EXPERIMENTS CONCERNING CORROSION 205 



green from C to D. If the surface, A, of the chloride solution is 

 very near to the top B of the iron, a little of the green rust 

 oxidises and the metal presents a very pretty appearance — 

 brown, green, and polished respectively. 



2. The Influence of Partial Immersion 



Quite a different type of oxidation takes place when iron is 

 only partially immersed in water. The portion of the metal not 

 touched by the liquid may remain quite bright, whilst the 

 submerged portion becomes covered with brown rust. But at 

 the surface of the water, where the air can dissolve most rapidly, 

 the corrosion is most vigorous, a thick mass of green and brown 

 rust being quickly formed. Fig. 4 shows this extremely well, 

 the metal there figured having been removed after about forty- 

 eight hours of suspension in distilled water, and gently rubbed. 

 The upper portion retains its polish, whilst the lower is some- 

 what tarnished. At the water line the metal is seen to be 

 heavily attacked. 



3. Pitting 



By the term " pitting " we understand the localisation of 

 severe corrosion at definite points on the surface of the metal, 

 whereby little hollows or pits are eaten out of the iron. This is 

 undoubtedly the most serious form of corrosion, and a simple 

 example will make this clear. Suppose we have a tank of water 

 built of steel plates. In all probability these plates might safely 

 lose a few ounces in weight through uniform corrosion without 

 seriously affecting the strength of the tank. But a quarter of an 

 ounce lost through the formation of a single pit might be 

 sufficient to perforate a plate and make the tank leak. 



In the experiments already described there is, or should be, 

 no pitting with Kahlbaum's foil, although pieces of the usual 

 commercial metal treated in the above ways will sometimes pit 

 and sometimes not. 



Some very beautiful pitting effects may be obtained, however, 

 with pure iron foil in solutions of mineral salts rendered weakly 

 alkaline. Fig. 5 shows the result of immersing a piece of foil for 

 several days in a beaker (as in fig. 1) containing a dilute solution 

 of potassium chloride in about one-twentieth normal potassium 

 hydroxide solution. Here the pitting is very pronounced, and 

 usually follows some scratch or irregularity in the metal, the 



