Corrosion engineering - domande e risposte in preparazione all'esame

Corrosion engineering

Atmospheric corrosion

Atmospheric corrosion .......................................................................................................................................... 2

List and describe the influencing factors. .......................................................................................................... 2

Compare the corrosion behavior of carbon steel, corten, galvanized steel and stainless steel. Give an estimation of the corrosion rate. ........................................................................................................................ 2

Describe the effect of coatings (metallic and non-metallic) used for prevention and control corrosion of carbon steel structures. ..................................................................................................................................... 3

Soil corrosion

Soil corrosion ......................................................................................................................................................... 3

List and describe (three) forms of localized corrosion on a carbon steel pipe. .................................................. 3

Describe the corrosion promoted by stray current. .......................................................................................... 4

Corrosion in water

Corrosion in water ................................................................................................................................................. 4

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. ......................................... 4

List and describe the influencing parameters to estimate the corrosion in aerated water. ............................. 4

Compare the corrosion behaviour of carbon steel, galvanized steel and stainless steel. Indicate and justify a possible interval of corrosion rate. .................................................................................................................... 5

Seawater: describe the corrosion mechanism and rate in the different zones. ............................................... 5

Oil&Gas

Oil&Gas .................................................................................................................................................................. 6

What does “sweet corrosion condition” and “sour service corrosion” mean? ................................................. 6

Describe in a qualitative way the mathematical approach used to estimate the corrosion rate in fluids containing CO₂. ................................................................................................................................................. 6

Describe the four localized corrosion forms in the presence of H₂S. ............................................................... 7

Which are the requisites of a carbon steel pipe to be used in sour condition? ................................................ 7

Cathodic protection

Cathodic protection ............................................................................................................................................... 8

Describe the three possible CP criteria. .............................................................................................................. 8

Describe the principle of CP potential monitoring. ........................................................................................... 8

How is it possible to eliminate the ohmic drop in the potential reading? ........................................................ 8

Describe the possible systems used to apply cathodic protection, highlighting advantages and drawbacks. . 9

Atmospheric corrosion

List and describe the influencing factors

• Meteorological factors
  • Time of wetness: corrosion rate depends on the contact between the metal and the electrolyte, thus if this factor is lower than 1 CR is reduced. Generally, the level is defined as the number of hours in a year. The condensation film (few µm = high enough to transport current but low enough to allow oxygen diffusion) has a high concentration of salts thus is really conductive.
  • Humidity: CR increases with higher humidity; hygroscopic salts reduce the critical RH (it is possible to have very high CR at 50% of RH only).
  • Rain: water is the electrolyte that allows corrosion. If they’re acids, corrosion products are also less protective and more soluble, increasing CR.
  • Temperature: it influences the oxygen diffusion coefficient (that increases with higher T), the humidity and condensation, the solubility of salts inside the electrolyte.
• Atmospheric contaminants
  • Sulphur oxides SO_x: water condensation increases and coatings can deteriorate (= increase of CR).
  • Acid rains: pH = 3 – 4, acidity deeply increases CR because corrosion products are more soluble and less protective.
  • Nitrogen oxides NO_x: can act as catalysts and transform SO₂ in SO₃ (more aggressive).
  • Chlorides: hygroscopic salts, reduce RH; corrosion products are less protective; the electrolyte is more conductive; promote localized corrosion (SS).
  • Solid particles: promote capillary condensation (higher time of wetness); cause shielding effects (crevice corrosion).
• Intrinsic factors
  • Material: all the metals with an equilibrium potential (Nernst) higher than the cathodic process of oxygen reduction (or hydrogen evolution) don’t undergo corrosion (of that kind).
  • Corrosion products: a passive metal covers itself with a protective layer of oxide which prevents corrosion; many corrosion products if adherent, non-porous, and mechanically resistant can act as a physical barrier towards oxygen diffusion, avoiding or slowing CR.

Compare the corrosion behavior of carbon steel, corten, galvanized steel and stainless steel

Give an estimation of the corrosion rate. Carbon steel has low corrosion resistance, its rust is expansive and not protective. It can stand up to 0,02 mg/L of oxygen in order to have negligible CR. If pH > 9 passivation occurs (as in concrete, where pH ≍ 13).

]= 10[ = 10 ∙ 0,1 =1 = 1,17

Corten is a low alloyed carbon steel: Cu, Cr, Ni, P promote a protective patina; V, Zr, Mo promote mechanical resistance. The patina is created alternating wet and dry conditions, thus high frequency cycles allow a rapid formation but more porous product. Corten cannot be used into permanent immersion and near the seaside (no chlorides). Acid rains (SO_x pollutant) dissolve the protective patina.

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. Zinc coating 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 used to prevent and control corrosion of carbon steel structures

Zn coating is an active coating that protects CS cathodically: Zn passivates, also in CO₂ atmosphere. The 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 (PbSO₄), very compact and adherent → self-healing, auto-protection.

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