ELECTROMOTIVE FORCE, TARNISH & CORROSION
ELECTROMOTIVE FORCE (EMF) also known as ELECTROMOTANCE, is a measure of the strength of a source of electrical energy, and it features in a number of areas of welding technology and fabrication. When using ELECTRIC ARC EQUIPMENT for example, the OPEN CURCUIT VOLTAGE (OCV) is the voltage available to initiate an arc when no CURRENT is flowing. This is an ELECTROMOTIVE FORCE, the source in this case being a welding generator. In more basic terms, a simple battery has a certain level of internal resistance available when the circuit it supplies is open (switched off); when the circuit is closed (switched on) and a current drawn, it leads to an internal drop in voltage. This drop in voltage is measurable and referred to known as POTENTIAL DIFFERENCE. [ref 1]
EMF & TARNISH
EMF is a feature of METALS, each of which has an ELECTRODE POTENTIAL VALUE (expressed in VOLTS). This value is either ELECTRODE POSITIVE or ELECTRODE NEGATIVE. The electrode potentials of various metals, which are collectively known as the EMF SERIES, are sorted according each metal’s decreasing tendency to enter an IONIC STATE when in SOLUTION. Metals that rapidly loose ELECTRONS in solution have an electrode positive potential, those with a lesser tendency to loose ions have an electrode negative potential. TIN and COPPER lie just either side of the neutral value of 0.00v at 25°C (HYDROGEN), tin has a positive potential in the series (+014v), copper a negative (-0.34v). LIGHT METALS such as ALUMINIUM (+1.33v), and ZINC (+0.76v) have high electrode positive potentials, NOBLE METALS such as GOLD (-1.36v) and PLATINUM (-0.86V), have high electrode negative potentials. [ref 2] This EMF data can be directly related to each metal’s tendency to TARNISH (DISCOLOUR) when exposed to the atmosphere. Metals with a higher electrode positive values (ALUMINIUM and IRON for example), will under normal circumstances develop a tarnish far more rapidly than metals with a high electrode negative value (GOLD and SILVER for example). The tendency for a metal to tarnish is an important consideration for artists, designers and architects alike. Metals and alloys are frequently selected for their ‘look’ as much as their structural properties, if tarnishing adversely affects the visual quality of an installed work over a period of time, the work may become less appealing unless regularly maintained.
EMF & CORROSION
In addition to being an indicator of TARNISH, a further consequence of each metal having it's own EMF value is that when two different metals are in contact with each other, the POTENTIAL DIFFERNECE between them causes ELECTRONS to flow from one metal to the other (functioning in much the same way as a simple battery, see above). This activity typically leads to CORROSION developing in one or both of the contacting metals. Known variously as ELECTROGALVANIC, GALVANIC or TWO METAL CORROSION; this corrosive activity can over time lead to a structural failing of critical joints or at least deterioration of the metal’s fabric. Where a NOBLE metal (with a high ELECTRODE NEGATIVE POTENTIAL), is in contact with a LIGHT METAL (with a high ELECTRODE POSITIVE POTENTIAL), for example, the noble metal becomes CATHODIC [-], and the light metal becomes ANODIC [+]. In these circumstances it is the anodic TERMINAL (eg the light metal), that suffers greater corrosion. [ref 3]
Of course this type of corrosion is not just restricted to extremes of LIGHT METALS and NOBLE METALS, alloys such as STEEL in contact with BRASS for example will also be exposed to this type of deterioration (in this case steel is ANODIC [+] and brass CATHODIC [-] with greater corrosion expected in the steel portion)
The EMF SERIES gives a good indication of any two metal’s likely behaviour in contact with each other, however a more specific series that relates to the particular environments that encourage ELECTROGALVANIC CORROSION is generally referred to when determining possible corrosive outcomes. This series, which is known as FREE CORROSION POTENTIALS (also expressed in VOLTS), is determined by testing metals and alloys in a saltwater and other specific environments (saltwater functions as an ELECTROLYTE that encourages corrosion when present in BI-METALLIC JOINTS).
If we return to the earlier example of STEEL (0.2%C) in contact with BRASS (70Cu:30Zn), mild steel has a FREE CORROSION POTENTIAL of -0.65v, admiralty brass a free corrosion potential of -0.30v. Admiralty BRASS being closer to NOBLE metal values than steel acts as a CATHODE. If Admiralty brass is in contact with STAINLESS STEEL (304 [0.05%C,18.40 Cr, 9.30 Ni]), with a free corrosion of -0.08v, the stainless steel acts as a cathode and the brass in this case functions as an ANODE and therefore becomes prone to corrosion. Metals and alloys with very close free corrosion values are unlikely to suffer from ELECTROGALVANIC CORROSION, for example the difference between grade 304 and 316 (0.02%C, 17.00 Cr, 11.90 Ni, 2.50 Mo), stainless steels is about 0.02v and therefore negligible, the difference between admiralty brass and copper is in the region of 0.04v and probably of little concern unless in a structurally critical joint.
INFO: A high profile example of electrogalvanic corrosion can be found in the 'Statue of Liberty' (Bartholdi/Eiffel 1886). Bi-metal corrosion was originally anticipated during construction and shellac impregnated asbestos was used as an insulating buffer between the iron sub-structure and copper cladding it supported. Due to deterioration in the water proof coating the asbestos absorbed salt water and the ensuing corrosion led to a severe deterioration in the fabric of the statue. This required extensive remedial work that was carried out in the mid 1980's [ref 4].