Corrosion is generally a problem in any system in which ferrous metals are in contact with aqueous solutions. Corrosion is the electrochemical reaction of metal with its environment. Corrosion is a destructive reaction which, simply stated, is the reversion of refined metals to their natural state. For example, iron ore is iron oxide. Iron oxide is refined into steel. When the steel corrodes, it forms iron oxide.
The metallurgy of boiler systems are predominantly iron-containing steel in contact with high temperature (approximately 200.degree. F., under 35-350 psig of pressure) aqueous solutions. Most industrial boiler and feedwater systems are constructed of carbon steel and sometimes with copper alloy and/or stainless steel feedwater heaters and condensers. Some have stainless steel super-heater elements.
Corrosion is one of the main causes of reduced reliability in boiler systems. The most common causes of corrosion are dissolved gases such as carbon dioxide and oxygen, under-deposit attack, low pH and attack of areas weakened by mechanical stress resulting in stress and fatigue cracking. Corrosion control in boilers varies with the type of corrosion encountered. Generally, corrosion in boilers may be controlled through procedures such as maintaining proper pH and alkalinity levels, reducing mechanical stresses, operating within design temperature and pressure specifications, by proper application of chemical corrosion inhibiting treatments and by controlling oxygen and boiler feedwater contamination. Generally boiler feedwater should be low in contaminants. The American Society of Mechanical Engineers (ASME) Consensus for Industrial Boilers specifies maximum levels of contaminants for corrosion and deposition control in boiler systems. The consensus is that feedwater oxygen, iron and copper content should be less than 7 parts per billion (ppb) oxygen, 20 ppb iron and 15 ppb copper for a 900 psig boiler and that pH should be maintained between 8.5 and 9.5 for boiler system corrosion protection.
It is commonly held that in boiler systems substantially free of oxygen (i.e., less than about 7 ppb oxygen) that ferrometal corrosion can be expressed in terms of the following equation: EQU Fe+2H.sub.2 O.fwdarw.Fe(OH).sub.2 +H.sub.2.
Therefore, Fe.sup.+2 (ferrous ion) is a useful material to determine the corrosion rate of ferrometals in substantially oxygen free systems such as boilers. However, in the presence of oxygen the following reaction occurs: EQU 3Fe(OH.sub.2).fwdarw.Fe.sub.3 O.sub.4 +H.sub.2 +2H.sub.2 O
Magnetite (Fe.sub.3 O.sub.4) is a complex of the hydroxides of iron (III) and iron (II) which can be expressed by the formula: EQU Fe.sup.3+ !.sup.IV Fe.sup.2+ Fe.sup.3+ !.sup.IV O.sub.4.
Other ferric-ferrous oxide species also exist, however, under the conditions present in most boiler water, magnetite is the most common species. As used herein the term "ferric-ferrous oxide" means magnetite as well as other ferric-ferrous oxide species such as maghemite (.gamma.Fe.sub.2 O.sub.3) and feroxyhyte (.delta.FeOOH) which are ferrimagnetic. Ferric-ferrous oxides are often introduced into a boiler system by boiler feedwater. Ferric-ferrous oxides can also be produced within the boiler through reaction of ferrous ions with trace free oxygen and via electrochemical reactions.
Ferrion chelation chemicals are compounds which form complex undissociated cations with divalent metal ions. Such chelation chemicals include 1,10-phenanthrolinre; 4,7-diphenyl-1,10-phenanthroline; 2,2.sub.1 bipyridine; 2,6-bis(2-pyridyl)-pyridine; 2,4,6-(bis(2-pyridyl)-1,3,5-triazine; phenyl 2-pyridyl ketoxime; 3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-nitrilotriacetic acid and salts thereof. Certain of these ferrion chelation chemicals react with ferrous ions to form colored species (ferroin reagents) which are useful in the coloromertric determination of ferrous ion concentration. It is generally believed that ferroin reagents do not change color when the reagent complexes with ferric (Fe.sup.+3) ions in water. However, some research suggests that oxides, such as magnetite, may be captured by the ferroin reagents and thus bias ferrous ion content determinations (see Matijevic, E. Han-Chyen, Chang, Interactions of Metal Hydrous Oxides with Chelating Agents: Part V Ferric-ferrous oxides-EDTA, J. Colloid Interface Sci., Fin. Chem. Lett. (1982); and Bohnsak, G. The Solubility of Ferric-ferrous oxides in Water and in Aqueous Solutions of Acid and Alkali (Essen: Vielkan-Verlag, 1987)).
Therefore, conventional techniques to measure ferrous ion content as an indicator of ferrous metal corrosion may inadvertently include some ferric-ferrous oxides concentration in the calculation of ferrous ion thereby yielding higher corrosion rate determinations then are actually occurring in the studied boiler system.
Thus a need exists for a method to remove ferric-ferrous oxides from a liquid containing ferric-ferrous oxides and ferrous ions.
A need also exists for a method to measure the ferrous ion concentration in boiler water without contamination of the rate determination results due to the presence of ferric-ferrous oxides in the boiler water.
It is, therefore, an object of this invention to provide a method for removing ferric-ferrous oxides from a liquid containing ferric-ferrous oxides and ferrous ions.
It is also an object of this invention to provide an accurate method to determine the corrosion rate of ferrous metals exposed to a boiler water in a boiler system which contains ferrous ions, ferric ions and/or ferric-ferrous oxides.