Corrosion engineering - domande e risposte in preparazione all'esame

Zinc coating so is an active (cathodic) protection, if there is a

small defect, also, CR of Zn doesn’t increase that much:

+

]

= 12[ ∙ = 12 ∙ 0,1 ∙ 1,2 = 1,44

Stainless steel: the metal is passive, thus no generalized

corrosion occurs. It is sensible to pitting corrosion if chlorides are

present above a critical threshold (depending on PREN).

Describe the effect of coatings (metallic and non-metallic) used to prevention and control

corrosion of carbon steel structures.

Zn coating is an active coating that protect CS cathodically: Zn passivates, also in CO atmosphere. The

 2

presence of small cracks creates negligible galvanic coupling; on the other hand, if the crack is big, ohmic

drop increases and at the centre of the defect there is not galvanic coupling but oxygen directly on CS.

Lead coating: cathodic protection is achieved; porosity is blocked by corrosion products (PbSO ), very

 4

compact and adherent → self-healing, auto-protection.

Copper coating is used for aesthetical reasons (brightness) but is a perfect protective coating only without

 defects (a small defect means very high cathodic area vs small anodic one, increasing up to 20 times CR).

Nickel behaves in similar way, but it is used only in contact with atmosphere.

Multi-layered system: combination of Cr (max 100 µm) on Ni, with a controlled equi-spatial porosity to

 have artificial corrosion spots only on Ni in well-known positions, few cm one from the other. Too much

porosity means distributed corrosion on Ni and detachment of Cr; on the other hand, too low porosity

means corrosion of Ni well localized and easier path to reach CS underneath. It is also possible to have

duplex and triplex coatings (respectively, Cr – NiS – Ni – CS and Cr – Ni – NiS – Ni – CS) in order to stop

corrosion in the NiS layer, because it is less noble than Ni.

Conversion coatings have to be avoided with chlorides, because they’re oxides. Phosphatation is a

 chemical conversion, it is the deposition of an insoluble salt on the metal surface due to a reaction

between metal ions of the substrate and an acid solution. The new surface is chemically inert and

electrically resistant, increases cathodic and anodic overvoltages, improves adhesion. Anodizing consists

in the formation of a very compact perfect barrier layer under a porous layer of Al O isolating Al surface.

2 3,

Painting cycle: min 250 µm (Zn-rich primer + intermediate organic layer + organic finishing). Paint should

 not be applied at low T, or when RH is more than 85%. Surface preparation is necessary to improve

adherence. Painting cycle is not an active coating, but it is just a physical barrier which impedes oxygen

diffusion and water contact. Maintenance is required (every 5 years).

Soil corrosion

List and describe (three) forms of localized corrosion on a carbon steel pipe.

Differential aeration: when a pipe is buried in different kind of soils, with different oxygen content, a macro

 cell develops, because the cathodic reaction concentrates in the soil more permeable to oxygen, transforming

in the anode the other part of the pipe in contact with the less-permeable soil. For example, if the pipe is

buried in sandy soil with clay islands, the clayey soil (being water saturated) doesn’t allow oxygen diffusion

and the pipe here is the anode. The cathodic area, therefore, is the surrounding pipeline in the sandy soil, very

permeable to air.

Galvanic coupling: metals with different practical nobilities in contact create a macro-couple current (there is

 a driving voltage, given by the difference between the free corrosion potentials between the two metals). The

anodic process concentrates on the less noble metal while the more noble one is protected (oxygen consumes

electrons “stolen” from the anode).

Electric interference: stationary interference (presence of an electric field, for example generated by a CP

 system nearby) or non-stationary one (DC traction system) cause the passage of high density of current inside

the pipeline. When the current enters the pipeline, nothing dangerous occurs; when the current exits – under

substations – a very high corrosion rate is achieved (small surface, very high current density). It is a local

perforation, no (or few) corrosion products are found.

The ground surrounding the rails can be viewed as a parallel conductor to the rails. The magnitude of stray

current flow in the ground conductor will increase as its resistivity decreases. Any metallic structure buried in

ground of this nature will tend to “attract” stray current, as it represents a very low resistance current path.

The highest rate of metal dissolution occurs where the current leaves the structure.

Tubercles: aerated soil + >200 ppm of chlorides + >1000 ppm of sulphates = formation of soluble iron salts,

 non-protective deposits, because porous and non-uniform. Oxygen content doesn’t decrease and corrosion

2+ 3+ - 42- +

continues inside the pit, which contains many ions (Fe , Fe , Cl , SO , H ) making the electrolyte really

conductive.

Describe the corrosion promoted by stray current.

Stationary interference (presence of an electric field, for example generated by a CP system nearby) or non-

stationary one (DC traction system) cause the passage of high density of current inside the pipeline. When the

current enters the pipeline, nothing dangerous occurs; when the current exits – under substations – a very high

corrosion rate is achieved (small surface, very high current density). It is a local perforation, no (or few) corrosion

products are found. The ground surrounding the rails can be viewed as a parallel conductor to the rails. The

magnitude of stray current flow in the ground conductor will increase as its resistivity decreases. Any metallic

structure buried in ground of this nature will tend to “attract” stray current, as it represents a very low resistance

current path. The highest rate of metal dissolution occurs where the current leaves the structure.

Corrosion in water

Describe how the following protection techniques act in order to control the corrosion of a

carbon steel pipeline transporting fresh water: coating, galvanization, inhibitor, stainless

steel.

A good coating should have optimum chemical and mechanical properties. It acts as a physical barrier,

 avoiding the contact between water and the pipe, and oxygen diffusion. Cathodic reaction occurs always in

defects, causing localized corrosion.

Galvanized steel: corrosion resistance is related to zinc passivation. It cannot be used if T>50 °C because of

 polarity inversion (zinc passivates). It can stand up to 0,1 mg/L of oxygen. Low rate uniform corrosion →

formation of a protective layer on residual Zn coating, which remains during full service life. High rate uniform

corrosion → complete loss of Zn coa ng.

Corrosion inhibitors make a change in the anodic behaviour (passivating) or in the cathodic process. They are

 characterized by efficiency and minimum threshold content, under which a negative effect (increase of CR)

can be achieved.

The consequent increase in reaction resistance sometimes results from the inhibition of the cathodic process

(for example by increasing the overvoltage of hydrogen or preventing oxygen from reaching the metal surface)

and/or the anodic process; in other cases, it results from the establishment of passive conditions.

SS can suffer only from localized corrosion as pitting and crevice, because it is passive. Generally, it is used

 instead of CS in transport of drinking water and in the food industry, where contamination is not allowed.

List and describe the influencing parameters to estimate the corrosion in aerated water.

4 ]+ [ ]) ( )

= ([ ( ) ( )

( )/

( ) ( )=

= 2

,

( ) =1+ 1 + 0,5

The oxygen content doesn’t exceed 11 ppm. The double layer δ is higher with stagnant water (up to 3 mm), and

goes down to 0,1 mm with high turbulence; turbulence forces some oxygen to enter in water, increasing the

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limiting current density. The diffusion coefficient of oxygen is ≈ 10 m /s, but it increases with higher temperature.

Temperature also influences δ, decreasing it for higher T.

The presence of bacteria increases CR, especially in anaerobic conditions, where SRB cause hydrogen evolution

and production of black rust, at CR ≈ 1 mm/y.

Depending on composition (hardness), T and pH, fresh water can form on metal surface a scale made of calcium

carbonate. The scaling tendency is governed by the chemical equilibrium of carbonate/bicarbonate (the former

insoluble, the latter not). The calcareous deposit is thick and impermeable, thus reduces oxygen diffusion. Langlier

2+

index LSI = pH – pHs where pHs depends on T, [Ca ] and total dissolved salts. If LSI > 0 there is deposition.

2+ 3-

Protective scales are formed when [Ca ] > 40 ppm, [HCO ] > 200 ppm, [O ] > 3 ppm and pH > 7.

2

Throwing power (the maximum current path from the anode to the cathode) depends on the conductivity of the

water, that is influenced by salinity (their ratio is usually 0,6). Salinity also influences solubility of oxygen: there’s

no oxygen when it exceeds 150 g/L. The presence of chlorides is important to evaluate corrosion of passive

materials, because above a critical threshold pitting starts.

Compare the corrosion behaviour of carbon steel, galvanized steel and stainless steel.

Indicate and justify a possible interval of corrosion rate.

CS undergoes generalized corrosion in neutral aerated water and in acidic de-aerated water; localized

 corrosion because of differential aeration, galvanic coupling, localized cross-section reduction with shallow

pits (when the surface deposits are only partially protective). At the beginning of the process, CR is very high

and rust forms. Then the rust reduces CR, but oxygen still diffuses inside it, until a new corrosion phenomenon

++

starts, increasing again CR. CS is safe when pH > 7 (passivation), [Ca ] > 20 mg/L to promote scale formation,

3- - 42-

oxygen concentration is negligible (below 0,02 ppm) and [HCO ]/[Cl + SO ] > 1,5 to avoid scale dissolution.

GS resistance to corrosion is related to Zn passivation. If T > 50°C polarity inversion occurs, Zn passivates and

 becomes more noble than Fe. Low rate uniform corrosion → forma on of a protec ve layer on residual Zn

coating, thus protection is achieved. High rate → complete loss of Zn coa ng (a ack on Fe, pollution of water).

The quantity of oxygen that can be stand is up to 5 times the one of CS (0,1 mg/L).

SS is a passive material that suffers from localized corrosion only, as pitting and crevice. Generalized corrosion

 is possible only in very acidic solution (pH < 2) or with organic acids (chemical dissolution). It doesn’t suffer

neither from bacteria nor differential aeration corrosion. Pitting potential depends on the kind of SS (different

PREN) and on the environment: critical chloride content, T (the higher T, the