Page 38 - MetalForming-Oct-2018-issue

36
MetalForming/October 2018
www.metalformingmagazine.com
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Corrosion Inhibitors: A Primer
Various classes of chemical compounds can prevent corrosion and staining on aluminum metal and alloys. Each has advantages and disadvantages.
Inorganic compounds
• React with ions in solution before they can reach the aluminum surface • Many of these compounds (including arsenates and hydrazine) are toxic
Heterocyclic organic compounds
• Good for copper-containing aluminum alloys
• Prevent galvanic corrosion caused by close contact between aluminum and
copper
Sulfonates and phosphates
• Work well as corrosion inhibitors
• Encourage microbial growth: biocides required
• Solutions with less than 2-percent phosphates effectively prevent stains
• Sulfonate-containing fluids can have an unpleasant odor, and they can corrode
copper
Amine carboxylates
• Very effective at keeping aluminum from staining
• Don’t require biocides (if the proper amine chemistry is selected)
• pH buffering action, which improves the overall stability of the MWF
• Relatively large amounts required, typically as much as 10 to15 percent of the
formulation
• Changing to a different amine has a big impact on the buffering and stain-
prevention properties of the fluid
Complex esters
• Small oxygen content requires a relatively large amount (6 to 7 percent) in the fluid to be effective at preventing stains
• High cost means that they are used mostly as lubricants rather than corrosion inhibitors
Silicates
• Similar chemically to phosphates and sulfates
• Microbes don’t use them as food
• Inexpensive
• High oxygen content means you can use less to get the same level of corrosion
protection
• Tend to form gels in water-based fluids
• 0.8-percent tetraethyl orthosilicate (TEOS) can prevent staining on aluminum
alloys without forming a gel
Silanes
• Lower oxygen content than silicates
• Less prone to forming gels but must use more to prevent corrosion
• Bonding at least one long-chain organic group to the silicon atom further
disrupts gel formation (but can drive up the cost of the additive)
Compounds with branched organic substituents
• Degree of branching has little effect on pH
• Highly branched organic substituents are significantly better at preventing
aluminum staining than their straight-chain counterparts
Amine-functionalized organosilicates
• Still under development as fluid additives
• Combine the beneficial properties of amines with those of silanes • Do not require biocide
• Good stain protection
Lubrication Challenges
Source: Hoon Kim, Chemetall