Methanolic Corrosion of Aluminum
Do Novinium’s Cablecure® 732/733 fluids interact with aluminum stranded cables?
In the illustration (taken from “Failures in Silicone-treated German Cables Due to an Unusual Aluminum-Methanol Reaction” by Glen Bertini, ICC, October 29, 2002), the gray material between the strands is aluminum methoxylate. The profuse formation of aluminum methoxylate caused the insulation to bulge in a way that was similar to when a snake eats a rat. An analogous phenomenon occurs in low-voltage cables in the presence of water. In both cases, the bulging occurs because the aluminum hydroxide or aluminum methoxylate has a very low density, or put another way, takes up a great deal of volume. Because rejuvenation is utilized to improve the reliability of power distribution cables, even a 1 percent induced failure rate is unacceptable.
Novinium was founded in 2003, so we enjoyed the benefit of hindsight into the methanolic corrosion of aluminum. As a result, we addressed this issue in all of our Cablecure 732/733 and Cablecure iXL technologies. Novinium hasn’t had a single incident of methanolic aluminum corrosion.
To understand why Novinium technology avoids methanolic corrosion, it is useful to examine the mechanism of methanolic corrosion. One compound and one element are required for methanolic corrosion to occur. The compound is methanol; the element is aluminum.
Cablecure XL fluid, Cablecure iXL fluid, and Cablecure 732 fluid all include methoxy silanes, which react with encountered water and produce methanol as a by-product. Cablecure 733 fluid and Cablecure DMDB do not produce methanol as by-products; instead these fluids produce larger, less chemically reactive alcohols that have a higher boiling point—namely 2-ethylhexanol and n-butanol.
The reaction of methanol with native aluminum (methoxylation) proceeds at a rate proportional to the concentration of the methanol. The concentration of methanol in the strands of a treated cable is influenced by four factors:
1. The amount of water that is present in the strands and the strand shield. Less water means less methanol; more water yields more methanol.
2. The stoichiometry of the silane water reaction. Stoichiometry deals with the ratios at which materials react. For the Cablecure XL fluid and Cablecure iXL fluids, which utilize the same monomeric silane, the maximum possible methanol concentration is about 25 percent by weight. For Cablecure 732 fluid, the maximum is about 20 percent by weight. All other things being equal, Cablecure 732 would enjoy about a 20 percent lower methoxylation rate because of the superior stoichiometry.
3. The rate at which methanol diffuses from the strand area out of the cable. The diffusion of methanol is quite fast, so the risk of methoxylation decreases rapidly for all technologies. A higher temperature accelerates the diffusion and dissipation of methanol.
4. The use of alkoxysilanes. Non-methanol-based alkoxysilanes reduce methanol concentrations beyond the 20 percent stoichiometric advantage described in factor 2. In a patented process (US Patent 7,611,748 and its foreign equivalents), Novinium adjusts the formulation with more and more non-methanol-based Cablecure 733 fluid as the anticipated temperature of the treated cable rises.
At first it may seem obvious that elemental aluminum is available in an aluminum stranded cable, but it is not. As soon as aluminum strands are drawn and laid into a strand bundle on the factory floor, the outside layer of aluminum reacts with oxygen to form aluminum oxide (Al2O3). Aluminum oxide forms a dense barrier that protects the underlying, native aluminum metal. This aluminum oxide layer is called a patina, and it protects the underlying aluminum from further corrosion.
If you take a piece of aluminum and scrape off the patina with a knife, you will see bright and shiny native aluminum underneath. In the presence of oxygen, the patina begins to reform immediately. The shiny surface will soon return to its dull gray appearance. Of course, in a power cable there are no knives scraping off the protective patina, so how did the Cablecure XL fluid penetrate the patina?
Cablecure XL fluid and Cablecure DMDB use a condensation catalyst called titanium (IV) isopropoxide. It’s also known as tetraisopropyltitanate, so for the purposes of this discussion, we will call it TIP. Over the course of Novinium’s research, we learned that TIP facilitates the degradation of the patina. Novinium does not use TIP in its Cablecure iXL or Cablecure 732/733 formulations. Novinium uses a patented catalyst (US Patent 7,700,871 and its foreign equivalents) that does not have the problem that TIP does.
A second way that the patina can be damaged is bubble nucleation or boiling. Bubbles form in microscopic cracks in the patina, and their rapid expansion and sudden disappearance mechanically perturb the patina. In the discussion above, we learned that Cablecure 732 fluids enjoy about 20 percent less stoichiometric methanol, and hence the boiling point of the mixture is higher. Put another way, it takes a greater temperature escalation for Cablecure 732 to produce bubble nucleation than it does for Cablecure XL and Cablecure iXL fluids. The patented silanes (US Patents 7,658,808 and 8,101,034 and their foreign equivalents) included in Cablecure 732/733 fluids by Novinium and our partners enjoy improved stoichiometry, which mitigates methanolic corrosion.
Cablecure XL fluid includes an ingredient called trimethylmethoxysilane (TMMS), which has a boiling point even lower than that of methanol. To mitigate the bubble nucleation problem in 2002 vintage Cablecure XL fluid, the concentration of TMMS was reduced in Cablecure XL by a factor of between 3 and 6. This problem with TMMS is well documented by US Patent Application 2009/0114882 and its international equivalent WO 2006/119196. Besides attacking the patina, the TMMS creates a fire and explosion hazard. Novinium does not use TMMS in Cablecure 732/733 or Cablecure iXL fluids.
In addition to mitigating the causes of patina damage, Novinium utilizes a patina stabilizer from BASF®, called Tinuvin® 123 hindered amine light stabilizer. In experiments undertaken at Novinium, Tinuvin 123 outperformed all other patina stabilizers by at least a factor of 2. Offering other beneficial performance attributes that help extend cable life, Tinuvin 123 is included in Cablecure 732/733 and Cablecure iXL fluids, and its use is protected by US Patents 7,658,808 and 8,101,034 and their foreign equivalents. In a patented process (US Patent 7,611,748 and its foreign equivalents), Novinium increases the supply of Tinuvin 123 by increasing Cablecure 212 fluid as the anticipated temperature of the treated cable rises.
To all but prevent methanolic corrosion of aluminum in its Cablecure 732/733 formulations, Novinium:
• Substantially reduces the methanol concentration by using proprietary silanes.
• Does not use TMMS, which has a low boiling point, is highly flammable, and is demonstrated to cause bubble nucleation even at moderate temperatures.
• Has eliminated from its fluids a patina-attacking catalyst that was utilized in the offending formulations.
• Adds a patina-stabilizing compound.
Cablecure iXL technology includes the improved catalyst and patina stabilization and does not use TMMS, which has a low boiling point. However, Cablecure iXL suffers from a higher methanol concentration than what is found in Cablecure 732/733 technology. Cablecure iXL technology should not be utilized in high temperature, aluminum-conductor applications.