In this portion will be covered the most common types of failures which occur with copper and its alloys. Most of the types of failures, however, may occur with other metals.

At the same time, the arrangement of the types of failure will indicate the manner in which the Research Department can identify the various types of corrosion that are encountered. Fortunately, the types of attack are usually typical enough so that they can be readily identified by the marks which they leave upon the metal. These characteristics, along with corrosion products which are usually left, serve as quite positive clues for identification of the trouble.

1.

Erosion-Corrosion (Impingement) —  This type of failure is most frequently encountered in condenser tubes, piping, or in such vessels as those where streams of liquid or gases emerge from an opening and hit upon the side wall, or within a tube where liquid turbulence occurs. The velocity or speed of the liquid which is required to cause failure of a given material will vary with composition of the metal and the composition of the liquid. The presence of entrained air bubbles, sand, paper pulp, or other solid particles will greatly accelerate the impingement failure, as well as contaminants of a chemical nature. In condenser tubes, obstructions such as mollusk shells or debris will cause local failure by producing a hole through the tube wall, long before the rest of the metal fails. The commonly accepted cause of failure by impingement is the inability of the metal to replace the thin film on the surface which is otherwise customarily present. There seems to be little or no relation between the hardness of the material and its ability to withstand erosion. We are not talking about resistance to wear which would be typical of pipe lines carrying large volumes of sand or other abrasives. The alloys most resistant to impingement attack are the Cupro-Nickels (706 and 715), which accounts for their widespread use as condenser tube material in warships and other marine installations where there is a combination of corrosion by salt water and erosion due to the velocity of that water. Other important alloys are Aluminum Bronze (608 and 614) and Aluminum Brass (687), which are intermediate between Admiralty Metal (443, 444, 445) and Cupro-Nickel in their ability to resist erosion. There is a growing tendency to use these alloys in modern power stations where salt water is used for cooling. Admiralty Metal, tonnage-wise, is most widely used since it offers a good resistance to many forms of water corrosion especially in the United States. In its inhibited form, the problem of dezincification is practically nil and is prevented by the use of arsenic, antimony or phosphorus in small amounts. Copper (arsenical) (142) may be used in inland waters and Deoxidized Copper (122) for many air conditioning and similar condensers using fresh water.

2.

Stress Corrosion —  In the ‘brass” business, stress corrosion is commonly called “season cracking.” The word season is defined here as “a period of time” and does not refer to any divisions of the calendar year. The requirements for season cracking to occur are three:

(a)	The alloy must be susceptible — In a practical manner, it has been long accepted that those alloys of copper and zinc with less than 20% zinc are generally immune from season cracking. This would mean that Gilding, 95% (210), Commercial Bronze, 90% (220), and Red Brass, 85% (230), commercially will seldom season crack. Neither will the Cupro-Nickels, Phosphor Bronzes (505, 510, 521, 524) and copper. There are exceptions to this which will be mentioned under corrosion—the second requirement.
(b)	Corrosion — Some type of corrosive attack must be in operation in order to cause stress corrosion cracking. Even though the alloy is a susceptible one and even though the third requirement of stress is present, season cracking will not occur unless there is corrosion. However, the mere presence of air (containing oxygen and C02) may be sufficient to provide this corrosion. Ammonia is a particularly active corrodent in regard to season cracking and will be discussed in a little more detail further on. Steam may attack Silicon and Aluminum Bronzes.
(c)	Stress — The third requirement is stress. Usually this stress is internal and within the metal, but there are also occasions when such stress can be induced by loads imposed upon this metal due to service conditions. It must also be a tensile stress, not a compressive one. Keeping these three requirements in mind, let us discuss further the factor of season cracking. Under certain conditions, any metal can be made to season crack. Industrially, however, there are few failures occurring with our metals rich in copper. The Silicon Bronzes with high silicon content, some Aluminum Bronzes, and High Zinc Brasses are generally susceptible to season cracking. With metals containing more than 15-2O% zinc, and with stress and a suitable corrodent present, season cracking can take place sooner or later. The actual time which may be required can vary from a few minutes to many years, depending upon the amount of stress and the corrosive attack of the surrounding medium. Metals other than copper and its alloys, are also subject to stress corrosion cracking, and this includes stainless steel, nickel alloys, magnesium, aluminum, lead, etc. The corroding compounds causing season cracking in these other materials may vary from those to which copper alloys are sensitive. The amount of stress required to cause cracking depends upon the corrodent and with the alloy in question. The most frequent cause of stress corrosion cracking is the presence of internal stress due to differential cold working between one part of the metal and another part. For failure to occur, it is important to note that this stress must be in tension and not in compression. While expert opinions differ, it seems pretty sure that most season cracking failures are due to ammonia in one form or another. It is quite probable that carbon dioxide, oxygen, and water vapor are also necessary for such action to take place. Ammonia can have many chemical forms, including organic compounds of the amine type, and certain plastics also have ammoniacal residues, so there are many potential places where ammonia can occur. There are other conditions also which may result in stress-corrosion cracking. It is obvious that protective coatings can prevent the action of the corrodent from taking place, or which will slow it down radically, will greatly extend the expected service life of any part exposed to season cracking influences. There are certain lacquers offering a dense and adherent coating that will prevent season cracking even when exposed to strong ammonia fumes. Decorative platings will also slow down or entirely eliminate season cracking. The obvious cure for season cracking is the use of some means to relieve the stresses which are inherently the cause. Either by the use of a relief anneal, that is, a temperature low enough to eliminate these stresses without softening, or a full anneal. Where the form is suitable, the use of proper straightening machines will reduce the stress below that level required for season cracking. Such protective measures are taken by Revere on those products whenever applications conducive to season cracking are known beforehand. An accepted test for season cracking is the mercurous nitrate test specified by A.S.T.M. It is important to note that there is no true correlation between the length of time required for cracking in this test and the expected service life. However, quick and sudden cracking in mercurous nitrate solutions is indicative of extreme susceptibility in service. The path of cracks due to mercury attack is invariably intergranular, as in many service failures. The presence of ammonia can induce transgranular cracking with the intergranular type.

3.	Corrosion Fatigue — Fatigue is sometimes erroneously known as “crystallization.” The reason for this erroneous term is the fact that most fatigue failures will show a bright crystalline type of fracture along with a very smooth section. Fatigue commonly occurs at stresses considerably below those otherwise required to break the material because a concentration of stress exists at some point due to either a notch, a corrosion pit, a sharp angle, or some other abrupt change in dimensions. The fatigue resistance of our materials is quite good, but in common with all other metals can be lowered by the presence of active corroding agents. Most typical examples of failure by fatigue will be in shaftings, condenser tubes, springs, bellows and similar places where corrosion is active and vibration can occur from one cause or another. Corrosion fatigue in condenser tubes may be caused by pump pulsations or by harmonic motion set up by high velocity steam being emitted from the turbine and striking the tubes. Identification of a corrosion fatigue failure under the microscope is usually by means of the crack which is typically across the crystals. Also,  there is found usually at the surface of the piece where the crack started, a corrosion pit which has served as a starting point.
4.	Dezincification — Dezincification is a word peculiar to the brass industry and denotes a type of attack which is most prevalent in the brasses containing more than 15-20% zinc. The Silicon Bronzes, the Cupro-Nickels, the Aluminum Bronzes, the Nickel Silvers, and the Phosphor Bronzes are not subject to dezincification. The Aluminum Bronzes, however, are subject to a similar type of attack which is referred to as “dealuminification”, but if the structure is all “alpha” it will not occur. Most frequently, dezincification takes place in acid solutions or others which are strongly conducting. The simplest explanation for the mechanism is that the copper-plus-zinc alloy is dissolved out of the metal and the copper is redeposited electrochemically. The zinc goes into solution and stays there or is precipitated as scale. Two common types of dezincification occur. One is the plug type wherein corrosion pits are filled with redeposited copper. The second is called the “layer type”. Here the entire metal surface is thinned to a more or less uniform depth and the copper redeposits over the entire surface creating a layer. The cure for dezincification is the use of inhibitors which are most effective in single phase alloys. In such duplex phase material as Naval Brass, Free Cutting Brass and Muntz Metal, the use of inhibitors does not entirely suppress dezincification but may retard it. Dealuminification can take place in Aluminum Bronzes, as mentioned beforehand, and again is due largely to the presence of acid or conducting solutions. The mechanism is much similar in that copper is redeposited upon the original base metal. Under certain special conditions Cupro-Nickel has been known to "denickleify” but it is not common, and does not occur in unpolluted salt water where much of the alloy is used.
5.	Pitting — Pitting is mentioned as a type of corrosion because it is unusually destructive, since one pinhole can frequently ruin a tube, pipe or vessel. Some corroding agents typically seem to cause pitting and our alloys are no more prone or immune to such action than any other of the commercial metals. For example, Monel Metal and Stainless Steel are subject to pitting under certain conditions. One of the worst actors with which copper alloys have to contend is carbon dioxide in natural waters. The type of attack is usually pitting and may be severe.
6.	Concentration Cells—  A concentration cell will leave a pit, and is in this respect a failure due to pitting. It is mentioned specifically, however, because concentration cells of one type or another are frequently encountered in service. A most common cause of pitting is due to a difference in oxygen concentration at a particular point. This can be caused by barnacles or other attachments to the metal which prevent oxygen from reaching the surface. A concentration cell is thus set up where a difference in potential exists, and corrosion can actively take place. As a matter of fact, most concentration cell action is quite rapid and causes early failure. Solutions containing mud or suspended solids may deposit coatings of them which will exclude oxygen. Gaskets, supports and other places which result in deficient oxygen will do the same thing. Alloys with their own oxide coatings as protective films are quite susceptible (aluminum alloys and stainless steel are examples).

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