated iron

oxide)


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Corrosion Cell

Need 4 components for a corrosion cell :

anode cathode

Electrolyte : water, acid/aqueous solution

electrical connection between the anode and cathode : metallic conductor,

metal


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6

Corrosion cell

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Basic Inspection & Corrosion @

INSTEP

Involve two chemical processes ; oxidation and reduction.

Oxidation electron is removed from an atom (anode)

Reduction - electron is added to an atom (cathode)

Electron move through a metallic conductor (either an external wire of thecorroding metal itself) from anode to the cathode

Positive and negative ions transmit current in solution


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For iron/steel in sea water :

Anodic Reaction

Fe ----> Fe2+ + 2e-

Cathodic Reaction :

Oxygen reduction (acid solution)

O2 + 4H+ + 4e- ---> 2H2O

Oxygen reduction (neutral solution)

O2 + 2H2O + 4e- ---> 4OH-

Hydrogen evolution

2H+ + 2e- ----> H2

Rust Formation : 4Fe2+ + 8OH- ---> 4Fe(OH)2

4Fe(OH)2 + O2 +2H2O ----> 4Fe(OH)3


Corrosion Reaction


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Corrosion Corrodant

Chlorides

Acid forming compounds

Hydrogen sulphide

Oxygen

Bacteria



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Chlorides- act as the conductive ion, severe with presence ofoxidizing agent

Source : sodium chloride / calcium chloride / seawater

-in solution / as products in the reservoir

Mode of attack - localized and external corrosion - pressure vessels,

structural components and piping system

Mitigation

materials (hastelloy B, tantalum) coatings or inhibitors

** tantalum almost parallels to corrosion resistance of glass


Corrosion Corrodant-chlorides


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Acid forming compounds

- Dry CO2 is non-corrosive. CO2 and water forms carbonic acid, which

is extremely corrosive

Source: present in some oil and gas reservoirs

Mode of attack-the exposed area is rapidly corroded

Mitigation : materials ( SS-nitric acid, steel - con. sulfuric acid),

coatings or inhibitors


Corrosion Corrodant-acid forming compounds


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Hydrogen Sulphide

- usually exists in gas phase

Source : reservoirs - designated as sour reservoirs

Mode of attack - corrosion cracking

Mitigation : Inhibitor- reaction with caustic (sodium hydroxide), lime,

iron oxide, sodium carbonate


Corrosion Corrodant hydrogen sulphide


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Oxygen - the oxygen will react with the ferric ions to form insoluble ferric

hydroxide - will easily combine with hydrogen atoms at the cathode

Without oxygen, the energy it takes to evolve hydrogen gas from the cathode is

a major bottleneck in the corrosion reaction


Anode : 4Fe - 8e ------- 4 Fe++

Cathode : 2O2 + 4H20 ----8OH

2H+ + 2e- ------ H2

4Fe++ + 8OH --------

4Fe(OH)24Fe(OH)2 + O2 +2H2O --4Fe(OH)3

Corrosion Corrodant-oxygen


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Source: partially treated surface water or through leakageinto low pressure tanks or vessels that are on vacuum,

separations facilities exposed to air

Mode of attack : surface corrosion / pitting (pinhole leak)

Mitigation

physical processes (gas stripping or vacuum

deaeration)

chemical processes (oxygen scavenging using

ammonium bisulphite)


Corrosion Corrodant-oxygen


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Bacteria

Aerobic (require oxygen) iron oxidising bacteria or

Anaerobic (not require oxygen) - sulphate reducing bacteria (SRB)

Source: seawater or hydrotest water

Mode of attack :

During fermentation, SRB produce lactate, a source of sulphur react

with iron

Electrochemical reactions take place at the anode - steel loses the

iron into solution -react w sulphate ions in solution (Fe + 2H+ Fe2+

+ H )

Iron sulphide - corrosion product

Iron sulphide scale adhere to the steel component -set up a

microenvironment where the bacteria will flourish

Mitigation : biocide treatment


Corrosion Corrodant-bacteria


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Corrosion Corrodant


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END



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Advantages of corrosion


Etching

key in high strength steel to

structural concrete.

Anodizing

Car Battery

Cathodic protection


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With some metals, like aluminum, surface corrosion forms an oxide layer

which protects the metal from being consumed by corrosion. This is the

principle behind "anodizing" steel to guard against rust.

Some metals, like zinc, attract corrosion due to their (di-) electrical properties.

That's why many metal structures (steel/copper) which are exposed to salt

water (spray) include sacrificial zinc "anodes" (basically a block of zinc screwed

to the structure) to divert the corrosion away from the main structure.

Anodizing also like the corrosion process,so it can be advantage of corrosion

process. Anodizing, or anodising in British English, is an electrolytic passivation

process used to increase the thickness of the natural oxide layer on thesurface of metal parts. The process is called "anodizing" because the part to

be treated forms the anode electrode of an electrical circuit. Anodizing

increases corrosion resistance and wear resistance, and provides better

adhesion for paint primers and glues than bare metal.

Advantages of Corrosion


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2. Corrosives are used to etch glass. Etching refers to the technique of

creating art on the surface of glass by applying acidic, caustic, or abrasive

substances. Traditionally this was done after the glass was blown or cast. 3. One such instance of corrosion being good is concrete to metal bonding

where slight or measured corrosion is required to key in high strength

steel to structural concrete.

corrosion fundamental

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