L3 Materials Topic 4 Corrosion of metals

SUBJECT : MATERIALS LEVEL 3 TOPIC: 4 ITEMOHELENG CAMPUS DISTANCE LEARNING 2020-MAY LECTURER: KHOSI MOHAPELOA Downloaded from hlayiso.com

TOPIC 4 : CORROSION OF METALS LEARNING OUTCOMES: • CAUSE OF CORROSION • METALS TO REVERT BACK TO THEIR ORES • METHODS OF CORROSION PROTECTION TO STEEL • BASIC CORROSION CELL AND FOUR CRITERIA PREREQUISITE FOR CORROSION (I.E. ANODE, CATHODE, ELECTROLYTE AND ELECTRICAL CONNECTION) • CORROSION PREREQUISITES TO EFFECTIVELY COMBAT CORROSION • ADVANTAGES AND DISADVANTAGES OF CORROSION PROTECTION TO STEEL • OXIDATION POTENTIAL OF METALS IN TERMS OF THE “GALVANIC SERIES” • METALS CONSUMED BY CORROSION AND METALS PROTECTED AGAINST CORROSION Downloaded from hlayiso.com

TOPIC 4 : CORROSION OF METALS LEARNING OUTCOMES: • SEA SALT AND STRONG ACIDS ON THE RATE OF CORROSION ON STEEL. • EFFECT OF STRONG ALKALIS ON METALS LIKE ALUMINIUM AND ZINC • CONSTITUENTS OF COMMON CORROSION RESISTING STEEL ALLOYS, EXPLAIN HOW THESE ALLOYS RESIST CORROSION • USE OF ZINC IN CORROSION PREVENTION • HOT DIP GALVANIZING TO PROTECT STEEL AND METALS FROM CORROSION • CARBON STEEL EXPOSED DIFFERENT ENVIRONMENTAL CONDITIONS • TYPES OF COATING BONDS THAT APPLY TO DIFFERENT TYPES OF PROTECTIVE COATINGS • FORMATION AND PREVENTION OF WHITE RUST ON GALVANIZED MATERIALS Downloaded from hlayiso.com

CAUSES OF CORROSION Corrosion is a natural process that converts a refined metal into a more chemically-stable form such as oxide, hydroxide, or sulphide. It is the gradual destruction of materials usually a metal by chemical and or electrochemical reaction with their environment. Corrosion is an electrochemical reaction that appears in several forms, such as chemical corrosion and atmospheric corrosion, the latter of which is the most common form. When acidic substances (including water) come in contact with metals, such as iron and/or steel, rust begins to form. Rust is the result of corroding steel after the iron (fe) particles have been exposed to oxygen and moisture (e.G., Humidity, vapor, immersion). Downloaded from hlayiso.com

METALS TO REVERT BACK TO THEIR ORES Metals have a tendency after production and shaping to revert back to their lower energy, more natural state of ore The tendency is known as the Law of Entropy Ore: Is a rock or earth from which you can get metals As all material in the universe strives to return to its lowest energy state, pure metals also strive to revert to their lowest energy state which they had as Downloaded fromOne sulphides or oxides. hlayiso.com of the ways in which metals can revert to a low energy level is by corrosion. The products of corrosion of metals are often sulphides or oxides.

CORROSION PREVENTATIVE PROCEDURES Types of corrosion: Uniform Corrosion, Galvanic Corrosion, Crevice Corrosion, Pitting Corrosion, Intergranular Corrosion, Dealloying Corrosion, Erosion Corrosion, Stress-Corrosion Cracking, Corrosion Fatigue, Fretting Corrosion. Forms of corrosion protection include the use of inhibitors, surface treatments, coatings and sealants, cathodic protection, and anodic protection. CORROSION PROTECTION METHODS ARE AS FOLLOWS: Drainage design method Painting Hot dip Galvanising Inherent corrosion resistance Encapsulation and insulation Downloaded from hlayiso.com

BASIC CORROSION CELL AND LIST FOUR CRITERIA PREREQUISITE FOR CORROSION Cathode: Is the part of a battery that collects electrons Electrolyte: Is a material that allows electricity to pass through it Anode: Is the part of a battery from which electrons leave to create an Downloaded from hlayiso.com electrical current Electron: Is a very small piece of matter that has a negative charge

CORROSION PREREQUISITES TO EFFECTIVELY COMBAT CORROSION • Anodic regions tend to develop at locations where the metal is stressed or is protected from oxygen • Cathodic protection using an external voltage source is widely used to protect underground structures such as tanks, pipelines and piers. The source can be a sacrificial anode of zinc or aluminum, or a line-operated or photovoltaic power supply • Anodic protection involves passivation of the metal to be protected. A passive film forms on the surface of the metal with the application of an electrical current. Once this film is formed, it acts to protect the metal from dissolution,Downloaded and the film itself isfrom hlayiso.com nearly insoluble in the environment which it formed. Passivation causes metals to become very non-reactive and consequently very resistant to corrosion.

ADVANTAGES AND DISADVANTAGES OF CORROSION PROTECTION TO STEEL Method Advantages Disadvantages Drainage design A simple way of reducing If the metal sections are corrosion and using not designed properly, physical design joints and bolts will corrode and be difficult and expensive to fix Painting A method that can be You cant get into all of the checked and maintained cracks and gaps in a metal regularly structure to paint it, so areas you cant see may be corroding without your knowledge. Hot dip Galvanising Long lasting and effective The metal components need to go to a factory Inherent corrosion No work needed, the metal Corrosion is not prevented resistance simply does not corrode completely. very much. Encapsulation and The metal does not get If the coating is defective, insulation Downloaded from hlayiso.com exposed to the environment the metal underneath may in the first place. be corroding without your knowledge.

OXIDATION POTENTIAL OF METALS IN TERMS OF THE “GALVANIC SERIES” The galvanic series determines the nobility of metals and semi-metals. When two metals are submerged in an electrolyte, while also electrically connected by some external conductor, the less noble will experience galvanic corrosion. The rate of corrosion is determined by the electrolyte, the difference in nobility, and the relative areas of the anode and cathode exposed to the electrolyte. The difference can be measured as a difference in voltage potential: the less noble metal is the one with a lower (that is, more negative) electrode potential than the nobler one, and will function as the anode (electron or anion attractor) within the electrolyte device functioning as described above a galvanic cell. Galvanic reaction is the principle upon which batteries are based. Downloaded from hlayiso.com

OXIDATION POTENTIAL OF METALS IN TERMS OF THE “GALVANIC SERIES” The lesser the voltage means, it gives away electrons quickly meaning they corrode easily. The more the voltage means, it does not give Downloaded from hlayiso.com away electrons quickly meaning they corrode with difficulty.

METALS CONSUMED BY CORROSION AND METALS PROTECTED AGAINST CORROSION Metals share more electrons easily than others, meaning they corrode quickly Steel: Give away electrons quickly meaning they corrode easily Gold: Is a metal that does not give electrons away very easily, so it does not corrode quickly Copper: Also does not give electrons easily but, gold has more resistance capability than it. Iron: Also does not give electrons easily but, copper has more resistance capability than it. Zinc: Gives electrons easily, however, copper has more resistance capability than it. Aluminium: Gives electrons easily, however, zinc has more resistance capability than it. Magnesium: Gives electrons easily, however, aluminium has more resistance capability than it. Metals share more electrons easily than others, meaning they corrode quickly Steel: Give away electrons away quickly meaning they corrode easily Those that do not give electrons easily, helps protect those that give electrons Downloaded from easily from corrosion, hlayiso.com sometimes vice versa

SEA SALT AND STRONG ACIDS ON THE RATE OF CORROSION ON STEEL Parts-per-thoudsnd It was observed that the corrosion rate initially increases, reaches a maximum, and then decrease with increasing salt concentration. The total salt content (calinity) of seawater may show variation with geography, depth, and temperature. If these variations were appreciable, they would have considerable affect on the corrosion rate of metals. However, if the average salinity of seawater is considered to be about 34 ppt, and if the variations in salinity are in the range from 32 to 36 ppt then the effect on corrosion rate would be relatively small, perhaps a few percent. Weight of Downloaded Effect of from on the hlayiso.com Salt Concentration Corrosion Solvent per-% Rate of Steel in Aerated Solution at Room Temperature

SEA SALT AND STRONG ACIDS ON THE RATE OF CORROSION ON STEEL Acids can corrode many different kinds of metals or wear them away through chemical processes. Not all metals react with acids in the same way, however, and some metals are more vulnerable to corrosion than others. Some metals react violently with acids -- common examples being sodium and potassium -- while others, like gold, do not react with most acids In most cases of metallic corrosion, the cathodic hydrogen ion reduction controls the rate of metallic corrosion in acidic solution, while in neutral solution, the cathodic oxygen reduction preferentially controls the corrosion rate. Anodic metal dissolution depends not only on the electrode potential but also on the acidity and foreign solutes present in the aqueous solution. acidic solution, hydrated chloride ions accelerate the anodic iron dissolution in relatively concentrated chloride solution, while they inhibit the iron dissolution in relatively dilute chloride solution. Downloaded from hlayiso.com

EFFECT OF STRONG ALKALIS ON METALS LIKE ALUMINIUM AND ZINC Aluminum should in theory be even more reactive than iron; in practice, however, the surface of the aluminum is protected by a passivating layer of aluminum oxide, which acts like a thin blanket to shield the metal beneath. Acids that form a complex with aluminum ions can eat their way through the oxide coating, however, so concentrated hydrochloric acid can dissolve aluminum. Zinc is also very reactive and lacks the passivating layer found on aluminum, so it reduces hydrogen ions from acids like hydrochloric acid to form hydrogen gas. The reaction is much less violent than similar reactions for the alkali and alkaline earth metals It is a common way of creating small amounts of hydrogen for use in a lab. Downloaded from hlayiso.com

PERIODIC TABLE Downloaded from hlayiso.com

ZINC IN CORROSION PREVENTION Zinc serves as a sacrificial coating interestingly, another reason that zinc coating is so effective at stopping corrosion is due to zinc's own corrosive properties. Through a process known as galvanic corrosion, zinc will “defer” to the metal that it's protecting. Zinc is more electrochemically active than iron. Zinc is currently the fourth most widely consumed metal in the world after iron, aluminium, and copper. It has strong anticorrosive properties and bonds well with other metals. Consequently, about one-half of the zinc that is produced is used in zinc galvanizing, which is the process of adding thin layers of zinc to iron or steel to prevent rusting. The next leading use of zinc is as an alloy; the zinc is combined with copper (to form brass) and with other metals to form materials that are used in automobiles, electrical components, and household fixtures. Downloaded from hlayiso.com

COMMON CORROSION RESISTING STEEL ALLOYS Alloy is a metal made by combining two or more metallic elements, especially to give greater strength or resistance to corrosion; normally; an alloy of nickel, bronze, and zinc Alloys can produce; stainless steels, Pig Iron, Wrought Iron, Cast Iron, Carbon Steels, Ferrous Metal- Steel, Bronze and brass, Amalgams, Pewter Aluminium alloys, Copper alloys, Magnesium alloys, Titanium alloys, Refractory metals, Noble metals. They have a variety of protection advantages from; -Improves resistance to reducing acids, and to pitting and crevice corrosion in aqueous chloride containing environments. It contributes to increased high- temperature strength. – Improves resistance to oxidation at elevated temperatures and promotes age hardening. -Improves resistance to reducing acids (particularly non-aerated sulfuric and hydrofluoric) and to salts. -Improves resistance to high-temperature carburizing environments, reduces alloy Downloaded costs, and controls from hlayiso.com thermal expansion.

HOT DIP GALVANIZING TO PROTECT STEEL AND METALS FROM CORROSION The hot galvanizing process consists of three basic steps: surface preparation, galvanizing and inspection. Surface preparation is the most important step in the application of any coating. Any failures or inadequacies in surface preparation will immediately be apparent when the steel is withdrawn from the molten zinc because the unclean areas will remain uncoated and immediate corrective action must be taken. GALVANIZING -In this step, the material is completely immersed in a bath consisting of a minimum of 98% pure molten zinc. The bath temperature is maintained at about 840 F (449 C). Fabricated items are immersed in the bath until they reach bath temperature. The zinc metal then reacts with the iron on the steel surface to form a zinc/iron intermetallic alloy. INSPECTION - A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance. Products are galvanized according to long established, well- accepted, and approved standards of ASTM A123/A123M, AS/NZS Downloaded 4680, and BS EN ISO 1461. from hlayiso.com

HOT DIP GALVANIZING TO PROTECT STEEL AND METALS FROM CORROSION CORROSION RESISTANCE-Galvanized coatings protect steel in three ways:: 1. The zinc weathers at a very slow rate giving a long and predictable life. 2. The coating corrodes preferentially to provide sacrificial protection to small areas of steel exposed through drilling, cutting or accidental damage. 3. If the damaged area is larger, sacrificial protection prevents sideways creep which can undermine coatings. Downloaded from hlayiso.com

CARBON STEEL EXPOSED DIFFERENT TO ENVIRONMENTAL CONDITIONS High Carbon Steels These are strongest and hardest of carbon steels, these are heat treatable, and mostly used in hardened and tempered conditions. • Very high wear resistance, and capable of holding sharp edges. • Used for tool application such as knives, razors, hacksaw blades, etc. • With addition of alloying element like Cr, V, Mo, W which forms hard carbides by reacting • carbon present, wear resistance of high carbon steels can be improved considerably Metals combines with carbon to reduce susceptibility to Intergranular corrosion due to chromium carbide precipitation resulting from heat treatments, improves resistance to pitting and crevice corrosion, and increases high temperature strength. Cr= Chromium metal V= Vanadium metal Mo = Molybdenum metal Downloaded from hlayiso.com W= Tungsten metal

TYPES OF COATING BONDS THAT APPLY TO DIFFERENT TYPES OF PROTECTIVE COATINGS Anodizing Galvanizing Electroplating Powder Coatings Porcelain Enamel Coatings Downloaded from hlayiso.com

FORMATION AND PREVENTION OF WHITE RUST ON GALVANIZED MATERIALS White rust: Is the white corrosion that normally appears on aluminium when is oxidises. White rust forms very quickly, however once it is formed it helps to protect the metal. White rust forms when zinc is in an environment with hydrogen and oxygen. Most frequently, it occurs when a freshly galvanized material is placed into contact with water, or hydrogen dioxide. White rust is especially likely to form on newly galvanized material. Ways of preventing white rust; • Eliminate exposure to water. • Eliminate the formation of condensation. • Allowing the zinc to form stable oxides. • Using passivating chemicals or oils. Downloaded from hlayiso.com

HOME ACTIVITY  1. Tabulate advantages and disadvantages of corrosion protection methods (10)  2. Describe the word corrosion (2)  3. Draw a Galvanic series diagram that details metals’ voltage needed to share electrons (7)  4. List five steps of types of coating bonds that apply to different types of protective coatings (5)  5. Mention 4 ways to prevent white rust from occurring (4) Total: 28 Downloaded from hlayiso.com

NEXT TOPIC: TOPIC 5; SOLDERING, BRAZING AND WELDING Downloaded from hlayiso.com

THANK YOU THANKS YOU Downloaded from hlayiso.com