Corrosion Engineering - Notes

Introduction

Lunedì 3 ottobre 2016 10:04 Ormellese 3 ottobre 2016 Pagina 13 ottobre 2016 Pagina 2 Faraday's Law - see slide.

Control the cathodic process

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• Apply a barrier (coating to avoid the penetration of oxide).
• Change the corrosivity by adding some chemicals into the solution.

Thermodynamic aspects

Cathodic process Anodic process NERNST LAW. Oxygen standard potential is very positive. Changing pH = different values Depending on T, species concentration. Neutral condition +0.820 V (most used oxygen reaction). Titanium will suffer corrosion from the thermodynamic point of view; kinetically, the question is different. Actually, deltaE with oxygen is about +2,4 V, which is much. Passive materials: not corroded by oxygen even if thermodynamically it can happen. Chromium is the most important element in stainless steel: SS suffers corrosion thermodynamically but kinetically it is very resistant. Pourbaix diagram - see slide.

Write the equation of metal dissolution, calculate the Nernst law -> find the Pourbaix diagram of every metal. Some corrosion products can form metal-ions complexes and so we can have precipitation. Big reduction on the equilibrium potential of the anodic reaction: diluted solution (complex precipitation) is more corrosive the more diluted with the complex because of the more negative value in log[Mz+], thus increasing the driving force. System with a complex: more corrosion because the driving force is higher. Lowering the concentration = more negative is the potential = greater the driving force.

Pourbaix diagrams of metals

• Corrosion area - the metal is stable as ion.
• Passivation area - corroded to form an oxide but this oxide forms a layer that protects further corrosion.
• Immunity area - no corrosion at all. Titanium has a very big passivation area, for example. See examples.

Amphoteric materials: immune only in a limited pH interval. Potential measurement: R>200 MegaOhm voltmeter high impedance. The reference value has to be fixed and a constant value stable everywhere (electrode potential). It is not easy to use a standard hydrogen electrode (that is already set to zero), so we need a specific reference electrode different from hydrogen. The reference electrode is made of a metal inserted in a container with an electrolyte. We have to compare every reference electrode with the hydrogen one, however.

Silver Chloride - Silver reference electrode +0.222 V with respect to hydrogen. E (V) +0,2 SSC green = interval of E, that is a function of the concentration. To fix the value, use saturation! Hydrogen 0 +0,318 CSE. 3 ottobre 2016 Pagina 3 Hydrogen 0 +0,318 CSE +0,244 SCE +0,2 SSC. Concentration of Chloride in seawater is constant -> constant solution -> control corrosion in seawater 0 Hydrogen.

Standard Calomel Electrode (SCE) -0,8 Zinc

Platinum wire dipped in pure mercury in a paste of mercury chloride (calomel) and a solution of KCl. +0,244 Copper - Copper sulfate electrode (CSE). Plastic container + pure copper road + copper sulfate solution saturated (with at the bottom the precipitated crystal). Considering carbon steel (-0,44) with hydrogen reference, we have a value; with CSE, we have -0,76 V, so we add the name after V, in this case, CSE. Putting carbon steel in seawater using the zinc reference electrode, we have +0,36.

Kinetic aspects

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Ohmic drop in the electrolyte. Anodic overvoltages Cathodic overvoltages. Contribution of current transported by electrons in metals is not considered here, because we cannot change the speed of electrons. Evans diagram (E vs current density - semi-logarithmic scale). Straight line if the metal is active (positive slope), dividing two areas (immune or not). The higher is the potential with respect to the Eeq, the higher is the current, so it is the corrosion rate. Example of carbon steel slope of … mV for each order of magnitude.

Passive material: very different Evans diagram. Passivity current density (?). Oxygen limiting current density Very negative potential also hydrogen evolution. It depends on pH. Changing the partial pressure of oxygen, … changes the parameter. D = diffusion coefficient of oxygen.

Increasing the turbulence, change of delta, higher current. See graphs on slides, always.4FD/delta = (usually delta of …) = 9,5 so 10 in stagnant condition at room temperature. Oxygen concentration in mg/L. Ohmic drop can be ignored in some cases. Carbon steel and stainless steel from thermodynamic point of view are the same (?) but one is active, the other passivates. Same driving force, completely different behaviour. All the driving force in the carbon steel is used to corrode.

Forms of corrosion

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Different kinds of localized corrosion

• Pitting: affects passive materials in most cases.
• Crevice: where 2 metals touch.
• Erosion corrosion: effect with fluid with high velocity.
• Mechanical stress (typically tensile) in a corrosion environment: very dangerous, less than 10 micrometer cracks, not visible with the naked eye, catastrophic failure.

Transgranular crack: the thickness lost is the same in the all structure: uniform metal thickness reduction. Zinc is not stable in acidic conditions. Chloride penetrates inside the material destroying the passive layer! The rust is inside the concrete + increase of volume of the metal inside the concrete = failure CR Fe/pH 7 = 12[oxygen](micron/year) (mg/L). Reduce the residual thickness not acceptable from the mechanical point of view.

Release of ions = pollutant in water.

Prevention of corrosion

• Add more calculated thickness (max 5 mm, economic reasons).
• Simply change the material, the best choice is a passive material.
• Inhibitors: reduce the rate of corrosion by changing the cathodic behavior (increase eta c) or diminishing eta anodic.
• Atmospheric corrosion -> coatings.
• Immersed structure -> cathodic protection.

Electrochemical procedure to reduce the amount of oxygen, thus reducing the rate.

Localized corrosion

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Galvanic coupling

Specific free corrosion potential: the point where the cat and an curves overlap in the Evans diagram. Galvanic coupling is higher when the material is coupled with a nobler material; the less corrosion-resistant is consumed more, and so on.

Geometrical effect: same surface, A less corrosion resistance, increase surface of B -> change of corrosion rate. Possibility to couple zinc with carbon steel: carbon steel is the cathodic material because it is more noble than zinc -> galvanized steel coated with a thin layer of zinc. Control generalized control of carbon steel.

Note that active stainless steel behaves like carbon steel. Passive stainless steel: 1 mA/m² = 1,17 micron/year. Creation of a galvanic couple. Same limit i because same situation. How many grams of iron are consumed? The same mass, because there is proportionality to oxygen. Oxygen is diffusing everywhere. But electrons are released only by Fe, so the penetration rate is doubled! Same mass but half surface! Localized effect due to the surface. 4 times I corr = 4 *100 mA/m² = 0.4 mm/y 8 times 780 g and i corr = 8*i(l).

Dangerous situations

Very big surface of noble metal with little surface of anodic surface. Fe coated with Zn, where there is a little crack: cathodic surface 10% -> 1.1 increase of corrosion rate, no big increase of i corrosion. Different final corrosion rate if we change the solution because of the ohmic drop! Distilled water: no ions transport current, the system is under ohmic control. Throwing power of the coupling: Fe coated with Zn, where there is a little crack: cathodic surface 10% -> 1.1 increase of corrosion rate, no big increase of i corrosion. Different final corrosion rate if we change the solution because of the ohmic drop! Distilled water: no ions transport current, the system is under ohmic control. Throwing power of the coupling: Driving force to drive the current everywhere. Throwing power L max = max current path from the anodic material (source of electrons) to the absorbing material. Sea water: lot of ions no Ohmic control = high L. Distilled water: ohmic control = min L, localized corrosion at the interface cat.-an.

Apply the paint everywhere not only where there is corrosion, because the paint is never perfect and the geometrical factor increases. Apply the paint on the more noble material theoretically in order to have a barrier to oxygen!

Pitting

First form of localized corrosion of passive materials. Different causes -> different name. Chloride corrosion (chemical) is pitting. In a gap between two materials: crevice. Thermal treatment changing the cathodic or anodic behavior: intergranular corrosion (anodic grain boundary + cathodic bulk after welding treatments). Mechanical: bubbles, …

Pitting

Mercoledì 5 ottobre 2016 15:31 See slides up to page 12 4 ottobre 2016 Pagina 11 Pitting mercoledì 5 ottobre 2016 15:29 Nickel not directly related to the capability of steel to resist chloride. Cathodic process: oxidizing power of the environment determines the threshold. Transpassive area High content of oxygen, high oxidizing power. Passive area Pitting as chlorine Pitting potential Biofilm stable on the surface. Free corrosion potential more noble of 300 mV Stainless steel in sea water with graphite (more noble): increase of free corrosion potential of stainless steel (dangerous!).

Specific micro environmental conditions on the surface determine the pitting initiation. Chlorides have more time to find the weak point of the passive layer when the solution is stagnant, so the situation is more dangerous. Laminar, turbulent condition: 200 mg/L critical chloride content.

Wetting permanency: the time of initiation of pitting is not immediate, there is a period of incubation. Leaving the cookware dirty after cooking with salts makes the initiation of corrosion (dots of rust at the bottom, generally after a day). Restriction on the quantity of sulfide and manganese. Welding area: pipes have to be welded, different techniques but with the heat treatment -> 1 inch of heat-affected zone: different microstructure, different passive layer formation (different Cr distribution) = disomogeneous corrosion behavior. If the weld is brown it is a very bad weld. Rainbow of colors: bad weld (related to the different thickness of the passive layer). The good weld has the same color of stainless steel (bright grey). Roughness increases the real surfaces in contact with solution: more corrosion.

Metallic composition, solution composition, … determine the pitting corrosion. PITTING PROPAGATION Local increase of chloride -> there initiates corrosion: dissolution of passive layer, macrocell corrosion. Anodic side: the pit; the cathodic surface is the rest of the surface. Current from the anode to cathode through the solution, … negative ions move towards the pit! Enrichment of Cl. Hydrolysis direction (Fe and water) so increase of acidity.

Local increase of chloride -> there initiates corrosion: dissolution of passive layer, macrocell corrosion. Anodic side: the pit; the cathodic surface is the rest of the surface. Current from the anode to cathode through the solution, … negative ions move towards the pit! Enrichment of Cl. Hydrolysis direction (Fe and water) so increase of acidity. The system is autocatalytic: increase of acidity keeps activation. Anode: pit; cathode: external surface (big). We have also geometric factor! Anodic surface is small. System conductive.

How to stop a pit initiated?

• Eliminate oxygen (no cathodic reactant).
• DO NOT REMOVE CHLORIDE. The pH is always acidic, nothing would change.
• Mechanically clean the pit (by "erasing" the pit itself; only with the external imperfect passivity surface actually).
• Reduce the potential of the stainless steel into the perfect interval. It is the only perfect passivity way from the electrochemical point of view. The repassivation potential is max 300-400 mV more negative than the pitting potential. Force the metal to work in passivity: no initiation, and the perfect means that the pitting stops. In the imperfect passivity, no initiation of pitting, but continuation of already begun pits.

(This diagram actually is valid for carbon steel in concrete) No stagnant water! No evaporation of water on the surface! = accumulation of chlorides.

Other kinds of corrosion

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Crevice corrosion

PREN: %Cr + 3.3*% Mo + const*%N The passivity is guaranteed if Cr is distributed in the steel. If there are precipitated there is no passive layer. Welding: avoid that range of precipitation temperature! We have to avoid entering the curves to avoid the Cr precipitation. Low carbon stainless steel is good for welding (AISI 304 L). Add alloying element to precipitate with carbon instead of chromium: stabilized stainless steel. AISI 304 L Ti Less thickness here: impact of liquid Laminar regime.

Typical passive metal

Few tens of micrometers Typical of hydrogen embrittlement. Typical of SSC AISI 4… suffer everywhere corrosion. Low toughness metal 1 mm/year.

Oxygen reduction

Acidic solutions is the cathodic reac. Embrittlement at the tip. Permeation of hydrogen (autocatalytic). Anodic Reduction of the stress is not site economic usually= bubbles (coalescence of cracks). Corrosive environment, different Wohler curve. Mechanical action + corrosion propagation. Number of cycles 20 Hz high frequency: no time for corrosion from the environment. Cracks propagate because of corrosion under 1 Hz. Between 1 and 20 Hz of frequency, we have fatigue corrosion. This condition is typical for platforms into the sea, because we have water motion and chlorides.

Atmospheric corrosion

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1. Electrolyte?
2. …
3. Which metals used? (standards - worldwide acceptable knowledge)

Copper alloys very stable in atmosphere. Carbon Steel has to be coated (barriers). Lead + SO₂ = very stable patina. Wrought iron: 600 years stable (superficial corrosion = perfect layer). Titanium not to resist corrosion but for aesthetic reasons. If the water condensates as droplets: very conductive situation because of high concentration of salts. Few micrometers 2-3 mm thickness of the layer Almost infinite amount of oxygen in the atmosphere that concentrates in few mm: very high concentration. Period of time in which the condensate is present on the surface Carbon steel generally.

Atmospheric corrosion

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Categories of severity of each kind of atmosphere

Define the aggressivity. Acidic gases, responsible for acid rains. Completely corroded from the acidic environment Along the year Dust on horizontal surfaces= more probability of condensation of water under the dust Carbon steel is passivated Marine-industrial atmosphere can be also present. Lower than 60% no condensation so no corrosion. More than 60% increase of probability of Chlorides have the condensation capability to dissolve any corrosion product! More than 80% surely condensation (no polluted atmospheres). Day by day we can use information about relative humidity to estimate the time of wetness. Among the parameters, Cl are the elements that mostly affect the relative humidity increasing condensation. Absence of Cl is impossible = reducing humidity = increasing wetting time.

10 ottobre 2016 Pagina 18 Number Relative humidity Five different levels of wetness: number of hours in a year. (Time of wetness)*(average corrosion rate). Time of wetness between 0 and 1 is good to reduce corrosion! Almost acidic rate Reduce RH No limitation in current circulation Distance from the sea Level of pollution, time of wetness -> different classes They increase time of wetness.

10 ottobre 2016 Pagina 19 Level of pollution, time of wetness -> different classes They increase time of wetness No pollution Categories of corrosion related to corrosion rates? Define the class -> CR1 corrosion rate of the first year -> used in the second standard to estimate the evolution considering that the rate is reduced (the next year's condensations have to face a physical barrier to a further condensation that is the first condensation in the first year). Law of reduction D = total penetration of corrosion. The coefficient depends on the chemical composition generally. Rural env. Using galvanized steel (with thin layer of zinc): corrosion rate of zinc with no defects is more than one order of magnitude lower than that of C steel. Al is a passive metal.

Indoor atmospheric corrosion assessment

Time of wetness 90%. ISOCORROSION CURVES OF CARBON STEEL Zinc more safe (anodic with respect to CS). Cr and Ni are cathodic with respect to CS and are used for aesthetic purposes, but with some defects, the substrate of CS is anodic, so we have penetration of corrosion. Prevention by:

• Physical barrier
• Change corrosion behavior

Enamel = smalto. Few micrometer thick Compact layer of alumina to max 30 nm. Alumina is an insulator. Only on aluminum. Insulator = no corrosion (high ohmic drop), but it has to be perfect, without defects. Very corrosive anyway, we need less than 10 micrometers/y. 10 ottobre 2016 Pagina 21 Very corrosive anyway, we need less than 10 micrometers/y Chemical reaction of the coating = complete hardening of the coating. Epoxy resin + hardener that catalyze the reaction. First layer + distribution of Zn powder (80%) -> the particles.