Corrosion Engineering - Notes

SOFT

HARD The most used membrane release Na ions.

(Made of NaCl)

Change of pH is also achieved: hard pH 7 -> soft

pH 9

Cleaning of the membrane in necessary because it is filled by Mg and Ca

ions that were blocked before during the softening. Regeneration of the

resin some times and then replace it. We have to take into consideration also the covering effect of the scale.

Few micrometer/y up to 1 mm/y

laminar

Active materials

Scale: porous coating. P is the porosity.

IL index is not infinite: max value 3.

IL = 3 -> p = 0,1 (90% of the surface is coated)

IL = 2 -> p = 0,6

IL = 1 -> p = 0,9

• CATHODIC REACTANT: Oxygen -> estimate limiting current density knowing concentration of

oxygen, T, v (m/s), Cl2

• pH

• Hardness

• Resistivity -> TDS factor

We need it in order to calculate the throwing power.

Note the 6 orders of magnitude of difference Relation: conductivity/salinity = 0.6 almost always constant.

18 ottobre 2016 Pagina 43 We need it in order to calculate the throwing power.

Note the 6 orders of magnitude of difference Relation: conductivity/salinity = 0.6 almost always constant.

18 ottobre 2016 Pagina 44

Corrosion in seawater

mercoledì 19 ottobre 2016 15:22 ASTM D1141: syntetic seawater regulations for production. Microorganisms have to be added.

Min pH 7,5.

Oxygen few mg/L

Fouling responsible of the severe corrosion on SS because increase the free corrosion potential of about

300 mV. More noble, more susceptible to localized corrosion.

Changing salinity -> change the amount of oxygen. More salinity = low oxygen. Caspian sea are more corrosive for CS for

example. Localized corrosion

Sulfate: important in bacteria corrosion. The salinity can change with the depth and the zone of the sea. Under some km the

composition is almost constant because there are less turbulences.

50 max geometrical

factor in general

Sea water more than

50

Distilled water is like 2

Increase of oxygen with biological reactions (organisms that consume CO2 for example).

No natural tendency to form scale, very hard water.

Force current into a metal surface in order to have a physical barrier: on that surface only cathodic reaction. In sea water:

cathodi reaction are hydrogen evolution (consume acidity) or oxygen reduction (produce alkalinity) -> pH > saturation pH:

calcareous deposit. Efficiency close to 99% (almost cover all the surface). Scaling condition created by cathodic protection!

Natural build up of a barrier.

19 ottobre 2016 Pagina 45 Most of the discussion is based on CS.

Atmospheric: same of atmosferic corrosion in marine environment.

316 SS with 3% of Mb is the best alternative in water (it remains also bright).

Maximum height to which a drop of water can arrive during thunderstorms. In nordic

seas the variation between the average height of sea and the upper zone can be also of

20 meters. Not treated bars, not coated.

19 ottobre 2016 Pagina 46 corrosion that produces rust + mechanical movement and fracture of the rust = continuous contact with

oxygen Consider also the piece of legs under the mud.

Divide the stucture, choose some parameters, …

Atmosferic zone

Splash zone

Immersed zone

Mud zone

Atmospheric zone:

Consider the time of wetness, according to RH, T, wind… Once estimated: oxygen -> 60-90%

(+ registrazione telefono!!!!)

CR = Dt^n where n is function of the material; n= 0,5 for CS. -> CR = 12(O2)*f(T)*tau Condensation is stagnant: no f(v). No geometric factor. Amount of oxygen in a

condensate is bigger than the bulk sea water (max 10 mg/L). 2 times the maximum oxygen is a good value. We can consider 15/20 mg/L of oxygen.

Temperature dependence: choose a temperature.

Corrosion

in seawater 0,1-04 mm/y generally.

Registrazione audio avviata: 16:36 mercoledì 19 ottobre 2016

Immersed part:

(once calculated the immersed, we know that in the splash zone the CR is greater! We can consider 30% more, 2 times, it depends.)

Time of wetness: 100%

How much oxygen? 100 mt total height of the structure. We can take O2 = 1-9 ppm.

Temperature? First meters: little possibility to have more than 25°C. Under 10 meters always under 25°C. We can consider f(T )=1 (conservative approach,

always, when the T is less than 25 choose 1).

Velocity? Laminar or turbulent.

CR = 0,2 - 0,3 mm/y generally.

Splash zone: CR > 0,3

-1 m under the sea, to the maximum level of the splash = apply a mechanical coating.

Mud zone:

No oxygen, no CR by oxygen. Localized perforation due to bacteria. Rate of corrosion close 1 mm/y. Differential aeration corrosion.

Which is the cathodic area? The upper part because it is receiving oxygen.

Anodic site

0,1-0,2 V

10*O2*f(v laminar)

0,2 ohm*m

19 ottobre 2016 Pagina 47

0,2 ohm*m

Explain and demonstrate why in the Dead Sea, Al and Mg corrode and CS does not (CR almost zero). 3 weeks to solve. Max 5 students.

19 ottobre 2016 Pagina 48

Corrosion in water

lunedì 24 ottobre 2016 09:06 EN 12502

CS is the most used material. It doesn't suffer from localized corrosion

phenomena typical of passive materials, as pitting for example. It can suffer from

other kind of localized corrosion. More severe; any rust formation is

possible, the metal is continuosly in

contact with the electrolyte. Oxygen is

limited by diffusion, hydrogen no.

of CS Differential aeration or galvanic coupling. If there are bacteria (SRB = sulfate

reducing bacteria) there is localized corrosion.

Never consider pitting even with kilograms of chlorides.

Localized cross section reduction: almost an open pit.

Laminar index greater than 1 (?)

There are enough sulfates to dissolve the scale!

Chlorides influence the current circulation in the solution,

increasing the conductivity.

Sweet corrosion with CO2: enter the solution and forms the carbonic acid = very corrosive.

Reduction of the generalized corrosion because of the film formation, but the film is porous,

cracked, not perfect. Iron sulfide is more noble than iron and galvanic coupling occurs.

Aerated water: calculate limiting current density, and so on. But the corrosion rate is not constant in

time! Very severe corrosion in the first period (first months): rust is formed. After the first year the

penetration of corrosion is reduced because of the physical barrier, the scale formed before.

The increase of corrosion rate then is constant

depending on the geometrical ratio when a defect

is formed.

Corrosion allowance: extra-thickness added to the

pipe in order to consider corrosion rate. Residual

thickness is enough to suffer the internal

pressure. If the residual thickness is not enough,

we have a mechanical collapse.

If a localized corrosion takes place, it is an error of

design generally.

This table define when CS is a safe material.

Bicarbonate in bigger quantity: formation of the protective calcareous scale.

24 ottobre 2016 Pagina 49 Real thickness loss/real time = 1 mm/y. It is too much. Velocity is zero (close

valve), max oxygen is 10 ppm. 12[O2] = 0,1 mm/y. Localized form of corrosion

cannot be determined only with oxygen! We have to consider increasing

factors as galvanic coupling, bacteria, differential aeration, … not Cl pitting with

CS. In this case study the valve was in SS.

Soft Differential aeretion in a vertical pipe? Typically on the horizontal pipe where

sand and particles deposit that create the differential aeration.

Here the cause was galvanic coupling.

Surface of SS valve (cross section of the pipe)

The rust of CS is voluminous: no more able to open the valve. We need SS. Metallic elements = electrical connection: we need electrical insulation between

valve and pipe = two insulating joints - polymeric - also bolts must be insulated otherwise we have a connection between SS and CS. Generally the bolt is

inserted into a polymeric ring! It cannot be used with water at T > 50 °C otherwise there is polarity

inversion: Zinc passivates and is more noble than CS in this situation.

Fifty times

greater the value

of oxygen than

CS

24 ottobre 2016 Pagina 50

White is the typical color of the Zn rust. Also calcareous deposit and Mg products are white, too. Here mixture of Zn corrosion product and calcium

carbonate. Brown deposit: iron oxide.

Thickness 4 mm; corrosion time 4 y. CR = thickness/time = 1 mm/y. No one is able to know when corrosion started, when the Zn is consumed, when

the galvanic coupling started. So assume that value as the MIN corrosion rate.

Measure how many square cm are suffering corrosion in order to calculate the geometrical factor. Shallow pit: 1 square cm of anodic surface.

Cathodic surface: maximum is the surface wetted of the pipe. We have to subtract the anodic surface, and also consider the throwing power! The

cathodic surface is less than all the wetted surface if the throwing power is small. Electrochemical energy of current. It depends on the

type of corrosion.

Differential aeration: max 0,2 V.

Pitting (SS): max 0,5 V.

Galvanic coupling: Ec - Ea = max 0,5 V generally

Rust formation: cause of differential aeration (different diffusion of oxygen). We have to consider all the (famous cases: CS-Zn 0,2/0,3 V; CS/SS 0,5 V; CS/Cu 0,5

possible causes and calculate throwing power etc. Then we'll have just one correct result. V).

If L >= 1/6 of the circumference: all the cathodic surface is all the total wet surface.

If L is <, the cathodic surface is lower.

(it is not always 1/6 obviously, it is just an example).

A more correct formula of throwing power of the pipe: diameter of electrolyte has to be considered.

If the pipeline is completely filled, the diameter of the

electrolyte is equal to the diameter of the pipeline.

Considering from 4 to 8 generally it is 1/4 of the diameter

of the pipeline. Current forced to circulate in small area.

Max value of partially filled: 1/3 of the pipe diameter.

24 ottobre 2016 Pagina 51

Stainless steel

lunedì 24 ottobre 2016 10:29 Very acidic solutions, pH < 2 or organic acids chemically dissolve SS: only cases of

generalized corrosion.

Localized corrosion: generally pitting and crevice. For sure not galvanic coupling

(graphite or some other very noble oxides, that are not typically used). No bacteria, no

differential aeration. Bacteria that corrode SS exist but aren't widespread.

Pitting Resistance Equivalent Number: if is high, high corrosion resistance.

Pitting potential only measured in laboratory never on field but is a good way to choose

the material. Higher T = higher

probability

More alkalyne = more

resistant (OH- is like an

inhibitor).

Higher velocity = lower

probability (worst:

stagnant)

Critical Cl content:

No deterministic approach; we obtain a distribution 3% of Mo

No Mo

Oxygen must be present to have pitting SS in seawater: bacteria formed on the surface

increase the oxidizing power of 300 mV.

If the metal is considered compatible for potable

water, i