The Forms of Corrosion

Note Some reference citations in this chapter refer to Handbooks containing several individual references. These are indicated in the text as (ASTM G 52004)4, for example, and the addition of R04 in the reference list stands for revised 2004. 

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Corrosion may take various forms and may combine other forms of damage (erosion, wear, fatigue, etc.) to cause equipment failure. The forms of corrosion most encountered in drilling equipment are uniform corrosion and galvanic corrosion. 

The Forms of Corrosion Recognition and Prevention, Corrosion Handbook No. 1, NACE, Houston, Texas (1982). 

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

Where particulate matter (in the form of corrosion products of iron oxide) is present in returning condensate, it often contains copper, nickel, and zinc oxides as well. This debris can initiate foaming (through steam bubble nucleation mechanisms) leading to carryover. It certainly contributes to boiler surface deposits, and the Cu usually also leads to copper-induced corrosion of steel. 

Reactions that involve the transfer of electrons are immensely beneficial in a vast range of chemical processes, from the operation of a battery to the transmission of nerve impulses. However, in the form of corrosion, electron-transfer reactions also can be highly destructive. The rasting of iron objects is a prominent example of corrosion. 

Owing to the tremendous economic damage it can cause, corrosion has and continues to be the subject of extensive study especially with a view to its minimization at acceptable expense—economic and environmental. We attempt to give an idea of the forms of corrosion, how to investigate it by electrochemistry, and how it can be minimized, or at least reduced and controlled. As will be seen, given the complexity of corrosion processes, the mechanism of which can alter significantly depending on the local environment, the more specialized literature should be consulted for details on specific cases, for example Refs. 1-6. 

Electrochemical noise can be characterized by some common statistical parameters including the mean, the variance, and the standard deviation. In particular, the standard deviation, o, is used as a measure of the amplitude of the variation in the noise signal. Skew and kurtosis sometimes give indications of the form of corrosion occurring (140). For unfiltered digitized noise data in a time record, the noise resistance, Rn, is... 

Depending upon the environment to which the copper and its alloys are exposed, various forms of corrosive attack occur. The environments of interest are (i) atmospheric (ii) fresh water and seawater (iii) soil and (iv) chemical solutions, including acids and bases. The forms of corrosion of copper and its alloys in different environments... 

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