CORROSION

1. BASIC PRINCIPLES OF CORROSION

2. WHAT IS CORROSION? • Corrosion is the degradation of metals and alloys as a result of their chemical and electrochemical reactions with their environment. Corrosion is a natural phenomenon that occurs as a result of the reaction of metallic materials with the environment they are in, without the need for external energy. Chemical Corrosion (Dry Corrosion): It is the oxidation of metals and alloys in gaseous environments. However, corrosion caused by the humid air that surrounds us is excluded from this recipe. Electrochemical Corrosion (Wet Corrosion): It is the degradation of metals and alloys in aqueous environment.

3. Losses Caused by Corrosion • Economic losses Facility out of service Loss of product Product contamination Loss of yield

4. ELECTROCHEMICAL FORMATION OF CORROSION • Anodic Reaction: It is the transformation of metal atoms into positively charged metal ions by losing a negative charge. As a result of this event, electrons are produced. Me Me+2 + 2e- Cathodic Reaction: The function of cathodic event is to expend the electrons produced in the anodic reaction. Me+2 + 2e- Me • Anode: Corroded (oxidized) metal Fe Fe+2 + 2e- Cathode: The surface of the metal where a reaction (reduction) occurs, which consumes the electrons released at the anode O2 + 2H2O + 4e- 4OH- Electronic Conductor: A metallic conductor that carries the electrons released at the anode to the cathode. The contact of the anode and the cathode with each other also provides this communication. Electrolyte: Electrolytic conductive, aqueous solution. Aqueous medium that provides ionic bonding between the anode and the cathode.

5. CORROSION Electrochemical occurrence pattern of corrosion (a) Battery, (b) Events involved in the corrosion cell

6. Electrochemical Reactions • One of the most important data in determining the tendency to corrosion is the standard electrode potential. The comparison is made with the standard potential of the hydrogen reaction, the potential of which we can assume to be zero. It allows all electrode events to be compared with each other electrochemically. This sequence is called the "electrochemical voltage sequence". In general, metals located above the knee show an anodic attitude towards the following. Standard ElectrodePotential Series ElectrodePotentialVolt(Oxidation) • Na = Na+ + e- 2,714 AKTIVE • Mg = Mg+2 + 2e- 2,363 • Al = Al+3 +3e- 1,662 • Mn = Mn+2 + 2e- 1,180 • Zn = Zn+2 + 2e- 0,763 • Fe = Fe+2 + 2e- 0,440 • Cd = Cd+2 + 2e- 0,403 • H2 = 2H+ + 2e- 0 • Cu = Cu+2 + 2e- -0,337 • 4OH- = O2+2H2O+4e- -0,401 • Ag = Ag+ + e- -0,799 • Au = Au+3 + 3e- -1,500 SOY

7. KEY EXAMPLES Corrosion in Oxygen-Free Acid Environments Let us take a look at the retention of zinc in hydrochloric acid: A.R: Zn Zn+2 + 2e- K.R: 2H+ + 2e-  H2 (gaz) Sum:Zn + 2H+ Zn+2 + H2 (gaz) Corrosion in Ventilated Neutral and Basic Environments A piece of steel in seawater is the best example: A.R: Fe  Fe+2 + 2e- K.R: O2 + 2H2O + 4e-  4OH- Sum: 2Fe + 2H2O + O2 2Fe+2 + 4OH- 2Fe(OH)2 2Fe(OH)2 + H2O + 1/2 O2  2Fe(OH)3

8. KEY EXAMPLES Corrosion in aerated acid environments The characteristic of such environments is that they contain oxygen molecules as well as hydrogen ions. The presence of hydrogen ions leads to the reduction of oxygen present in the environment by its reaction. O2 + 4H+ + 4e- 2H2O

9. CORROSION TYPES Homogeneous Corrosion It is the most common type of corrosion. Metal loss is very high compared to other types. It walks at the same speed at every point of the metal surface The thickness of the metal becomes thinner at every point Surface coatings can be controlled with cathodic protection and corrosion rate limiting agents added to the aggressive environment. Pit Corrosion It is the concentration of corrosion on very narrow areas. The loss of metal is very small. However, the parts are punctured in a short time. It occurs in neutral environments containing chlorine and bromine ions. As the pH drops, it is replaced by general corrosion. It occurs in stagnant solution. It is found in stainless steel and aluminum alloys with a high tendency to passivate It is evaluated by measuring the average number of pits and the maximum pit depth. MeCl2 + 2H2O  Me(OH)2 +2H+ + 2Cl-

10. CORROSION TYPES Galvanic Corrosion It is the corrosion of different types of metals and alloys in contact with each other in the same environment. The pairing of metals or alloys that are far from each other in the electroplating array should be avoided. The lineage one is the cathode, and the active one is the anode. If the electrolyte conductivity is high, the corrosion rate increases Care should be taken to ensure that the one with the smaller surface area of the paired metals is the lineage compared to the other. (The As/Aa ratio should be small) The two metals must be insulated. Cathodic protection can be applied by attaching a third metal with anodic character to the system. Addition of corrosion retardant to the environment. Selective CorrosionIt is the corrosion that occurs as a result of the corrosion of one of the elements in an alloy. It is the corrosion of zinc in brass alloy before copper. Brass: 70% copper and 30% zinc alloy As a result of corrosion, the porous structure gains and the strength decreases As the zinc in the alloy increases, corrosion resistance decreases Reducing the zinc content below 15% or adding 1% tin to brass increases corrosion resistance.

11. Underground AndUnderwaterCorrosion • Underground Corrosion Corrosion caused by galvanic action Corrosion due to difference in soil structure Corrosion caused by different ventilation Corrosion caused by coating defects Biological corrosion Leakage current and interference corrosion Aqueous Media Corrosion The most important factor determining the corrosion rate of metals in natural waters and the sea is the amount of oxygen dissolved in the water. The amount of oxygen dissolved in both fresh water and marine environment is around 8 ppm (5.6 ml/lt) under normal conditions.

12. Formation of galvanic corrosion betwePotential difference between old and new pipeen old and new pipes Galvanic corrosion caused by contact of different metals Potential difference between old and new pipe

13. Corrosion of the pipe base due to different ventilation Anode and cathode regions due to the difference in ground structure Corrosion due to coating defect

14. Corrosion in a pipeline entering concrete from the ground Corrosion caused by clay soils adhering to the pipe surface

15. Potansiyel-pH Diyagramları • In the most general sense, it shows the areas where metal ions and oxides have thermodynamic stability, that is, it has the nature of a phase diagram. The potential-pH diagram for the Fe-H2O system was developed taking into account the reactions between ions and oxides that occur as a result of the contact of iron with aqueous media. Therefore, it can provide information about the corrosion of iron in aqueous environments.

16. POLARIZATION • When current flows through a galvanic cell, a negative change in the cathode potential and a positive change in the anode potential (polarization) occurs. • As a result of polarization, the cathode and anode potentials get closer and closer to each other, reaching an equilibrium potential. • Initially, the cathode potential is ECorC and the anode potential is ECorA. After the anode and cathode are connected to each other, the EDenge potential is formed with the effect of the current flowing through the cell. Polarization of the galvanic cell

17. POLARIZATION • A metal that corrodes in aqueous solutions also becomes polarized like a galvanic cell. The cathode reaction of such a corrosion cell is hydrogen ion reduction, and the anode reaction is oxidation of the metal. • Initially, the cathode potential is EH+/H and the anode potential is EM/M+. With the onset of corrosion, the cathode potential increases in the negative direction and the anode potential increases in the positive direction, reaching an equilibrium potential Ecorr. V ioH+/H EH+/H icorr Ecorr c ioM/M+ iapp ia ic EM/M+ logi Polarization of a corroded electrode

18. POLARIZATION • If an external current equal to iapp is applied in the cathodic direction to an electrode that is in equilibrium at the Ecorr potential, the electrode potential shifts towards the negative direction by c. • The difference between the potential measured under current and the equilibrium potential is called overvoltage and is denoted by ; •  = Ei - Eo V ioH+/H EH+/H icorr Ecorr c ioM/M+ iapp ia ic EM/M+ logi Polarization of a corroded electrode

19. CATHODETIC PROTECTION

20. CATHODETIC PROTECTION • Cathodic protection is the process of removing anodic currents on the metal surface by turning the metal to be protected into the cathode of an electrochemical cell to be formed. As an example, let's look at the iron electrode in neutral aqueous solution: • A.R: Fe  Fe+2 + 2e- K.R: O2 + 2H2O + 4e- 4OH- Corrosion occurs when these two reactions run together. Electrons flow through the metal from the anode to the cathode, and the cathode reaction proceeds using these electrons from the anode. • If the electrons required for the cathode reaction are given from an external source, electrons cannot be produced by the anode reaction. In this case, the corrosion event in the anode is stopped. The electrons given to the metal by the external current completely stop the anodic reactions that are going on on the metal surface, while increasing the speed of the cathodic reaction. • The anode reaction no longer runs on the surface of the metal being protected, but on the anode located in the cathodic protection circuit. • The protected metal surface is now exactly the cathode.

21. CATHODIC PROTECTION FROM EXTERNAL CURRENT • Externally, direct current is applied through a transformer rectifier. The (-) end is connected to the metal, and the (+) end is connected to the anode. The current strength depends on the surface area of the metal to be protected and the degree of corrosiveness of the environment in which the metal is located.

22. CATHODE PROTECTION WITH GALVANIC ANGIODE • A galvanic cell is formed by attaching a metal that is more active than itself to the metal structure to be protected. In order for current to flow through the cathodic protection circuit, there must be a potential difference between the anode and cathode that can overcome the circuit resistance. The current drawn from the galvanic anode depends on the magnitude of the circuit resistance with the open-circuit potential of the galvanic anode.

23. GALVANIC ANODE CATHODIC PROTECTION • There is no need for an external current source. The required current is supplied from galvanic anodes. It is the only option where there is no electrical energy. The current cost is high. For this reason, it is not preferred in pipelines with high current demand. • Since the circuit potential is low, high resistance ground is not applied. It is suitable for floors with a resistance of up to 5000 ohm.cm. It is very easy to apply. If there is an increase in the current requirement, an anode can be added to the system later. It is not possible to adjust the current drawn from the anode. Galvanic anodes self-regulate the current required for cathodic protection. If there is an increase in the current requirement of the structure, its potential decreases, so there is an increase in the potential difference between the anode and the cathode, and more current is drawn from the anode. There is no peeling on the pipe surface close to the anode due to excessive voltage. Since the anode/ground pot.is low, the interference effect is insignificant.

24. Cathodic Protection From External Current • It cannot be applied in places where there is no electric current. The cost of electric current is cheaper. There is no limit to the need for current. Initial facility costs are high. High resistance is not an obstacle. The desired amount of current can be applied by lowering the anode bearing resistance and increasing the number of T/Rs. The current capacity of the transformer unit cannot be exceeded. The anode bearing resistance cannot be lowered during operation. In case of change in current requirement, the current and potential need to be adjusted manually or automatically in the T/R unit. Otherwise, when the current requirement of the structure increases, the potential may fall below the protection criterion. Peeling of the pipe coating may occur due to overvoltage in the area close to the anode bearing. Interference may occur in foreign pipelines in the vicinity of the anode bed and intersecting with the cathodic conserved pipeline.

25. CATHODIC PROTECTION CRITERIA -850 mV CriterionThe potential of the steel structure being protected measured under current relative to the saturated copper/copper sulfate electrode must be -850 mV or a negative value. The pipe/ground potential is measured after applying current to the structure for a sufficient period of time (at least four hours) and while the structure is under current. For this reason, it is necessary to make a correction by taking into account the IR ommic potential drop in the measurement circuit. (200-300 mV difference, especially in high resistance floors) 300 mV Potential Shift When applying cathodic protection current in the potential of the steel structure, a shift of 300 mV in the negative direction from its static potential (the equilibrium potential measured before applying current) must be achieved. 100 mV Polarization Shift This value is the difference between the "off" potential value measured after a current in the cathodic direction is applied to the structure for at least 4 hours and the equilibrium potential of the structure before the current is applied. IR ohmic decline is not included in dying.

26. CATHODIC PROTECTION CURRENT REQUIREMENT AMBIENT CONDITIONSApproximatecurrentrequirement (mA/m2) • Baresteel in movingseawater 100-160 • Baresteel in stagnantseawater 55-85 • Bare steel in mud like the sea 20-30 • Baresteel in dampground 10-20 • Poorly coated steel in ground or water 1-2 • Zemin veya su içinde iyi kaplanmış çelik 0,05 • Polyethylene coated steel in ground or water 0,005

27. CORROSIVE PROPERTIES OF SOILS Soil pH The degree of acidity of the soil pH<5 acidic ground High corrosion rate pH>8 alkaline ground (calcareous floor) RedoxPotential <100 Severe Corrosive 100-200 Korozif 200-400 MediumCorrosive >400 TheCorozif Ered = Ep + Eref + 60 (pH-7) Ered=Ground redox potential (mV) Ep=Pot measured between platinum electrode and ref.electrodeEref = Potential of the ref electrode used compared to the hydrogen electrode(Cu/CuSO4for316 mV) GroundResistivity <1000 Verycorrosive 1000-3000 Korozif 3000-100000 MediumCorrosive >10000 Not Corrosive  = 2aR ( ohm.cm)

28. GALVANIC ANODES AnodePotential It must be negative enough to cathodically polarize the metal to be protected. The potential difference between the anode and the cathode is the executing force that allows the cathodic shielding current to pass. This potential difference must be large enough to overcome the cathodic protection resistance. It is not possible to use low-potential anodes on high resistivity floors. Anode Current Capacity and Efficiency The amount of current in A.hour that 1 kg of anode metal can produce is called current capacity. (A.hour/kg) ACC = Anode Current Output (A.hour) AnodeMass (kg) Anode Current Yield It is the ratio of the actual current capacity to the theoretical current capacity ACY= Actualcurrentcapacityx 100 Theoreticalcurrentcapacity Yield Mg anode: 50-60% Yield Al anode: 90%

29. GALVANIC ANODES ANODE LIFEBy using the anode capacity, the current generation time of an anode of a certain mass, that is, its life, can be calculated. In practice, it is not possible to use the entire anode mass. Only a certain percentage of a galvanic anode can be used to generate current. Current cannot be drawn from the remaining part at the desired intensity. The percentage of anodes that can produce current is called the "utilization factor". Anode Life = Anode weight, kg x Utilization Factor x Anode Yield Current strength, A x Current Capacity, kg/A.year

30. Galvanic Anode Bearings • Galvanic anodes are not placed directly in the ground, but in an anode bearing filler. So: Within the anode bed, the anode dissolves uniformly. As a result, the percentage of availability of the anode increases. The periphery of the anode remains constantly damp. Thus, the anode resistance is reduced and the current output is increased. Galvanic anodes can also be used in high resistivity floors. Anode bearing filler material

31. ANODE BEARING RESISTANCE Anode bearing resistance (Dwight formula) Vertical:Rd =  (ln8L -1) Horizontal:Ry =  (ln 4L -1) 2L d 2L d Rd: Singleanoderesistorplacedvertically, ohmRy: Singleanoderesistorplacedhorizontally, ohm : Anodebearingresistivity, ohm.cm L: Anodelength. Cm (includinganodebearing) d: Anodediameter, cm (includinganodebearing)

32. ELECTROPLATING ANODE BEARING EXAMPLES Galvanized anode assembly with and without package Connection of a large number of galvanic anodes at one point Binding of magnesium anode in packets Placement of galvanic anodes below pipe level

33. Magnesium Anodes • It is at the top of the electrode potential series and is extremely active. The potential of the high-potential magnesium anode is -1.75 Volts. They can also be used in high resistivity floors and fresh waters. The current efficiency of magnesium anodes is between 50-60%. • As the current strength drawn from the anode increases, the current efficiency increases. • The current efficiency of AZ63 (6%Al-%3Zn) anodes is higher than that of high-potential anodes. • The theoretical current capacity is 2200 A.h/kg, the actual current capacity is 1100 A.h/kg. Change of magnesium anode current yield with respect to current density Chemical compositions of magnesium anodes *Yüksek pot.Mg anotlarda mangan yüzdesi Al yüzdesine bağlıdır.

34. Zinc Anodes • It is used in low resistivity soils and in the protection of marine structures. However, it becomes difficult to draw current on soils with a resistance of more than 2000 ohm.cm. The current efficiency is 90%. However, the increase in temperature causes the electrode potential and current capacity of the anodes to decrease. (Especially >600C) A potential difference of 250 mV occurs between pure zinc and protected ferrous metal. However, iron reduces this potential difference. Iron should be less than 0.0014%. Aluminum (0.1%) is added to it to form an alloy with iron and iron is bonded. Cadmium, on the other hand, destroys the harmful effect of lead. Çinko anotların elektrokimyasal özellikleri Çinko anotların kimyasal bileşimleri

35. Aluminum Anodes • It is more active than zinc. It is used in sea water or in salty waters with low resistivity. Copper and nickel shift the Al potential in a positive direction. Zinc, magnesium and cadmium, on the other hand, reduce passivation. Mercury, tin and indium metals keep Al anodes constantly active. Since mercury creates environmental pollution, indium alloy anode is used, which provides the same effect. Passivation is prevented and it dissolves uniformly in seawater. The current capacity is 2.4 times larger than Mg anodes and 3.6 times larger than zinc anodes. Alüminyum anotların elektrokimyasal özellikleri Alüminyum anotların potansiyelinin resistiviteye göre değişimi

36. External Current Source Anodes • In cathodic protection systems, about half of the initial plant cost is spent on anodes. Therefore, it is economically important that anode metal is cheap. The current that can be drawn from the anode unit surface should be as high as possible and the anode resistance should not increase too much over time. The anode mass loss per unit current drawn from the anode (A.year) should be as small as possible. Anodes must be able to produce current in the time and amount expected of them.

37. External Current Source Anodes Metal Oxide Coated Titanium Anodes By coating conductive metal oxides on titanium, anodes that do not show any passivation are obtained. The most important are nickel-ferrite (NiO+Fe2O3) coated anodes. This type of anodes is not affected by chlorine or oxygen output as a result of the anode reaction and is based on acids up to pH = 1. Mass loss is very small. 600 A/m2 can be drawn in sea water and 100 A/m2 can be drawn in the ground using a coke powder anode bed.

38. External Current Source Anode Bearings Anode Bearing FillerAuxiliary anodes used in external current-induced cathodic protection systems are placed in an anode bed. While the anodes are placed in the bed, the anode circumference is filled with coke powder. The coke powder filling placed in the anode bed increases the anode effective dimensions, reduces the anode bed resistance and plays a role in reducing the anode mass loss. Graphite, petroleum coke or coke are used as fillers. Anodebearingfillingmaterial-CokePowder

39. CARE AND ATTITUDES TOWARDS CORROSION ON BOARD

40. Corrosion is the loss of metallic properties by entering into chemical or electrochemical reactions with the environment in which metals are located. • Most of the metals are not resistant to the effects of water and atmosphere and corrode even under normal conditions. • With the exception of some noble metals, all metals and their alloys corrode to a greater or lesser extent. • Corrosion manifests itself in every part of the industry. • Tanks, warehouses, masts, guardrails, vehicles, ships, many machine parts in the open atmosphere are faced with corrosion events. All these structures are out of operation in a shorter time than expected due to corrosion and great economic losses occur.

41. Corrosion, Which Begins To Occur On The Surfaces Of Metals, Occurs Under Certain Conditions And Is Seen At Certain Points. These Points Are: • The material is corroded from the surface. The reason for this; The first points where the material encounters external effects are its outer surfaces. In alloys, one of the alloys may be corroded. Although the other alloy is not damaged, all of the material becomes unusable. It is more common in points such as screws, welds and rivets. This may be due to the fact that the oxide environment is formed more easily at the junction points, or it may be that the chemical properties of the junction points change with the type seen at the source.

43. Factors Causing Corrosion Corrosion is a dynamic cycle process. In other words, metal or non-metal materials can be exposed to a corrosive effect in any environment at any time. Factors that cause corrosion; It can be grouped under four main groups as electrochemical, chemical, physical and environmental.

45. CORROSION BY PHYSICAL FACTORS: • High pressure and temperature provide a more favorable environment for corrosion of substances. • If a part is loaded much more than its strength or is under the influence of multidirectional forces, corrosion is inevitable. • With friction, the protective surfaces of the materials are destroyed and they can be exposed to corrosion. • For example, an environment with extreme temperature differences during the day causes corrosion.

46. CORROSION BY ENVIRONMENTAL FACTORS • Climatic conditions also play an important role in terms of causing corrosion according to geographical location. • The temperature-humidity effect increases the corrosive effect especially in regions with tropical maritime climate. • In addition, corrosive compounds such as sulfur dioxide (SO2) and hydrochloric acid (HCl), which are densely found in the atmosphere of industrial zones, penetrate the material and cause great damage to them.

47. CORROSION BY ELECTROCHEMICAL AGENTS • Electrochemical corrosion is the most dangerous corrosion. • Electrochemical corrosion is simply the corrosion of metals with electric current. • However, it is not enough to have electric current alone for corrosion to occur.

48. CHEMICAL CORROSION: • If a metal wears out by forming chemical compounds without any intermediary, it is called chemical corrosion. It is the etching of mineral materials by oxygen, sulfur, nitrogen, dense acids, bases and salts.

49. WAYS TO PREVENT CORROSION • By removing the substances that make up the corrosion, it is possible to remove some of the oxygen in the water that causes the corrosion. • In particular, the oxygen of the water used in the heating installation can be reduced in this way. • Corrosion of automobile radiators can be prevented by using water with potassium added. • Coating the Surface with Other Material: (Coating) The most important method of protection from corrosion is coating. The material to be protected from corrosion is covered with a suitable coating material and protection from corrosion is provided. Cathodic Protection Today, many metallic structures such as pier piers, ships, water and oil storage reinforced concrete iron, etc., can only be operated cathodically and pipelines can only be operated safely by performing cathodic protection.

50. Cathodic protection is divided into external current source and galvanic anode: • Cathodic protection from external current, cathodic protection from external current is made by applying an external direct current to the metal. The direct current (-) end obtained from a transformer rectifier system is connected to the metal to be protected, and the (+) end is connected to an auxiliary anode. • Cathodic protection with galvanic anode creates a galvanic battery by connecting a metal (anode) with a more negative potential to the metal structure to be protected in its systems. Thus, the metal structure is turned into a cathode. Galvanic anodes dissolve on their own and produce current, just like a battery. The electrons released as a result of the dissolution of the anode are transported from the external junction to the cathode (protected metal structure), providing electrons required for the cathodic reaction.