Now, shipbuilding quality mild steel is a type of carbon steel containing a very low amount of carbon and the amount of carbon typically found in such metal is only 0.23% by weight. Steel is not an iron alloy and, further, does not contain large amounts of other elements such as chromium, molybdenum or manganese, although all of those can be found as trace elements together with small amounts of sulphur and phosphorus. Since its carbon and alloying element content are relatively low, there are several properties it has that differentiate it from the higher carbon and alloy steels. Less carbon means that mild steel is typically more ductile, machinable and weldable than high carbon and other steels, but that also means it is nearly impossible to harden and strengthen through heating and quenching. The low carbon content also means that the carbon and other alloying elements do not block dislocations in its crystal structure, generally resulting in a lower tensile strength than the high carbon and alloy steels. Mild steel also has a high amount of iron and ferrite, making it magnetic. The carbon is usually found in carbide crystals of pearlite and cementite. Both those crystals are cathodic to those of the ferrite, so that when unprotected steel is placed in an electrolyte such as canal, river or sea water, a galvanic cell is set up between them causing the ferrite to dissolve forming deep pits over the steel surface. In order to prevent the resulting damage, the steel has to be properly coated with a protective paint and fitted with a suitable type and number of anodes. The strength of that galvanic current is, inter alia, a function of the conductivity of the water and that, in turn, is a function of the water’s salinity or impurity. The particles that make up that current also possess mass and are, therefore, controlled not only by Ohm’s law but also by Newton’s laws of motion and a properly designed (not guessed) cathodic protection scheme has to take that fact into account.
Ohm’s law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit and is named after German physicist Georg Ohm (1789- 1854). It addresses the key quantities at work in simple electric circuits.
Symbolically, it states that: V
= where I
R V
The formula is
usually demonstrated graphically as a triangle as in Figure 1.
Volts Amps Ohms
Figure 1 - The Ohm’s Law Triangle
R where
A L
R ρ
= = = =
the area of the electrical path The length of that path the resistance
the specific resistance
Effectively that means that the area over which the anode provides cover is limited and that fact decides the distance over which an anode will work and, therefore, the distance apart that anodes must be fitted.
Sir Isaac Newton (25th 1642 – 20th
December March,1726) who is
widely recognised as one of the greatest of mathematicians and most influential of scientists, first published his three laws of motion in his book Philosophiae Naturalis Principia Mathematica in 1687. The first of those laws states that if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by another force. The postulate is also known as the law of inertia and was first formulated by Galileo for horizontal motion on Earth and later generalized by Descartes. Before Galileo, it had been thought that all horizontal motion required a direct cause, but Galileo deduced from his experiments that a body in motion would remain in motion unless a force (such as friction) caused it to come to rest.
• a moving object continues to move at the same velocity and in the same direction.
Those two laws have a number of clear implications for the effectiveness of an anode.
Ohm’s law means that as the only force acting on one of the particles making up the current is the algebraic sum of the potential difference between the anode, the cathode and the resistance offered by the electrolyte (the water in which the vessel floats), there is a definite limit to the area over which the anode can be effective. It must also be clearly understood by the marine surveyor that that effective area over which the anode works, is a function of the wetted surface area of the anode
The Report • June 2021 • Issue 96 | 105
mm2 m
ohms ohms/mm.m
According to that law, an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, then:
• a stationary object remains stationary,
= ρL/A
If two of those three factors are known, then the third is easily calculated.
The resistance in the circuit is governed by the equation: -
ohms (2)
= = =
The current in the circuit
The resistance encountered by the current ohms The potential difference in the circuit
amps volts
I x R
(1)
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136
