In this case the of copper to aluminium fitting or joining - big copper area and small aluminium area should be bad as there is a large galvanic potential difference between copper and aluminium ( approx. - 0.50 Volts , refer our galvanic potential table /chart sent in our earlier mail ) . The more is the voltage difference between two metals in the chart , more is the chance of higher galvanic corrosion in anodic side Aluminium