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Marcus Thielen Discusses LED Corrosion

Winter, electrolytic effects and humidity play roles

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This month, I’ll deal with corrosion – especially the electrolytic effects in LED signs, because humidity is one of the worst wintertime enemies of electric signage. Corrosion, from the Latin corrodere, which means “eating away,” describes how some chemical agents (like acids) can change an object’s constitution and finally dissolve it. I’ll confine myself here to metal corrosion, because it’s the most prevalent type of corrosion in signs.
Rusting iron is the most common form of metal corrosion. However, iron won’t rust automatically; kept dry or in absolutely pure water, it’ll stay shiny forever. But in a humid environment, iron transforms into a brittle, brown-grey substance: rust, chemically called iron oxide (more precisely: a mixture of different iron oxides, oxide-hydrates and/or iron carbonates). To understand corrosion – and prevent it – we need insights into its surface chemistry.

Reasons for rust
Oxidization (the formation of an oxide compound from metal and oxygen) of a metal is the removal of electrons; the reverse effect, reduction, captures electrons. Every atom has a specified-strength affinity to electrons (i.e., a strength to hold or repel them), sometimes called “electron pressure.”
So what happens if iron is put into water?
As said, pure water won’t cause rusting, because it’s electrically neutral and insulating (absolutely pure water doesn’t conduct electricity!). It has no free electrons nor free charged atoms, which are called ions.
In contrast, common water always contains dissolved gases and minerals. For example, carbon dioxide (CO²) from the air forms H²CO³, carbonic acid, which in water exists in two separate parts, H+ and CO³- , as electrically charged ions. Chemists say the carbonic-acid molecule dissociates into ions. When iron and water interact, the iron attracts the CO3- ions, generates surplus electrons, and becomes negatively charged.
If I immerse copper wire into the water, it tends to repel electrons and attract positive, hydrogen ions. Thus, the copper wire becomes positively charged in water.
The iron’s charge, for example, is created by the CO³- ion being pulled to the iron’s surface. It’s broken up into CO² + O-, and the O- is discharged to form FeO, plus one electron e-. This electrical field repels further CO³- ions; so reaction stops when the charge is created.
If we connect a voltmeter between the two immersed materials, we can measure the difference in electron affinity – called electrochemical potential – in the form of electrical voltage (Photo 1). Here, theoretically, 0.06V (iron) – 0.52V (copper) = 0.58V.
When no current flows, the charge repels further CO3- ions, and the situation is purely static. Thus, no further oxidization occurs. If the two metals contact each other, the voltage difference becomes zero. Other ions move to the iron surface, and more oxygen gives off electrons. An electric current results between the materials.
Thus, the two different metals in water act as an electric battery. This current remains as long as there are enough ions in the water to attract the surface. Corrosion greatly increases when two different materials are in contact in water. The metal with the more negative, electrochemical potential is dissolved, while the other is preserved. The effect is also called local element.
Even if no macroscopic electricity is involved, when in-contact, differing metals get wet, “galvanic corrosion” occurs. The principle of “galvanized” iron (the trade term even if no galvanic reaction occurs between the coating and zinc) is that the zinc is more negative than the iron, and dissolves instead of the iron.

Helpful hints
The more the materials differ in their electrochemical potential, the higher the voltage and the faster the corrosion. The more conductive the water, the worse it is (thus seawater or salt spray on boardwalk signs requires special attention). When constructing signs in humid conditions, avoid these material combinations: stainless-steel screws or rivets in galvanized iron; brass screws in aluminum; pig iron and aluminum (Photo 2); and, stainless-steel screws in mild steel.
Anodized aluminum generates a very thin layer of electrically isolating, aluminum oxide on the surface, which prevents humidity from attacking the underlying metal. However, in sign construction, this oxide layer may be penetrated (for example, when self-tapping or sheetmetal screws are used), or if the metal structure must be electrically grounded for safety reasons.
Here, toothed washers are mandatory, so the oxide layer is definitively broken to make electrical contact. These breaks in the protective layer are very prone to corrosion by humidity. Usually, they must be protected against the elements, but the applied coating (paint) is usually permitted only after the electrical inspection has occurred.
Our discussion has covered only electricity generated by different materials in humidity, but it hasn’t focused on electric signs. LEDs in signs have posed new problems for the industry, because most LEDs operate on direct current (DC), compared to old-style electric signs, which worked mainly on alternating current (AC).

LED problems
Although most LED circuits operate on a low voltage (12- or 24VDC), it’s still much higher than the electrochemical potentials (EP) of the materials involved (for a quick EP reference chart for numerous metals, go to www.thelen.us/1galv.php). So if electrical circuits, contacts or wires are exposed to humidity, the DC greatly increases the strength of the ion movement.
To experiment, I took two common, low-cost LED modules. One had potting shallow enough to expose the screws that connect the circuit board to the wires. In the other, the circuit board was coated with varnish, and the components were additionally covered by a drop of polymer, but the wire solder points were exposed. Both run on 12VDC.
To create corrosive effects, I applied a few drops of tap water on top of the modules. As soon as I switched on the LEDs, you could see gas bubbles developing on the negatively charged wire. Less than eight minutes later, severe corrosion and conductive material surrounded the positive conductors, while the negative ones were heavily pitted and nearly destroyed. DC electricity accelerates corrosion.
With alternating current, the ions would simply be pushed forward, and, when polarity reversed, back in the next fraction of a second. Hence, AC won’t cause the extreme corrosion DC does. Remember, standard acrylic can absorb up to 10% of its own weight in water. Thus, acrylic enclosures don’t make a circuit waterproof!
Here’s some advice for outdoor LED signs:
• Prevent rain or condensed water from accumulating in a channel letter or sign box. Make sure drain holes are large enough not to become obstructed by debris.
• If stranded wires carry DC power, they should be watertight lengthwise.
• Printed circuit boards, including all connections, should be potted or coated with a material impermeable to water and water vapor.
• Don’t use exposed conductors (like support rails) to supply DC voltage.
• And, make sure connector blocks inside channel letters or sign boxes are individually waterproofed – don’t use wire nuts.
 

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