Introduction

Properties of Electrical Materials accounts for approximately 4 to 6 questions on the Electrical FE exam. The NCEES outline covers the following topics for electrical materials: chemical, electrical, mechanical and thermal. Remember that these topics are focused from an electrical engineering perspective. So under the chemical properties, the test will focus on corrosion because an electrical engineer must understand this topic in order to design sacrificial anode or cathodic protection systems. The electrical subtopic will cover the material properties that lead to the common electrical concepts like resistance, inductance and capacitance. The mechanical and thermal topics discuss how materials used in electrical engineering last and react in various conditions like under heat or cold or stress and strain.

2.0 CHEMICAL

The information you need to complete the problems on chemical properties of electrical materials can be found in the NCEES FE Reference Handbook – Chemistry Section.

There are a few chemical terms that you should be familiar with to make it easier and quicker to complete these few problems.

Cathode is where reduction occurs, which means it gains electrons. By definition, the movement of electrons is the opposite of current flow, so current flows away from the cathode. The cathode has the positive potential. Corrosion does not occur at the positive potential (i.e. voltage).

Anode is where oxidation (also known as rusting) occurs, which means it loses electrons. Current flows to the anode, which means the anode has the negative potential. The metal with the most negative potential (voltage) will corrode.

One way to remember these terms is through the term sacrificial anode. Sacrificial anodes are used to promote oxidation (rusting) on this material only. It sacrifices itself, in order to protect nearby metals from rusting. Once you know that the anode is where oxidizing occurs, then you know that the cathode must be where reduction occurs. Oxidation or rusting deteriorates the metal, so the anode is where it loses material and electrons. Then, reduction must be where the gaining of electrons occurs. Lastly, current always flows opposite to the flow of electrons. If the anode is losing electrons then current is flowing to this material and away from the cathode. Thus the cathode has a positive voltage and the anode has a negative voltage.

NCEES FE Reference Handbook Discrepancy: The NCEES FE Reference Handbook has a table in the chemistry section called, Standard Oxidation Potentials for Corrosion Reactions. This table lists the potential (volts) for various materials undergoing corrosion reactions. The discrepancy which is shown in the note below indicates that the volt values are reversed. Please remember that those values should be reversed on the exam, meaning that the values should be negative when shown positive and should be negative when shown as positive. For example, iron (Fe) is shown with a potential of +0.440 V and water (H2O) has a potential of -1.229 V. These values should be reversed, Fe: -0.440 V and H2O: +1.229 V.

Another way to remember these terms is to know the common metals and which metals will corrode first. The most common metal used in engineering is Iron (Fe) [-0.44 V]. Copper (Cu) is stronger [+0.337 V], that is one of the reasons it is used for electrical wiring. Copper is mainly used for its electrical conductivity. Gold is much stronger at [+1.50 V]. Aluminum (Al) is weaker at [-1.662 V]

Corrosion is the deterioration of a material over time. Corrosion is accelerated when exposed to hostile environments or in contact with certain materials. Being able to understand which metals are more or less susceptible to corrosion is important for selecting materials.

The corrosion rate is determined by the thickness surface deterioration over time, typically measured in mils/year, where 1 mil = 0.0254 mm or one-thousandth of an inch (.001 in)

Corrosion rates are measured by finding the weight difference of a metal over time to calculate the material loss.

The equation above uses dimensional analysis to convert lbs of material lost per year to mils of thickness lost per year.

The most common types of corrosion occur when metals are exposed to salt laden or acidic air. Corrosion can also be accelerated when dissimilar metals are in contact with each other. This is known as galvanic corrosion. The farther apart the materials are on the galvanic series chart, the more likely the metals are to exchange electrons between each other, causing corrosion.

The galvanic series tables rate which metals are more likely to undergo corrosion. It is measured based on a metal’s electric potential compared to a reference material. A metal that is anodic (a metal that loses electrons, i.e. negative voltage potential) is more vulnerable to corrosion, while a cathodic material (the metal that receives electrons, i.e. positive voltage potential) is more stable and is better at resisting corrosion. For example, steel and cast iron will corrode faster than titanium and stainless steel. Furthermore, 316 stainless steel resists corrosion better than 304 stainless steel.

Figure 1: Galvanic Series in Salt Water Chart

Sometimes it is not feasible or cost effective to select a material that is resistant to the corrosive agents of your application. For example, a certain material strength is required and cannot be sacrificed by replacing one type of metal for another, or an inert metal is much too costly for your project. If this is the case, then you must pursue other options of corrosion protection. Some of these methods are shown in the next few paragraphs.

Paints and Coatings: For metals exposed to harsh environments, paints or coatings can be applied. Common coating types include epoxy, phenolic, zinc, and polyurethane. The specific types of coatings are beyond the scope of this book. The application and testing methods for coatings are governed by ASTM, the National Association of Corrosion Engineers Standards (NACE), and the Society for Protective Coatings (SSPC).

Isolation Kits/Dielectric Unions: Galvanic corrosion between dissimilar metals is prevented by the following methods: (1) using metals that are similar in electrode potential on the galvanic series chart, (2) coatings, and (3) dielectric unions or isolation kits.

Isolation kits use gaskets and sleeves to isolate one metal from the other and prevent the electrode transfer that causes the galvanic corrosion.

Figure 2: An isolation kit uses a non-conductive gasket to stop the electrochemical reaction.

Dielectric Unions act in a similar manner. A dielectric union is a fitting with two different metals on opposite ends and an internal material that isolates the dissimilar metals from each other.

Figure 3: A dielectric union separates two connected pipes of dissimilar metals with a non-conductive washer shown in blue. Normally an electrochemically reaction will occur from the more noble metal to the less noble metal with the water serving as the path connecting the two metals. The dielectric union stops this reaction by physically separating the metals.

Another cause of corrosion is due to oxidation. Oxidation is a chemical reaction where a molecule loses electrons to another molecule. For the purpose of the exam, the focus is on the oxidation between metal and air. The metal oxidizes (loses electrons and corrodes) in the presence of air (the oxidizing agent that receives the electrons). It will be difficult to test this topic numerically, but you should be familiar with the overall concept of oxidation and how oxidation chemically occurs.

Each element has a typical oxidation state that represent number electrons transferred. See the table in the Chemistry chapter of the NCEES FE Reference Handbook for standard oxidation states.

A metal (M) with two valence electrons (divalent) will likely use all of its electrons and be combined with ½ oxygen (O2) molecule to form a metal-oxide.

If the metal loses more electrons, then the same ratio will be used to complete the above reaction. For example, the following shows the reaction for aluminum.

Aluminum loses 3 electrons, thus the least common multiple is 6 electrons, which requires 2 aluminum molecules and 3 oxygen molecules. More examples of the oxidation reaction are shown in the table below.

The following figure shows how a metal oxide film forms on the surface of a metal with 2 electrons.

Figure 4: Oxidation occurs when electrons flow from the metal to the air. This causes a film to form on the top layer of the metal. This film is a metal-oxide.

One way to prevent oxidation is to provide cathodic protection. Cathodic protection uses a sacrificial anode that is more prone to oxidation than the metal it is protecting, i.e. it is more prone to corrosion. This allows the sacrificial metal to serve as the anode and the metal that is being protected will serve as the cathode. Sacrificial anodes have a more negative electrochemical potential. [Note: In the NCEES table, you must choose a metal that has a higher positive potential, but in practice you must select the metal that has a more negative potential, since all other tables show the potentials opposite from the NCEES FE Reference Handbook.]

The two metals are connected, typically by a wire, and ultimately the corrosion occurs at the sacrificial anode and not the protected metal. Typical sacrificial anode materials are zinc, aluminum, or magnesium.

Another method of cathodic protection is by galvanization. In this process, a metal is coated with zinc, typically by hot dip galvanizing, which submerges the metal into liquefied zinc. The protective coating then acts as the sacrificial anode to the metal beneath it. A common material is “galvanized steel,” this is steel that has gone through the galvanization process. Even if part of the metal is exposed, the zinc will still act as the anode and protect the adjacent metal. It is important to note that galvanization does not protect against acidic corrosion.

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3.0 ELECTRICAL

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4.0 MECHANICAL

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5.0 THERMAL

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6.0 Practice Problems

6.1 PRACTICE PROBLEM 1 – MATERIALS

6.2 PRACTICE PROBLEM 2 – MATERIALS

6.3 PRACTICE PROBLEM 3 - MATERIALS

Practice Problems 4 through 6 and all solutions included in the technical study guide, along with many more practice exam problems.