Basic Corrosion Theory Basic Chemistry and Basic Corrosion Theory

Matter, Elements, Atoms, and Ions Matter is anything that has mass and occupies space. All matter is made up of substances called elements, which have specific chemical and physical properties and cannot be broken down into other substances through ordinary chemical reactions. The structure of the atom An atom is the smallest unit of matter that retains all of the chemical properties of an element.

Electron, Proton, and Neutron Electrons are present outside the nucleus of an atom and carry negative charges. They also hold atoms together. The mass of an electron is considered to negligible. They are spread around the nucleus Proton Protons are present in the nucleus of an atom and carry positive charges. They are closely bound. The mass of a proton is approximately 1840 times as the mass of an electron. Neutron Neutrons are present in the nucleus of an atom and they are neutral. They are closely bound. The mass of a neutron is nearly equal to the mass of a proton

Electron, Proton, and Neutron

The Periodic Law The periodic law states that the properties of elements recur in a repeating pattern when arranged according to increasing atomic number.

Hydrogen on the Periodic Table Hydrogen occupies a special position on the periodic table. It is a gas with properties similar to nonmetals. It also reacts by losing one electron, similar to metals. We will place hydrogen in the middle of the periodic table to recognize its unique behavior.

Valence Electrons When an atom undergoes a chemical reaction, only the outermost electrons are involved. These electrons are of the highest energy and are furthest away from the nucleus. These are the valence electrons.

Ionic Charge Recall, that metals tend to lose electrons and nonmetals tend to gain electrons. The charge on an ion is related to the number of valence electrons on the atom. Group IA/1 metals lose their one valence electron to form 1+ ions. Na → Na+ + e- Metals lose their valence electrons to form ions.

Ions, Cations, and Anions  

Chemical Compound Chemical Compound: Any substance consisting of two or more different types of elements in a fixed proportion of its atoms

Ionic Charges

Basic Chemistry Corrosion is defined by NACE International as the deterioration of a material, usually a metal that results from a reaction with its environment. Understanding corrosion and cathodic protection requires a basic knowledge of chemistry and electrochemistry. Electrochemistry is a branch of chemistry dealing with chemical changes that accompany the passage of an electric current, or a process in which a chemical reaction that produces an electric current. Pertinent terms and descriptions are given in this chapter.

Acidity and Alkalinity (pH) When discussing an aqueous medium (including soil), the question often arises as to how acid or alkaline the solution is. This refers to whether there is an excess of hydrogen (H+) or hydroxyl (OH-) ions present. The strength of an acid is a measure of the hydrogen ion concentration in an aqueous solution and is classified according to the pH scale. The definition of pH is: pH = - log [H+]

Oxidation and Reduction Oxidation is the term applied to the loss of one or more electrons from an atom or molecule. which then forms a positively charged ion. An oxidation reaction occurs any time electrons are given up by an atom or molecule. The atom or molecule decreases in negative charge.

Oxidation and Reduction Reduction is the tenm applied to the gain of one or more electrons to an atom or molecule, which then forms a negatively charged ion or neutral element. A reduction reaction occurs any time that electrons are gained by an atom or molecule. The atom or molecule increases in negative charge.

Corrosion Cell Corrosion is an electrochemical process involving the flow of electrons and ions. Corrosion occurs at the anode, Protection occurs at the cathode. Electrochemical corrosion involves the transfer of electrons across metal/electrolyte interfaces. Any corrosion cell consists of four parts: Anode; Cathode; Electrolyte; Metallic Path.

Conventional Current Flow Sketch below shows the actual electrochemical current flow that exists in a corrosion cell. In corrosion and cathodic protection work, conventional current flow is used. This is a flow of current in the direction of the positive ion transfer (so called positive current).

Use of Voltmeters An analysis of meter polarity connection and sign displayed allows the determination of the direction of conventional current flow. When measuring voltage across a circuit, the voltmeter is connected in parallel across the element. For example, the voltmeter in sketch below is connected in parallel to Resistor R2 of the external circuit.

Use of Voltmeters The measurements commonly made in cathodic protection surveys are: Structure-to-electrolyte potential Driving voltage of a galvanic anode system Rectifier voltage output Voltage drop across a pipe span Voltage across a current shunt

Polarity Sign Most digital meters will display a negative sign for a negative reading and no sign for a positive reading. When a voltmeter is connected across a metallic element, the voltage display is positive when the positive terminal of the voltmeter is upstream of the current flow as illustrated in sketch below. When measuring the voltage difference of dissimilar metals, the sign is positive when the positive terminal of the voltmeter is connected to the more noble metal

Polarity Sign

Sign Convention When the positive terminal of voltmeter is connected to the more noble metal and the negative terminal to the more active metal, then the reading is positive. Current flow from the active to the noble metal through the electrolyte and from the noble to the active metal through the metallic path. Structure-to-electrolyte readings are considered negative to the reference electrode. When the reference electrode is connected to the negative terminal, the voltmeter produces a negative reading.

Reference Electrodes (Half-Cells) Reference electrodes, or half-cells, are important devices that permit measuring the potential of a metal surface exposed to an electrolyte. An example is a structure-to-soil potential measurement. There are several potential benchmarks in common use, but all of them are related to a basic standard. The standard hydrogen electrode half-cell is awkward to use in most circumstances in which potential measurements are to be made. Other combinations of metal electrodes in solution with a specific concentration of ions are used. The reference cell must be stable and capable of producing reproducible data.

Copper-Copper Sulfate Electrode Copper sulfate reference electrodes (CSE) are the most commonly used reference electrode for measuring potentials of underground structures and also for those exposed to fresh water.