eBook T7 CORROSION

ROHANA BINTI SEMAAIL@ISMAIL SOFEA LING ABDULLAH JABATAN KEJURUTERAANMEKANIKAL POLITEKNIK KOTA BHARU

C O R R O S I O N PAGE I Copyright © 2023 – Politeknik Kota Bharu MATERIAL SCIENCE AND ENGINEERING CORROSION ROHANA BINTI SEMAAIL@ISMAIL SOFIA LING ABDULLAH POLITEKNIK KOTA BHARU

C O R R O S I O N PAGE I Copyright © 2023 – Politeknik Kota Bharu Jabatan Kejuruteraan Mekanikal Politeknik Kota Bharu KM 24, Kok Lanas, 16450 Ketereh, Kelantan. CORROSION First Edition 2023 © 2023 Rohana Binti Semaail@Ismail, Sofea Ling Abdullah All right reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronics, mechanical, photocopying, recording or otherwise without prior permission of the publisher. CORROSION / Rohana Binti Semaail@Ismail, Sofea Ling Abdullah

C O R R O S I O N PAGE II Copyright © 2023 – Politeknik Kota Bharu APPRECIATION Thanks to Allah SWT for the strength that has been given to us in preparing this book. I would also like to take the opportunity to express my gratitude to the Head of Mechanical Engineering Department, Tuan Haji Zuhairy Bin Zahari for the trust given in carrying out this task. A word of thanks also to Puan Ruzila Binti Mat Ghani, JKM's E-Learning Coordinator, who jointly made revisions and edits, as well as comrades-in-arms who contributed their thoughts and time directly and indirectly in strengthening the content of this book. Not forgetting the family and friends who have given a lot of support to make this e-book a success. CORROSION/Rohana Binti Semaail@Ismail, Sofea Ling Abdullah Jabatan Kejuruteraan Mekanikal Politeknik Kota Bharu KM 24 Kok Lanas 16450 Ketereh, Kelantan

C O R R O S I O N PAGE III Copyright © 2023 – Politeknik Kota Bharu AUTHOR BIOGRAPHIES ROHANA BINTI SEMAAIL@ISMAIL was born on July 1972 in small town, north Perak called Bagan Serai. She went to Sekolah Kebangsaan Matang Gerdu, Bagan Serai and then to Sekolah Rendah Sri Labis, Labis, Johor to receive her early education. She was then accepted as a student of Sekolah Raja Perempuan Ta’ayah, Ipoh, Perak to continue her studies at secondary level. She holds a Degree in Mechanical Engineering after completed her diploma in the same field from Universiti Teknologi Malaysia. She currently works as a lecturer of Mechanical Engineering Department at Politeknik Kota Bharu, Kelantan. She had experienced in teaching Material Science since 2012 and her interest includes Material Science & Engineering and Vehicle Dynamic. SOFEA LING BINTI ABDULLAH was born on May 1983 in small town called Pendang, Kedah. She received early education at Sekolah Kebangsaan Sungai Tiang, Pendang, Kedah. She was accepted as a student of Sekolah Menengah Kebangsaan Pendang, Kedah to continue her studies at secondary level. She holds a Degree in Mechanical Engineering from Universiti Teknologi Tun Hussein Onn Malaysia. She currently works as a lecturer of Mechanical Engineering Department at Politeknik Kota Bharu, Kelantan. She had experienced in teaching Material Science since 2014 and her interest includes Material Science & Engineering and AutoCAD Design.

C O R R O S I O N PAGE IV Copyright © 2023 – Politeknik Kota Bharu CORROSION This book was written for topic 7, Corrosion in subject Material Science and Engineering. Subtopics discussed include introduction to corrosion, types of corrosions, which are uniform attack, galvanize, pitting, crevice, intergranular and stress corrosions. Further discussion in this topic is the corrosion remedial which include, materials selection, product designs, coatings, cathodic protection, alloying and inhibitors. The titles found in this book refer to the course curriculum DJJ 30113-Material Science and Engineering, Polytechnic Malaysia. The content summary has been formulated by the lecturer who teaches the subject and translated as scientific writing. Therefore, this book is very suitable for students who are new to the field of Corrosion in Material Science and Engineering.

C O R R O S I O N PAGE V Copyright © 2023 – Politeknik Kota Bharu C O N T E N T S 1.0 INTRODUCTION .................................................................................................................. 8 1.1 What is corrosion? CLO-C1.................................................................................................8 1.2 Why the study of corrosion is very important?..............................................................10 2.0 CORROSION CLASSIFICATION.................................................................................... 11 2.1 Dry Corrosion.......................................................................................................................12 2.1.1 Passivation in Corrosion............................................................................................13 2.1.2 Exercises.....................................................................................................................14 2.2 Wet Corrosion......................................................................................................................16 2.2.1 Galvanic cell and Electrolytic cell.............................................................................17 2.2.2 The different between Galvanic Cell and Electrolytic Cell .................................17 2.2.3 Electrochemical Cell Parts ........................................................................................18 2.2.4 Electrochemical Series ..............................................................................................18 2.2.5 Exercises......................................................................................................................19 3.0 Types of Corrosions ............................................................................................................. 21 3.1 Uniform attack (Uniform Corrosion) ................................................................................22 3.2 Galvanic Corrosion ............................................................................................................23 3.3 Pitting Corrosion.................................................................................................................24 3.4 Crevice Corrosion ..............................................................................................................26 3.5 Intergranular Corrosion .....................................................................................................27 3.6 Stress Corrosion.................................................................................................................27 3.7 Exercises.............................................................................................................................28 4.0 Remedial Action ................................................................................................................... 31 4.1 Material Selection................................................................................................................32 4.2 Design...................................................................................................................................32

C O R R O S I O N PAGE V Copyright © 2023 – Politeknik Kota Bharu 4.3 Metal Coating.......................................................................................................................33 4.3.1 Noble Coating..............................................................................................................35 4.3.2 Non- Noble Coating....................................................................................................35 4.4 Non-Metal Coating ..............................................................................................................36 4.4.1 Porcelain enamel ........................................................................................................38 4.4.2 Wet Process Enameling .............................................................................................39 4.4.3 Thermoplastic Coating ................................................................................................40 4.4.4 3-Layer Polyethylene coating ...................................................................................41 4.5 Oxide layer ...........................................................................................................................42 4.5.1 Stainless Steel Passivation ........................................................................................43 4.5.2 Aluminum Passivation.................................................................................................44 4.6 Cathodic Protection.............................................................................................................45 4.7 Alloying .................................................................................................................................46 4.7.1 Alloying Processes .....................................................................................................46 4.7.2 Mixing raw metals by melting process. ...................................................................47 4.7.3 Mixing raw metals by compaction process .............................................................48 4.8 Inhibitor .................................................................................................................................49 4.9 Exercises ..............................................................................................................................50 5.0 REFERRENCES..................................................................................................................... 52

C O R R O S I O N PAGE 8 OF 54 Copyright © 2023 – Politeknik Kota Bharu 1.0 INTRODUCTION Referring to the Oxford dictionary, corrosion is a slow chemical reaction process between a material and its environment. This process will cause a change in the quality of the material in terms of its physical, mechanical, and chemical properties. According to the free encyclopedia, Wikipedia, corrosion is a natural process that transforms a metallic substance into a stable oxide. However, this oxide layer will slowly deteriorate until the material can no longer be used. In simple words, corrosion can be defined as a chemical reaction process that changes the state of a material from usable to unusable. The corrosion process that occurs on material is also known as the electrochemical oxidation process. This oxidation usually involves the reaction of metals and elements in the surrounding air such as oxygen, hydrogen, and nitrogen. An environment with high humidity contributes to the increased rate of corrosion on a material. Rusting is typical term used to describe a metal that has been corroded. But the term rust present especially in iron or steel and the color is red. Other terms used are patina for aluminum and verdigris in copper which usually in green color. 1.1 What is corrosion? CLO-C1 ● Corrosion is a process of reducing a metal from a usable state to an unusable state. ● Occurs when a metal is damaged, changes in appearance and material decreases after the occurrence of a chemical reaction. ● The picture below shows the changes of color on a set of lustrous, shining gear and after experience corrosion process, the gears covered of red oxide layer called rust. ● Metal that undergoes corrosion is said to have rusted as in the following example, Figure 1 and Figure 2.

C O R R O S I O N PAGE 9 OF 54 Copyright © 2023 – Politeknik Kota Bharu Figure 2: A coated of shoe saw machine changed from black paint color to red rustic color. Figure 1: A color of a set gear changed from shining and lustrous color to red rustic color.

C O R R O S I O N PAGE 10 OF 54 Copyright © 2023 – Politeknik Kota Bharu 1.2 Why the study of corrosion is very important? Metals and alloys naturally react easily with their environment to form protective bulwarks. But the chemical reaction that occurs tends to cause severe corrosion to the material if not controlled. This damage has a negative impact on production, health and safety and environment. From a study related to the maintenance cost of corrosion in the United States, it was found that the cost of corrosion increased from 276 million USD in 1998 to 1.1 trillion USD in 2016, which is 86% higher. From investigation conducted by NACE impact studies, 2017, the global cost of corrosion was estimated to be US$2.5 trillion, which is equivalent to 3.4% of the global GDP. These the data, obviously indicates the cost of global corrosion increasing gradually each year. Table 1 shows global corrosion costs in 2013 which Arab countries dominate with 5.0% GDP. Arab countries well known as the world's largest producer of oil. Thus, the maintenance cost of oil and gas pipelines as well as oil mines is indeed higher. Table 2 shows the cost of anti-corrosion method in year 2002, in China. From the survey, coatings became most used method to prevent damages due of corrosion which is 75.63% from all corrosion preventing cost. Table 1 – Global Corrosion Costs (2013)

C O R R O S I O N PAGE 11 OF 54 Copyright © 2023 – Politeknik Kota Bharu 2.0 CORROSION CLASSIFICATION Corrosion can be classified into two categories. The first one is dry corrosion and the other is wet corrosion. Severe corrosion usually occurs due to the present of air with higher moisture OR fluid. The elements in the higher moisture air like oxygen, hydrogen and nitrogen contribute corrosion acceleration rate. Figure 3 explains clearly both water and air contribute to corrosion. But the severe of the corrosion also induced by other factors such an element reactivity in the liquid which will Table 2- The cost of corrosion in the 2002 national survey in China (Uhlig method) Figure 3: Dry and Wet Corrosion

C O R R O S I O N PAGE 12 OF 54 Copyright © 2023 – Politeknik Kota Bharu discuss in next subtopic. Figure 4 below, shows offshore oil rig and drillship. Oil and gas industries facing high risk of both dry and wet corrosion. 2.1 Dry Corrosion ● Dry corrosion also known as a direct corrosion. Most metals undergo dry corrosion due to chemical reactions with oxygen gas in the atmosphere. ● A metal like iron, aluminum etc, difficult to corrode; it needs a present of both air and moisture to rust them. Figure 4: Types offshore oil rigs and drillship

C O R R O S I O N PAGE 13 OF 54 Copyright © 2023 – Politeknik Kota Bharu ● A reaction between metals and oxygen, a layer of oxide forms on its surface. If the layer remains on the surface without any maintenance, over time it will corrode all metals. But in certain metals, this oxide layer act as a protector to the metals. ● Figure 5 illustrates how the oxide layer thicken after exposing to environment for a long time. 2.1.1 Passivation in Corrosion The chemical reaction forms an oxide layer that protects the alloy and preventing corrosion from getting worse. This process is called PASSIVATION and the layer called PASSIVATION LAYER. Figure 5: Dry corrosion develop oxide layer. Figure 6: Chromium oxide as a passivation layer.

C O R R O S I O N PAGE 14 OF 54 Copyright © 2023 – Politeknik Kota Bharu In common case of ferrous metal like low carbon steel, corrosion can occur at any time if the surface directly contacting to humid environment. The present of air and water in atmosphere speeding the rate of ferrous metal to corrode. Chromium oxide layer in Figure 6, formed of elements Chromium and Oxygen. In steel passivation rarely occurs due to the nature of iron oxides which tend to damage the metal. But some steel alloys like stainless steels, Figure 7, create this passivation layer. Non-ferrous like aluminum, titanium, silver, copper, cadmium, magnesium, tin etc. have tendency to create this oxide layer when expose to environments. 2.1.2 Exercises Figure 7: Chromium oxide rebuilt when the indentation exposed to environment. Question 1 Draw a suitable diagram and explain what is dry corrosion? CLO1 – C3

C O R R O S I O N PAGE 15 OF 54 Copyright © 2023 – Politeknik Kota Bharu Question 2 From the diagram above, name the process and explain step by step how the process protect the metal alloy from corrosion. CLO1-C3 Answer: From the sketched diagram above, metal expose directly with its environment. The element in the air like oxygen or hydrogen react to the metal and form an oxide layer. This layer will be thickened over time and reduce the performance of the metal from useable to unusable.

C O R R O S I O N PAGE 16 OF 54 Copyright © 2023 – Politeknik Kota Bharu 2.2 Wet Corrosion ● In certain case wet corrosion also known as electrochemical corrosion. ● Usually, this corrosion occurs at normal temperature (room temperature - 24oC) in the presence of moisture or electrolytes. ● This corrosion involves the positioning of two dissimilar metals in an electrochemical series, which causes damage to the metal slowly but effectively. ● In this process the metals will turn into ions or chemical compounds and with the help of electrolytes, it becomes a solution that conducts an electric current that is positive and negative ions. ● This corrosion is due to the transfer of positive ions and negative ions between two metals of different potentials. Answer. The process involves called Passivation process. When metal alloy, stainless steel expose to air, chromium in the alloy, automatically combine with oxygen and form a passivation layer. This layer may damage because of scratch or indentation. To prevent from corrosion, again chromium around the pitting area react to oxygen in the air to build new passivation layer to protect the alloy from further corrosion damages.

C O R R O S I O N PAGE 17 OF 54 Copyright © 2023 – Politeknik Kota Bharu 2.2.1 Galvanic cell and Electrolytic cell Electrochemical corrosion can be shown as Electrolytic cell or Galvanic cell. To build the cells 3 components must be considered. There are: ● 2 electrodes with metallic potential difference in electrochemical series - anode and cathode ● Electrolyte solution (water / liquid salt and containers) ● Connecting wire (to connect anode and cathode) Figure 8: Galvanic and electrolytic cells 2.2.2 The different between Galvanic Cell and Electrolytic Cell GALVANIC CELL ELECTROLYTIC CELL Electrical energy Chemical energy Anode is -ve Anode +ve Cathode is +ve Cathode is –ve Spontaneous reaction occurs Non-Spontaneous reaction occurs Does not require external voltage source Require external voltage source Table 1: Galvanic cell versus electrolytic cell cells

C O R R O S I O N PAGE 18 OF 54 Copyright © 2023 – Politeknik Kota Bharu Table 1 shows the difference between galvanic and electrolytic cells. There are 5 points that could be highlighted. Galvanic cell producing electrical current moving from anode to cathode and voltmeter used to indicate the volt potential. Unlike electrolytic cell which produce chemical energy and the direction is opposite to galvanic cell. The chemical reaction in galvanic cell is spontaneous without a voltage source. But electrolytic cell needs energy source needed to ensure chemical reaction occur. 2.2.3 Electrochemical Cell Parts In electrochemical cells there are important features to know and their functions: ● Electrodes are rods, usually made of metal, which are connected by a conductive wire. There are two electrodes used in a cell. ● Anodes are the electrodes where oxidation occurs. The oxidation at the electrode can be represented by an oxidation half-reaction. ● Cathodes are the electrodes where reduction occurs. The reduction at the electrode is represented by a reduction half-reaction. ● Reducing Agent is the chemical component which donates electrons. Since, the reducing agent loses electrons, it is the oxidized species. ● Oxidizing Agent is the chemical component which accepts electrons. The oxidizing agent gains electrons and is reduced. ● Electrolyte solution is a conductive solution. The electrolyte solution allows electric current to travel between the two electrodes in a cell. 2.2.4 Electrochemical Series Electrochemical Corrosion can occur when 2 different metals in electrochemical series connect to each other with metal wire and dip in electrolyte liquid. The metal which anodic will corrode faster compared to less anodic metal. The potential of metals can be seen in Table 2. Using this series, the rate of corrosion can be predicted. For example, iron less anodic compared to zinc. Thus, zinc will corrode faster compared to iron.

C O R R O S I O N PAGE 19 OF 54 Copyright © 2023 – Politeknik Kota Bharu 2.2.5 Exercises Question 1 Sketch a suitable diagram and label it to show Electrolytic cell and galvanic cell. CLO1 – C3

C O R R O S I O N PAGE 20 OF 54 Copyright © 2023 – Politeknik Kota Bharu Answer: Answer: Electron Flow Anode (Oxidation) Zn → Zn 2+ + 2 eCathode (Reduction) Cu 2+ + 2 e- → Cu Salt Bridge Zinc Electrode Copper Electrode Electrolyte Solution Galvanic cell e - e - Electrolyte Solution Magnesiu m Electrode Chlorin Electrode Cathode (Reduction) Mg 2+ + 2 e- → Mg Electrolytic cell Anode (Oxidation) 2Cl → Cl2 + 2 eM + A - Electron Flow Question 2 Sketch a suitable diagram and explain electrochemical corrosion. CLO1 – C3

C O R R O S I O N PAGE 21 OF 54 Copyright © 2023 – Politeknik Kota Bharu 1. In this process, as sketched diagram, two metal plates of different potentials, namely zinc plate and copper plate, are placed in a container, containing electrolyte. The two plates are connected to each other with one wire. 2. The metal in the electrolyte liquid will be a positive ion (+ve/cation) and a negative ion (-ve/anion). 3. The metal that releases electrons will be positively charged (cation) which is zinc, and the metal that receives electrons will be negatively charged (anion) which is copper. 4. From the electrochemical series, it was found that zinc is more anodic than copper. Thus, the zinc plate will corrode, while the copper plate will not corrode. 3.0 TYPES OF CORROSIONS Corrosion is classified according to the state of the corroded metal. There are 6 types of corrosions, namely as in Figure 9, uniform attack, galvanic, pitting, crevice, intergranular and stress corrosions. Figure 9: Types of corrosions

C O R R O S I O N PAGE 22 OF 54 Copyright © 2023 – Politeknik Kota Bharu 3.1 Uniform attack (Uniform Corrosion) What is uniform corrosion? This is a uniform and general attack, in which the entire metal surface area, Figure 10, exposed to the corrosive environment is converted into its oxide form. It is the uniform thinning of a metal without any localized attack, corrosion does not penetrate very deep inside, and the most familiar example is the rusting of steel in air. Uniform corrosion is assumed to be most common form of corrosion and particularly responsible for most the materials loss. Traditionally, however it is not recognized as dangerous form of corrosion, because: 1. Prediction of thickness reduction rate can by means of simple tests. Corresponding corrosion allowance can be added, taking into an account strength requiring and lifetime. 2. Available protection methods are usually so efficient that the corrosion rate is reduced to an acceptable level. Actual methods are application of coatings, cathodic protection or possibly change of environment or material. Uniform corrosion adds color and appeal to whole surface. Two classics in this respect are the patina (green) created by naturally tarnishing copper roofs and the rust (copper red) hues produced on weathering steels. Figure 10: Uniform attack or uniform corrosion

C O R R O S I O N PAGE 23 OF 54 Copyright © 2023 – Politeknik Kota Bharu 3.2 Galvanic Corrosion What is galvanic corrosion? Galvanic corrosion or bimetallic corrosion is an oxidation and reduction processes. The corrosion occurs when two dissimilar metals come in contact in the presence of an electrolyte or salt solutions. A galvanic cell is created and the most active anode of the two metals corrodes to protect the least active cathode metal. In Figure 10, the carbon steel fastener, a less noble metal will corrode instead of the stainless steels screw fastener or in stainless steels assembly which more noble metal. Figure 11 shows copper plate and aluminum plate attached to each other by steel fastener. Aluminum corroding due to less noble compared to copper dan steel. List and explain 4 elements that needed to galvanic corrosion occurs. 1. Anode: A negative electrode where an oxidation occurs, or negative ions are discharged, and positive ions formed. 2. Cathode: A positive electrode where a reduction occurs, or positive ions are discharged, and negative ions formed. 3. Electrolyte: A fluid acts as a conductor which carries the current. 4. Return current path: The metallic pathway that connects the anode and cathode. Figure 10: Galvanic corrosion on carbon steel

C O R R O S I O N PAGE 24 OF 54 Copyright © 2023 – Politeknik Kota Bharu 3.3 Pitting Corrosion What is Pitting Corrosion? The effect of pitting corrosion can be seen as in Figure 12, where it usually attacks metals and alloys such as iron-carbon, aluminum, copper, and others. Pitting usually begins in a small area where the passive coating layer is physically damaged or chemically attacked. Damage such as fine cracks can collect liquids such as rainwater, so that the area becomes anodic or weak. Chemical reactions occur between metals and elements present in the liquid. This corrosion is considered dangerous because early damage is difficult to detect and is often ignored. The attack occurs rapidly, especially on surfaces directly exposed to a corrosive environment. If the maintenance lightly taken and not done immediately, it will cause severe destruction such as severe damage to the metal structure or building roof system. Figure 11: Galvanic corrosion on Aluminum plate. Figure 12: Pitting corrosion

C O R R O S I O N PAGE 25 OF 54 Copyright © 2023 – Politeknik Kota Bharu Explain how Pitting Corrosion occurs. From Figure 13, pitting corrosion begin with a small hold or cavity on metal like steel surface. The cavities are obscured by small amount of corrosion product on the surface. When a cathodic reaction in a large area sustains an anodic reaction area, the pit will form. Oxidation occurs in the metal even when there is no oxygen. High electron demand by the large cathode is put on the small anode, the result is intense pitting corrosion. It will be subtle and happen rapidly with a very harmful effect. Only a small spot of rust visible on the surface while damage happens deep in the metal structure below. What Causes Pitting Corrosion? Pitting corrosion occurs when the anode (exposed metal) is small, and the cathode (damaged coating) is large. The surface protection film or layer becomes the cathode when it is damaged and cracked. A small area of metal under the protective layer is then exposed and becomes the anodic. Pitting usually vigorous and absolutely severe when the solution on the metal surface contains hypochlorite, chloride or bromide ions. Other harmful solutions are those that contain fluorides and iodides. Sulphides and water are also known to enhance the pitting process. List down 7 the most common pitting corrosion causes. 1. Protective coating layer damage or crack. 2. The metal surface has scratches, dents, or small chips. 3. Metal experiences non-uniform stress. 4. There is damage or roughness to the metal substrate. 5. The metal is in a turbulent fluid flow. 6. Non-uniform protective coating layer. 7. Chemical attack on protective coating. Figure 13: Pitting corrosion process

C O R R O S I O N PAGE 26 OF 54 Copyright © 2023 – Politeknik Kota Bharu List down 8 typical metals or alloys prone to pitting corrosion. 1. Stainless steel 2. Chromium 3. Passive iron 4. Mercury 5. Cobalt 6. Aluminum 7. Copper 8. Associated alloys. 3.4 Crevice Corrosion What is crevice corrosion? Crevice corrosion, Figure 14 is a localized attack on a metal adjacent to the crevice between two joining surfaces (two metals or metal-nonmetal crevices). The corrosion is generally confined to one localized area to one metal. This type of corrosion can be initiated by concentration gradients (due to ions or oxygen). Accumulation of chlorides inside crevice will aggravate damage. Various factors influence crevice corrosion, such as. 1. Materials: alloy composition, metallographic structure. 2. Environmental conditions such as pH, oxygen concentration, halide concentrations, temperature. 3. Geometrical features of crevices, surface roughness. Metal to metal or metal to nonmetal type. Figure 14: Crevice Corrosion

C O R R O S I O N PAGE 27 OF 54 Copyright © 2023 – Politeknik Kota Bharu 3.5 Intergranular Corrosion ● This corrosion occurs along grain boundaries that are sensitive to corrosion. ● Generally, it occurs due to different potentials between atoms at one boundary with adjacent boundaries and will create anode and cathode boundaries. ● This corrosion usually starts on the metal surface and will degenerate quickly inwards due to the poor internal structures. For example, Figure 15, stainless steel that has gone through the welding process and forms a chromium carbide precipitate. These carbides do not dissolve but instead are at grain boundaries that have lost chromium elements. This causes the grain boundaries to be weak (anodic) compared to the grain. 3.6 Stress Corrosion Stress corrosion involves metals that are similar but have different stress regions. The parts with high stress values are anodic and the other parts are cathodic. When these goods are exposed to electrolyte fluids, then the high part of its stress will be eroded due to the difference of stress area. Figure 16, shows failures of pipe and stainless steels plate due to stresses and corrosive environment. It can be avoided through a good and suitable design that can avoid the presence of significant stress area differences. Figure 15: Intergranular Corrosion

C O R R O S I O N PAGE 28 OF 54 Copyright © 2023 – Politeknik Kota Bharu 3.7 Exercises Figure 16: (a) The pipe failed due to chloride-induced stress-corrosion cracking (CSCC). (b) Stress Corrosion Cracking of 316L Stainless Steel in H2 S due to tensile stress and corrosive environment. Question 1 From the given picture, a copper plate attached to aluminum plate with aluminum fastener and copper nut. (CLO1 C3) a. Label the picture correctly. b. Name the type of corrosion. c. Based your answer in a and b, explain your answer using your knowledge about electrochemical series.

C O R R O S I O N PAGE 29 OF 54 Copyright © 2023 – Politeknik Kota Bharu Answer: a. Label the picture correctly. b. Name the type of corrosion. Crevice corrosion Copper plate Aluminum plate Corrosion product Electrolyte Copper screw

C O R R O S I O N PAGE 30 OF 54 Copyright © 2023 – Politeknik Kota Bharu c. From the diagram given, the copper and aluminium plates are fastened together using copper screws. In the electrochemical series, it was found that aluminium is more anodic when compared to copper. In the presence of electrolyte, aluminium will corrode easily. Answer: Question 2 Corrosion is a reduction of metal from good to worst condition. List 6 types of corrosions and draw a picture to show each type of corrosion. (CLO1 C3) Types of corrosions 1. Uniform attack 2. Galvanic corrosion 3. Pitting corrosion 4. Crevice corrosion 5. Intergranular corrosion 6. Stress corrosion

C O R R O S I O N PAGE 31 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.0 REMEDIAL ACTION Metals are naturally easy to react with elements in their environment such as oxygen, nitrogen, and hydrogen. The reaction between metal and oxygen is known as the oxidation process. The result of the process will form an oxide layer that can generally be protective for the metal. However, the resulting oxides cause severe damage to the metal, making it unusable if not properly maintained. Therefore, to reduce the maintenance cost of corrosion problems, engineers are always looking for the best method to take when designing a product. In this subtopic, 8 methods, design, material selection, metal coating, non-metal coating, oxide layer, alloying and inhibitors, are used to reduce damages due to corrosion. MATERIAL SELECTION DESIGN METAL COATING NON-METAL COATING OXIDE LAYER CATHODIC PROTECTION ALLOYING INHIBITORS

C O R R O S I O N PAGE 32 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.1 Material Selection ● Is the earliest and easiest stage in preventing corrosion. ● High resistance to corrosion factors should be taken into accounts in selecting the materials. ● The position of the material in the electrochemical series should be taken into accounts. 4.2 Design Corrosion can be avoided by choosing an appropriate design. The design must take into account the following factors: ● Avoid using different materials to prevent galvanic corrosion. ● Avoid the presence of cracks. ● Sharp corners should be avoided (causing stress to occur). ● Replace the rivet connection with a welded connection.

C O R R O S I O N PAGE 33 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.3 Metal Coating ● Coating an item with a certain metal can also prevent the effects of corrosion. ● Coating is done through several processes such as metal spraying, dipping into hot metal, and electroplating plating as shown in Figure 17, 18 and 19 respectively. ● The types of plating materials that are often used are gold, silver, chromium, cadmium, tin and zinc. ● Metal coatings can be classified into, namely: 1. Noble Coating 2. Sacrificial Coating Figure 17: Metal spraying

C O R R O S I O N PAGE 34 OF 54 Copyright © 2023 – Politeknik Kota Bharu Figure 18: Metal dipping Figure 19: Metal plating

C O R R O S I O N PAGE 35 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.3.1 Noble Coating The coating is noble in nature to the base metal. The nobility of the metal can be checked in electrochemical series. The higher metal in the series, the less noble the metal. Thus, this type of coating will not protect the base metal if there are holes in the coating. Cu as in the Figure 20 more noble compared to Fe. The corrosion product could be seen on the metal surface. This is because the base metal, Fe more anodic. Figure 21 shows tire rim using Cr as a coating. Other examples of this type of coating are Cu, Ni, Al etc 4.3.2 Non- Noble Coating Non-noble coating also known as Sacrificial Coating, where the base metal is protected at the expense of the coating metal which acts as the anode. In electrochemical series, the metal became more anodic in ascending order. Zinc as in Figure 22, much more anodic compared to iron. Zinc naturally will sacrifice itself to protect iron from degrading. Figure 20: Copper noble coating. Figure 21: Powder coating for automotive parts

C O R R O S I O N PAGE 36 OF 54 Copyright © 2023 – Politeknik Kota Bharu This method still protects the base metal even if there are holes in it. An example of a coating of this method is the plating of a roof made of steel with a layer of zinc. As shown in Figure 23. 4.4 Non-Metal Coating Coating metals with non -metallic materials can also prevent corrosion. It acts to protect metal surfaces from contact with oxygen or provides basic protection with a coating of a stable material that is not easily penetrated by moisture. ● It consists of organic and inorganic materials. ● Examples of organic coatings are paints, tar, oil and varnishes. ● Paints are a good example of barrier protection. ● While the inorganic coating is enamel. ● Plastics and oils are among the two main non-metallic materials used as coatings. Figure 22: Zinc acted as non- noble coating to iron. Figure 23: Roofing steel coated with zinc.

C O R R O S I O N PAGE 37 OF 54 Copyright © 2023 – Politeknik Kota Bharu Figure 24: (a) Pipe with inorganic polymer coating. (b) Kitchen utensils with enamel coating. (c) Fluids tank with paint coating. (a) (c) (b)

C O R R O S I O N PAGE 38 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.4.1 Porcelain enamel Porcelain enamel is produces from natural, inorganic, and raw material as Figure 24(b). It is exceptional durable finishing, 100 % recyclable and inherently environment friendly product. A blend of minerals is melted in much the same manner as common window or container glass; this is the beginning of the process. Once all the ingredients are fully melted— at temperatures over 1093 degrees Celsius—the molten mixture is poured from a smelter and quenched between water-cooled rollers. This process is known as “fritting.” The quick-cooled ribbon of glass is then shattered into flakes, or frit. The chemical composition of the frit or glass is tailored to the end-product performance requirements and metal substrate used. Table 4 : Frit Compositional Ranges

C O R R O S I O N PAGE 39 OF 54 Copyright © 2023 – Politeknik Kota Bharu For the frit formulator, there are a few key things the enamel coating must accomplish. For a ground coat, the first layer of glass on the metal, the porcelain enamel glass must adhere to the metal (bond), match the thermal expansion of the metal substrate, and have a smooth, defect-free surface. For the second layer, the cover coat, the enamel must have the desired surface color and texture, and have the appropriate chemical durability for its intended use. In special cases, a single coat of enamel must accomplish all of these things, and in other cases, three enamel layers are required. Table 4 shows the common chemical compositional range of both ground coat and cover coat enamels, along with the basic function of the chemical constituents. 4.4.2 Wet Process Enameling Wet process enameling is to coat enamel slurry to metal body. There are mainly 3 ways for wet process enameling: dipping process in Figure 25, spraying process in Figure 26, and flow coating. Dipping is the most primitive and easy way in enamel coating. It’s fit for product in single color. If metal body is small and with simple shape, we just need to dip it in enamel slurry and take it out after it adsorb enough enamel slurry, then wipe off redundant enamel slurry. If metal body is complicated, we will need turn it in enamel slurry and swing, turn or hook it to wipe off redundant enamel slurry. To achieve a uniform enamel coating. Figure 25: Dipping enameling process.

C O R R O S I O N PAGE 40 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.4.3 Thermoplastic Coating This coating provides the substrate with a thick corrosion resistant and insulating film which can be given properties according to the application and project needs. Figure 26: Spray enameling process Figure 27: Polymer dip coating process

C O R R O S I O N PAGE 41 OF 54 Copyright © 2023 – Politeknik Kota Bharu Dip coating metal, Figure 27, is done in 6 phases which together result in a plastic coating layer. 1. Removing contaminants from the objects 2. Applying a primer and baking it in a curing oven 3. Dipping the objects into the fluidized bed; note that the object is still warm from the baking of the primer. 4. Moving the coated objects to a curing oven to bake the coating. 5. Cooling down the objects before handling them further 6. Inspecting the finished products, packaging and shipping them. The film thickness is dependent on the immersion time, temperature and the thermoplastic powder used. The higher the temperature and the longer the immersion, the thicker the film. 4.4.4 3-Layer Polyethylene coating Figure 28: 3-Layer PE system non-metal coating

C O R R O S I O N PAGE 42 OF 54 Copyright © 2023 – Politeknik Kota Bharu 3-Layer PE System, Figure 28, provides extremely effective defense against the hazards of transportation and installation, the stresses of temperature change in addition to water, organic acids, alkalis, bacteria, and galvanic action. The epoxy primer layer provides reaction sites capable of chemically bonding with the reactive group contained within the terpolymer adhesive layer. This chemical bonding between layers provides very high peel strength adhesion. The effectiveness of epoxy powder differs substantially when used as primer. Other than PE, PP (polypropylene) also used as non-metal coating for pip. Figure 29 shows pipe used PP as anti-corrosion coating. 4.5 Oxide layer Oxide layers such as zinc and aluminum oxide layers are among the corrosionresistant materials. This type of oxide layer is high in density and thus can prevent oxygen and water from corroding. Oxide coatings are often found on items that are usually made of zinc, aluminum, stainless steel and lead. Figure 29: 3LPP API %L Pipeline Coating API %L PSL2 X60 Polypropylene Anti-corrosion Coating

C O R R O S I O N PAGE 43 OF 54 Copyright © 2023 – Politeknik Kota Bharu Stainless steel tanks, Figure 30, are widely used in food, beverage, dairy, medicine, cosmetics, and other manufacturing processes where cleanliness and purity are important. These are also used in industrial plants for storing chemicals and gases where strong resistance from chemical degradation is required. 4.5.1 Stainless Steel Passivation Stainless steel comprises about 50 % iron, and anywhere between 10.5 % (12 percent is a typical minimal amount) and 30 % chromium, depending on the grade. The chromium oxide layer forms, Figure 31, on the stainless steels surface when chromium reacts with oxygen. This happens instantly, with formation speeds measured in nanoseconds and film thicknesses in microns. Why this occurrence become important in industry? Stainless steels most prominent stainless property is corrosion resistance, which results from its ability to form and regenerate a chromium oxide layer in the presence of oxygen. However, stainless steel does not provide corrosion resistance below the oxide layer. Which mean, once corrosion initiates, it progresses rapidly. Figure 30: Stainless steel tank

C O R R O S I O N PAGE 44 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.5.2 Aluminum Passivation Aluminum does corrode and form a patina on its surface. This oxide sometimes feels a powdery finish on aluminum and that is the patina. Now this patina also forms a protective layer, see Figure 32, that prevent further breakdown or corrosion. Compared to many other metals aluminum has good corrosion resistance. This is because aluminum develops a thin oxide layer on the surface when the metal contact with oxygen. The oxide layer protects the aluminum against corrosion and if it is damaged, it will immediately regenerate, provided there is oxygen present. If aluminum is stored in environments without major temperature fluctuations and not exposed to moisture, the oxide layer without further surface treatment will protect the metal against corrosion. Figure 31: Chromium oxide formation on stainless steel surface.

C O R R O S I O N PAGE 45 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.6 Cathodic Protection This protection means that the material that needs to be protected from corrosion is converted into a cathode due to the anode metal undergoing corrosion. Use Sub-marine, Figure 33, as an example. The hull is usually made of steel and the propeller is made of bronze. Steel is anodic while its propeller is cathodic, and both are in seawater which is an electrolyte solution. The hull will corrode due to its anodic properties, so to overcome this problem a material that is more anodic than bronze and steel is used as a corrosion resistor such as zinc metal. Zinc sheets are attached on the hull so that during the process of electrochemical corrosion occurs, only zinc is corroded because it is the most anodic. These zinc sheets need to be replaced from time to time due to wear due to corrosion. Figure 32: Superplastic alumina self-healing process

C O R R O S I O N PAGE 46 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.7 Alloying The purpose of alloying is to alter or to enhance certain metal properties to meet the industrial demands. Alloying steel with chromium, silicon, manganese, and vanadium will enhance mechanical properties such as strength, hardness, corrosion resistance, toughness, and wear resistance. This method also can change the appearance of the alloy, like stainless steel which is more shine and lustrous. Table 5 shows some alloying elements used in enhancing certain mechanical properties such as strength and corrosion resistance. 4.7.1 Alloying Processes Alloying is a process can be done in 2 ways, namely: 1. Mix the raw metals together and heat up until its melt. 2. Mix and blend the raw metals together, compact and sinter. Figure 33: Sub-marine

C O R R O S I O N PAGE 47 OF 54 Copyright © 2023 – Politeknik Kota Bharu ALLOYING ELEMENTS CHARACTERISTIC EFFECTS Aluminum Hardness Increased Strength Increased Ductility Decreased Beryllium Oxidation Decreased Calcium Oxidation Decreased Cerium Corrosion resistance Increased Yield strength Decreased Copper Strength Increased Ductility Decreased Manganese Corrosion resistance Increased Nickel Yield & Ultimate strength Increased Ductility & corrosion resistance Decreased Silicon Corrosion resistance Increased Silver Tensile strength at elevated temperature Increased Zinc Corrosion resistance Increased 4.7.2 Mixing raw metals by melting process. Table 5: Alloying elements characteristics and effects. Figure 34: Hot-rolled Zn-Mn-Fe alloy plate fabrication process.

C O R R O S I O N PAGE 48 OF 54 Copyright © 2023 – Politeknik Kota Bharu Fabrication of hot-rolled Zn-Mn-Fe alloy plates begin with inserting the raw metals of zinc, iron, and manganese together in a vacuum induction furnace as shown in Figure 34. The furnace heated up to 700°C and kept at the temperature in 10 minutes. The molten alloy then poured into high purity graphite cylinder and cooled in moderate rate. The product then through several heat treatment processes before rolled in hot rolling plan to reduce it thickness. 4.7.3 Mixing raw metals by compaction process This process well known as conventional powder metallurgy process, Figure 35. Combination of fixed compositions of metal powder mixed and blended together before compressed into desired shape. The product then sintered in certain temperature before finishing process. Figure 35: Conventional powder metallurgy process.

C O R R O S I O N PAGE 49 OF 54 Copyright © 2023 – Politeknik Kota Bharu 4.8 Inhibitor Inhibitors are chemical substances applied to environment (liquid or gas) in contact to the metal surface. The reaction between metal and the environment formed a thin layer protecting the metal from corrosion or reduce the rate of corrosion, Figure 36. Inhibitors can be categorized into 3 groups namely, 1. Anodic inhibitors. These inhibitors are usually crystalline salts such as nitrates, chromate, and molybdate. The reaction formed a protective oxide layer or passivation region, which help reduce material corrosion potential. However, only negative anions in this compound can act to reduce corrosion. 2. Cathodic inhibitor. These inhibitors applied to reduce the rate of corrosion by retarding cathode reaction. Insoluble compounds precipitation formed to decrease corrosion rate. Example of cathodic inhibitors are zinc phosphate, calcium carbonate, calcium phosphate and polyphosphates. 3. Organic inhibitor. This inhibitor formed a protective compound film that effect the entire of corroded metal surface if present in sufficient concentration. The effectiveness of inhibitors depended on their chemical compositions, molecular structures and surface contacted. Since the formation of film obtained from absorption process, the temperature and pressure in the system became important factors. Examples of organic inhibitors are amines, benzonate and phosphonates. Figure 36: Inhibitors coating