Corrosion: Cause, Effect and Control
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orrosion both affects and is affected by deposition and microbiological concerns. Just as fuel, oxygen and an ignition source are required to start and sustain a fire, so the water treatment triangle (Figure 1) requires the water treater to keep all three of its legs
This paper defines corrosion, with a primary focus on mild steel corrosion. The types of anodic and cathodic inhibitors are described, as well as the types of systems where each approach is most appropriate from the standpoint of results and cost.
under control. Corrosion’s place at the top of the water treatment triangle may make it “first among equals,” however, with respect to the steel industry. Corrosion byproducts tend to be the root cause of deposit problems, especially within a continuous caster (closed mold system and spray water system). Iron in a spray system is more frequently the result of corrosion, rather than particulate from the slab or billet. Corrosion byproducts also promote the proliferation of sessile (surface-dwelling) bacteria. These bacteria, in turn, will act to entrap other foulants, and create under-deposit corrosion. Iron will almost always be a major component of any microbial deposit.
focus of this paper is corrosion and corrosion prevention of mild steel. As Chemical Principles states concerning iron, “The curse of rust is not that it forms, but that it constantly flakes off and exposes fresh iron surface for attack.” It is interesting to note that aluminum is more susceptible to oxidation than iron on the reduction-potential scale, yet aluminum is relatively inert to corrosion (as long as pH does not exceed 8.5 or if it is a potential galvanic cell to a more noble metal like copper). The reason is that aluminum oxide adheres tightly to its base metal, because the packing distances of aluminum and its oxide are very similar to one another. In contrast, the packing dimensions of metallic iron and its oxide layer are not particularly close. Therefore, the iron oxide layer will not adhere to metallic iron on its own (Chemical Principles, Chapter 19-7, page 725).
There are several definitions of corrosion. It can be defined as part of the third law of thermodynamics — everything in life seeks to return to its most stable state. The Betz handbook defines corrosion as “the destruction of a metal by chemical or electrochemical reaction with its environment.” Chemical Principles, third edition (1979), defines corrosion as a redox reaction gone astray. One definition of corrosion can even be found in the Bible: “Remember, man, that you are dust, and to dust you shall return” (Genesis 3:19). Although there are many metallurgies encountered in the steel industry, the major
In order to have corrosion, there must be an anode, a cathode and an electrolyte, which is usually water. When pH is below 4.3, hydrogen ions are in sufficient concentration to reduce electrons at the cathode, and drive the corrosion cell. This explains why acid overfeeds are so detrimental to a cooling water system. For pH ranges above free mineral acidity (4.3), there must be oxygen in the water to drive the corrosion cell. This is the reason that boiler water treatment focuses on oxygen removal from feed water at elevated pH in order to prevent corrosion within the de-aerator or boiler tubes. Some closed cooling loops do use oxygen scavenging technology, usually coupled with a mild steel passivator, to inhibit corrosion. There are some high-temperature, high-pressure (even low-temperature, lowpressure) BOF hoods that use this type of treatment and get excellent results. Another way to inhibit corrosion is to remove all dissolved solids from the cooling water. This means more than simply removing all hardness from water. Removing hardness significantly reduces the tendency for calcium-based scales, but actually makes water
Jim Gleason, GE Water and Process Technologies, Trevose, Pa. (firstname.lastname@example.org) 34 ✦ Iron & Steel Technology