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| CAUSES OF CORROSION |
| Corrosion is a complex process but which can, for our purposes, be broken down into three basic conditions: |
1. Atmospheric
2. Water Solutions
3. Elevated Temperatures
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These will be discussed in more detail in the following:
| 1. |
In the atmosphere - of which most important factors are oxygen and water, plus carbon dioxide
for non-polluted air. In industrial or city environments there may be sulfur dioxide and other sulfur compounds; nitrogen oxides and ammonia to name a few. As a practical matter, copper and its alloys do not corrode in most atmospheres. Copper forms a greenish coating after a while, called “Patina”, considered architecturally desirable. Other colors of browns and blacks may form if sulfur is present. There are many copper roofs still in service after several hundred years. Previous service life of copper metals is often the best way of deciding what to use.
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| 2. |
Corrosion in Water Solutions
- This is by far the most common form of corrosion and usually occurs in one of the following ways: |
(a) Galvanic Action
(b) Replacement of one metal by another or by one metal and hydrogen
(c) By an oxidation process
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To further explain:
(a) Galvanic action occurs only in a conducting solution when two dissimilar metals are placed in it. A conducting solution usually means an acid, alkali or a compound of either is present. (Distilled or de-ionized water is not conducting). This allows an electromotive force to be established so that one metal is corroded. In Table I many common metals and alloys are listed. Materials close to each other will not corrode the other, such as zinc and aluminum alloys or copper alloys and nickel alloys. On the other hand, steel and copper results in corrosion of the steel. Of the two metals involved the one nearer the top of the list will be corroded. As an example, an iron coupling used to attach two lengths of copper pipe together will eventually result in the coupling corroding thru if the water is at all corrosive, such as one containing carbon dioxide.
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(b) Chemical Replacement. When chemical compounds are dissolved in water, a phenomenon takes place called dissociation, wherein the atoms of one part of the compound will assume positive valence and the other portion will assume negative valence. As an example, when copper sulphate is dissolved in water, the copper has a valence of plus two, that is there are two positive charges, and the sulphate, which is composed of sulphur and oxygen, acts as an entity and has two minus charges. It is chemically written as Copper (Cu+ +) and Sulphate (SO4 - -) These ions, with their positive and negative charges, explain many of the reactions which take place in water solutions. The mere fact that ionization takes place does not necessarily mean, however, that a chemical reaction will occur, because there are many factors which will affect the reaction.
Similarly, when acids are made into water solutions, dissociation takes place and in the case of sulphuric acid there are two hydrogen portions, each carrying a positive charge, and the sulphate again acts as an entity with two minus charges. This is written 2H+ and S04 - -. The degree to which this ionization takes place determines, in a great measure, how strong the corrosive attack will be. Relatively weak acids which, however, have a strong tendency to dissociate, can cause rapid corrosion.
To determine some of the relative tendencies for metals to be corroded by acids which dissociate, Table II is useful, which is much similar in character to Table I, but provides additional information.
The metallic elements are listed by name and by their chemical symbol and also have a figure following each one of them which is in most cases a minus, but in some is plus. It will be noted that all of them are in reference to hydrogen, which is rated as zero. The figures represent the electrochemical voltage, which is established when compared to hydrogen in normal solutions.
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TABLE II
ELECTROMOTIVE SERIES
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The following are the approximate single potentials of metals toward solutions with normal metal ion concentration, based on the normal hydrogen electrode as zero:
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NAME |
SYMBOL |
POTENTIAL |
| Lithium |
Li |
-2.96 |
| Potassium |
K |
-2.92 |
| Barium |
Ba |
-2.80 |
| Sodium |
Na |
-2.71 |
| Strontium |
Sr |
-2.70 |
| Calcium |
Ca |
-2.50 |
| Magnesium |
Mg |
- 1.55 |
| Manganese |
Mn |
-1.00 |
| Zinc |
Zn |
-0.76 |
| Chromium |
Cr + + |
-0.60 |
| Chromium |
Cr + + + |
-0.50 |
| Iron |
Fe |
-0.44 |
| Cadmium |
Cd |
-0.40 |
| Cobalt |
Co |
-0.29 |
| Nickel |
Ni |
-0.22 |
| Tin |
Sn |
-0.14 |
| Lead |
Pb |
-0.12 |
| Iron |
Fe + + + |
-0.05 |
| Hydrogen |
H |
0.00 |
| Antimony |
Sb |
+0.10 |
| Bismuth |
Bi |
+0.20 |
| Arsenic |
As |
+0.30 |
| Copper |
Cu |
+0.35 |
| Mercury |
Hg |
+0.80 |
| Silver |
Ag |
+0.80 |
| Gold |
Au |
+1.40 |
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For the conditions under which the above table was determined, all metals above hydrogen are capable of displacing hydrogen from solutions containing hydrogen ion - i.e., they are soluble in acids.
Those below hydrogen are (usually) unable to displace hydrogen directly.
Those with minus signs are attacked by air-free acids. The hydrogen is evolved as a gas. As an
example, with iron and sulphuric acid a reaction takes place so that hydrogen is evolved and iron sulphate is formed.
It will be noted that copper is below hydrogen in this table. Copper will not evolve hydrogen when immersed in acids except at high concentration.
Table II also shows why gold and silver are not readily attacked by certain acids, that is, they are essentially noble”.
Complicating these statements above is the fact that certain acids are of so-called oxidizing character; that is to say, they act as if oxygen were present. This may completely change the action which might otherwise be expected from the position of the various metals in the electromotive series. |
| (c) Oxidation. Oxidation, chemically speaking, is the raising of the valence. If this power exists, then oxidation takes place even though there may not be any oxygen itself involved. Oxygen can also raise valence. Other substances such as sulphur or chlorine or sodium bichromate, and many others, have this same power to raise valence. |
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As we have said, valence is the number of charges carried on the ions when in solution and changing this number of positive charges, to let us say from one to two, or from two to three, would be an oxidation process.
There are some acids which not only will dissociate to form hydrogen ions, but also act as oxidizers at the same time, such as nitric acid. Another example is phosphoric acid which contains an appreciable percentage of iron. These compounds of iron, which are in such a state of valence as to be called ferric as contrasted with ferrous, are highly oxidizing in character. The combination of active acids in oxidizing agents is a very destructive one for many commercial metals.
Some other materials which are oxidizing in character and, therefore, corrosive to copper and its alloys are nitrates, chromates, chlorates, permanganates, and cupric (copper) and stannic (tin) compounds.
Oxidation is most evident in the case of heating metals for hot rolling sheer, strip or plate; hot piercing of tubes; hot extrusion of tube and rod. In this case oxygen in the air reacts directly to form copper oxide on electrolytic tough-pitch copper or slight variations thereof while brasses, aluminum bronzes and copper-nickel alloys will also have some oxides of the added elements used to make the alloy.
3. High Temperature Corrosion — We mean higher than room temperature, for the most part, when we speak of this form of corrosion. In modern power plants, steam may be as hot as 1050°bf, which requires very special alloys to handle—and very special treatments to exclude various corrosive substances, including oxygen.
There are very definite limits to the temperatures to which copper metals may be exposed, partly because they lose strength and partly because of oxide formation which may waste away the base metal and hence reduce the thickness or section below a safe strength level.
Generally speaking 800°F. is maximum for hi-strength alloys and 400°F. for electrolytic copper.
Other than oxygen there are various gases which attack copper metals at elevated temperatures, such as those containing sulfur, chlorine and the like. Scales or films may be formed which are loose and not adherent, and hence not protective to the metal.
In addition to these, molten salts may be corrosive as well as molten metals. Liquid tin, soft solders, and some silver solders are corrosive when in contact for a certain length of time because they form new metal alloys, which may not be strong and ductile, but rather weak and brittle.
Liquid mercury is a corrosive metal at room temperature. |
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