Intergranular Corrosion

Intergranular corrosion

Intergranular corrosion is a form of attack in which corrosion proceeds preferentially along grain boundaries with the result that relatively little total corrosion can cause serious loss of strength. As a possible problem with brasses in service it is restricted to aluminium brass of abnormally high phosphorus content in service in sulphide-polluted water. Provided that the phosphorus impurity level is below 0.015% no trouble is experienced. No maximum for phosphorus is given in standards for aluminium brass but commercial material is normally well below the 0.015% level.

Pitting corrosion

Pitting corrosion, which some years ago was a rather common cause of failure of copper water pipes in some districts, is not a serious problem with brasses. Alpha brasses inhibited against dezincification can however suffer pitting under some circumstances. As in copper, the pitting produces very localised attack often in approximately hemispherical form beneath a small adherent mound of green corrosion product. If this mound is carefully removed, crystals of red cuprous oxide can usually be seen in the cavity. Service in slow-flowing sulphide-polluted sea water is most likely to produce pitting in aluminium brass but this alloy sometimes develops pitting corrosion in fresh water service. The number of examples of this that have been reported are too few to establish the range of water composition and service conditions that are necessary to cause it and it is therefore advisable to use admiralty brass instead of aluminium brass for all fresh waters.

Galvanic corrosion

When different metals or alloys are in contact with one another in an electrolyte (sea water, fresh water, rain, dew, condensation, etc.) they affect one another’s resistance to corrosion. Usually one - the more "noble" - will cause some degree of accelerated attack (galvanic corrosion) on the other and will itself receive a corresponding degree of protection. Figure 8 shows lists a number of common metals and alloys in their order of nobility in sea water and may be used to give some indication of the possible galvanic corrosion effects of coupling brasses to other metals. In general the further the other metal is from the brasses in the electrochemical series the greater the effect will be.

Fig 8: Galvanic series for common metals and alloys in sea water

The relative positions of the different metals indicated in Table 29 are different in a different environment or even under prolonged stagnant conditions in sea water, where the passive films on stainless steels could break down and sulphide films could form on the copper alloys. The series shown can however be taken as representing the majority of service conditions.

Among the brasses themselves there are small differences of electro-chemical potential, those of highest copper content being more noble. In particular, the alpha brasses are somewhat more noble than the beta brasses; this shows itself in the tendency for the beta phase in alpha-beta brasses to suffer preferential attack but the difference between the two is not great.

Relative area effects

In a water system with brass valves and fittings and copper or stainless steel pipes, the total area of the cathodic metal greatly exceeds that of the brass but, because of the limiting influence of the electrical resistance of the water, significant corrosion currents flow only between the brass and the copper or stainless steel very close to it. Consequently the effective areas of brass and copper or stainless steel are not very different and the extent of any galvanic action between them is small. A brass fitting in a copper or stainless steel tank, on the other hand, would come under the influence of a much larger area of cathodic metal and severe galvanic attack would be expected.

Similarly, naval brass tubeplates can be used with copper-nickel tubes in sea water cooled condensers because the effective cathodic area of the tubes does not extend more than a few tube diameters from the tube plate surface. If, however, the copper nickel tubes are replaced by titanium, which is much further from the brasses in the galvanic series, deep attack on the tubesheet will occur.

Beneficial effects

Just as coupling to a metal above it in the galvanic series will generally cause additional corrosion of brass, coupling to a metal below it can reduce attack. The prime example of this is the use of galvanic anodes for cathodic protection. A less obvious example is the successful use of high tensile brass spindles in cast iron valves. In gunmetal valves the galvanic action between an HTB spindle and the valve body causes accelerated dezincification of the spindle, but in a cast iron valve the galvanic action reduces the corrosion of the spindle and this combination is generally satisfactory in service.

Prevention by insulating or coating

It is possible, but often wrongly assumed to be easy, to prevent galvanic corrosion by electrically insulating the more noble and less noble metals from one another. The difficulty arises because the metallic connection (more accurately, the electronically-conducting connection) between the two members of the galvanic couple does not have to be by direct contact between them. The possibility of galvanically accelerated dezincification of an alpha-beta brass valve bolted to a flange on a large copper vessel is not eliminated, or even reduced, simply by fitting an insulating gasket between the two, since they will remain connected through the bolts. It is necessary also to fit insulating washers under the bolt heads and insulating bushes in the holes drilled in the flange of the valve. Even then there remains the possibility of the valve and vessel being in electronically-conducting connection with one another though the pipework and supporting steel work. Whenever steps are taken to insulate the two members of a potential galvanic couple from one another it is important to check, before they are brought in contact with the water or other electrolyte for which they are to be used, that the desired absence of electrical continuity between them has been achieved.

As an alternative to insulating the two members of a couple from one another, one or both of them can be isolated from the electrolyte by coating or painting. In some cases the anodic member needs to be painted or coated to protect it from corrosion that would take place even in the absence of the galvanic effect - for example ferrous water boxes of condensers with brass tubes and tubeplates. In principle, however, galvanic corrosion is more safely prevented by painting or coating the cathodic member. This follows from the relative area effect. If a coating applied to the anodic member is only 90% complete, the total amount of galvanic corrosion will remain the same but it will all be concentrated on the exposed 10%, i.e. the situation will actually have been made worse. If, on the other hand, a coating applied to the cathodic member is 90% complete the total amount of galvanic corrosion will be reduced by 90%.

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