Galvanic coupling 6061 aluminium alloy

Table 7.3 shows an example of galvanic corrosion rates of aluminium alloys in 3.5% NaCl solution when coupled to different materials. For instance, it is seen that the contact with low-alloy steel gives considerably higher galvanic corrosion rates on aluminium than does contact with the - from a practical point of view - more noble stainless steels as well as Ni- and Ti-based alloys (regarding material descriptions, see Section 10.1). The table reflects the cathodic efficiency of the various materials coupled to aluminium (with the exception of cadmium, zinc and aluminium alloys) in the actual environment. 

Some of the CRP participants saw no corrosion of their aluminium alloy coupons, while others saw significant pitting. Pitting, crevice and galvanic corrosion were the main forms of corrosion observed. Crevice corrosion was not always accompanied by pitting of the aluminium surfaces within the crevice. Bimetallic corrosion of aluminium alloys coupled to stainless steel generally resulted in accelerated corrosion with pitting. 

Galvanic corrosion occurs when a metal or alloy is electrically coupled to another, usually dissimilar, metal in the same electrolyte. Severe localized attack is often seen when aluminium alloys are coupled with more noble metals. As this... 

The IPEN rack exposed aluminium alloys 1060,6061 and 6262, used in the fabrication of fuel assemblies for IPEN s lEA-Rl research reactor. The composition of these alloys is given in Table 4.4. Besides 80 mm diameter coupons of the three alloys, the rack included coupons of 1060 in the processed and scratched condition (to simulate the effect of scratches formed on fuel assemblies during handling in the reactor) and various combinations of galvanic and crevice couples. Table 4.5 lists the sequence of the coupons in the IPEN rack. 

The aluminium coupon surfaces gradually turned dull and this increased with immersion time. The outer surface of almost every aluminium alloy coupon was covered with a dark grey layer consisting of a relatively thin oxide film. This oxide film protects the coupon from further corrosion. The facing surfaces of all the crevice and galvanic couples appeared stained or showed white/grey spots that were loosely adhered. This could have been a kind of scale formed by the water. 

Two racks (racks 2 and 3) containing aluminium and stainless steel coupons were received during the second RCM in S3o Paulo. These coupons consisted of three pairs of large discs of the aluminium alloys 1100,6061 and 6063, and two pairs of small discs of the aluminium alloys 6061 and 6063, in galvanic contact with 316 type stainless steel. The large discs were used to form the crevice couples, while the smaller discs were used to form both crevice and galvanic couples. The chemical compositions of the coupons were assumed to... 

At a magnification of 100 x, etching along grain boundaries was observed (Fig. 11.6), indicating the onset of intercrystaUine corrosion. A white film was observed on the surface of aU aluminium alloy coupons (Fig. 11.7). A significant amount of this oxide formed on contact surfaces of crevice and galvanic couples. 

The first example is a ship. The propeller shaft is very often made of stainless steel, because for reasons related to mechanical resistance, it is impossible to manufacture it in aluminium alloys, except perhaps in 7075. The propeller is very often made in brass, and sometimes in cast aluminium, for small crafts. This has not been a hindrance for the construction of ships in aluminium, even very large ones, because it is possible to control galvanic coupling in the hull. 

This method is widely used in ship building for the neutralisation of galvanic coupling with a sacrificial anode, generally made in zinc or in a specific aluminium alloy (Figure B.5.4). In order for this protection to be effective, all metallic parts must be at the same potential and the anodes must not be painted they should aU be inspected regularly to check whether they are in perfect working order and to replace them as necessary. In seawater, such anodes are effective up to a distance of 10 m. 

Anodising aluminium does not prevent galvanic coupling. The use of fittings made of aluminium alloys with high mechanical characteristics (6060, 7075), anodised and sealed with dichromate, considerably simplifies bolted assemblies of aluminium components. 

On the other hand the localized corrosion and more specially the role of galvanic coupling is largely discussed for aluminium alloys. In these alloys, microstmctural corrosion e.g. pitting or intergranular corrosion (IGC) can be initiated at the interface between constiment irrtermetallic (IM) particles and the matrix. Most of the constituent particles contained in stractural alloys have a simple cathodic behavior towards the matrix and support reduc-tioa This reduction can be corrsidered, in a simple way, as a fotu-electron process ... 

Deshpande, K. B., Validated numerical modelling of galvanic corrosion for couples Magnesium alloy (AE44)-mild steel and AE44-aluminium alloy (AA6063) in brine solution. Corrosion Science, 52, 2010, 3514. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

An IPEN rack containing 1060, 6061 and 6262 alloys, used in fuel assembly manufacture, was also immersed in the basin. After 16 months of exposure, it was observed that some pitting had occurred on the uncoupled coupons, mostly on the top surfaces. The aluminium couples were stained inside the crevices but were not pitted. The stainless steel-aluminium galvanic coupons were much more severely corroded. Additional laboratory tests were conducted to determine whether increased levels of silver in the basin water could have increased the corrosion of the aluminium cladding in the IPEN basin. Results indicated no pitting but an increase in darkness of the surface oxide colour with the increase of silver concentration. 

---