tion before the feed water enters the boiler. The capital investment' in these installations is high and the steam is often used for producing power and in large scale continuous processes w-hich require uninterrupted steam supplies. The prevent'ion of deposits on metal surfaces in these units is especially iniport'ant since high availability and reliability are essential t'o justify the capital investment, and to avoid costly shutdowns of production equipment. The chemical requirements of these units are considerably smaller than the first group, but they are more exacting. Boiler pressures and heat transfer rates are generally higher: and periods between scheduled shutdoms are longer. The concentration of potential deposit and sludge forming solids present in the feed water entering these boilers is generallv quite low. However, the actual pounds of solids entering a boiler will in time add up t o a troublesome deposit if t,hey are not properly treated, since these boilers usually evaporate large quantities of feed water. Internal treat,ment to prevent scale deposits in t,hese boilers must do two things. I t must rapidly precipitate. t o an insoluble form, the small amounts of hardness which enter the boiler, and it must condition these nreciDitated sludges to make them nonadherent. Various phosphates are used for precipitation. conditioning o f these phosphat,e sludges i? accomplished by the use of organic dispersant? which have been specially processed t'o improve their stability a t the high temperature in the boiler water. The int,roduction of electronic induction heat'ing as a method of applying heat to experimental boilers has made possihle precisely controlled laboratory tests of organic dispersants under presssure and heat, transfer conditions which closely simulate artual hoiler conditions. As a result of these tests, processing conditions have been modified t o obtain dispersants having greater effectiveness and stability. The limiting conditions of pressure and heat) transfer which these dispersants will n-ithstand have also been det ermincd. The effect, of different, scale deposits on tube m e h l tcmpera-
tures at, different pressurrs and heat transfer rates was also studied recently in an induction heated experimental boiler. Heat t'ransfer coefficients of the four deposit8 tested were found to increase in the order of analcite, magnesium phosphate, magnetic iron oxide, and calcium phosphate (Figure 2). During this serieii of tests, an actual tube metal failure due t,o overheating caused by analcite scale format,ion was produced in the laboratory (Figure 1). CONDENSATE ALKALINITY
I n the third group of boilers where make-up and leakage t o t'he system are very low, iron and copper picked up from the turbine and condemate &em represent a major portion of the deposit forming materials entering the boiler. In some instances organic dispersants have been effective for keeping these materials suspended in t,he boiler water so they could be removed through the hlowdorm system. The inore recent approach to t,he solution of this problem is to add volatile ainines to raise the p€I of the condensate in all parts of t,he system. Since iron pickup hp pure -rater is at a minimum when the pH is about 9.0, it is both pract~icaland economical t,o control the amount of iron pickup by this mct,hod. The exact pK which should be carried depends on several factors and should be determined by careful evaluation of these factors for each individual system. Recent experimental st'udies of these volatile amines have shown that, at least one of them may be relat,ively stable at pressures up t o 2500 pounds per square inch and steam temperatures up to 1200' F. This means that an effective method of controlling iron pickup is available to practically all steam generating plants in operation today. Improvements in the conditioning of feed water by modern external treating met'hods have not been t8he final answer to wat,er conditioning. Each of these improvements has emphasized the need for careful attention to the entire water-steam cycle to ensure that the problems are not eliminat'ed from one spot, only to be transferred to another.
Corrosion in the Boiler R . F. ANDRES Dayton Power and L i g h t Co., D a y t o n , Ohio
scuision piesents a resume of some recent theoiies
TH and dl I developments S in the field of chemical treatment of boiler wateis for corrosion control. The chemical reactions of iron in aqueous media are both complex and controversial For this papcr the electrochemical explanation for corrosion of iron in water based on the following general equation is used. 3Fe
+ 4H2O .+ FesOa -+ 4H2
>himixing corrosion on boiler metal surfaces is still pi edicated on controlling this reaction through the formation and maintcnaiire of a continuous adherent protective magnetic iron oyide film of minimum thickness. This protective film formation is quite rapid i f the water in contact with the metal surface is a k a line and free of disqolvcd oxygen. Coriosion OCCUIY if, and on11 if, the protective film is destroyed ( 9 ) . Any disruption of the continuity of this protective film either by mechanical means or chemical rcnriion tends to promote corrosion. Alechaniral de990
stiuction of the film may result from erosion, tcrnperature changes, stresses, etc., and chemical destruction of hlms may result fiom action of dissolved oxygen, carbon dioxide, galvanic coi iosion. excessive caustic concentration, and reaction of iron nith supelheated Jtearn. The solutions to the problems of chemical film destruction present a very real challenge to chemists and engineers involved in preventing internal boiler corrosion It has long been recognized that an1 chemical treatment that tends to leave boilei water osygrn free, maintains proper alkalinity in rontacr with all wetted surfaces, and reduces or eliminates sludge and scale deposits is a nisterial aid in reducing internal boiler coirosion. Troublesome boiler deposits may be formed from thrre distinct sources-salts noniinally present or added t o boileI reed water, inetals and metallic oxides resulting from corrosion and Prosion in the preboiler cycle, and metal oxidee formed intrmdllq in the hoiler from corrosion of boiler metal. The prevention and elimination of sealrs and sludges formed from
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Boiler Water Chemistrysalts nominally present in feed water have already been discussed in the previous paper. Our attention is directed to the prevention of the other types of deposits. pH CONTROL
Corrosion of iron, copper, and nickel by the relatively pure feed water passing through preboiler piping and equipment has assumed major importance in recent years. Elimination of dissolved oxygen and elevation of feed water pII values to 9.0 reduce preboiler corrosion appreciably (2). Early attempts to raise boiler feed water pH were based on recirculation of alkaline boiler &ater or addition of sodium hydroxide. Recirculation of boiler water presents problems in getting sufficient quantity to gain the desired pH elevation, and there is always the possibility of fouling feed water heaters by deposition of insolubles from the boiler water. TVater from condenser leakage also may react with the softened boiler water to give troublesome heater deposits. The use of caustic mag result in increased blowdown requirements. The disadvantages of these methods have been recognized, and in recent years attention has been directed to the use of ammonia and amines for preboiler corrosion control (3, 4, 18). Chemical treatmtlnt with these materials permits precise control of pH values throughout the water-steam cycle and, in the absence of dissolved oxygen, there is no indication that copper pickup in the feed Rater system is increased to any appreciable extent. The following concentrations in parts per million will give pure water a pH of 9 0, ammonia, less than 0.5; cyclohexylamine, 2.0; and mor pholine, 4.0. The trend towards the increased use of ammonia and amines is apparent from a recent survey of central steam generating stations which indicated that roughly three fourths of the companies contacted are controlling feed water pH chemically, with 21 using ammonia or amines for this purpose, nine using recirculation of boiler water, and six feeding caustic. OXYGEN SCAVENGERS
For control of dissolved oxygen in boiler feed water, the use of sodium sulfite has been almost unanimously accepted. I t is well to point out, however, that with improvements in mechanical deaeration equipment the influence of dissolved oxygen in feed water corrosion has been markedly reduced, and many companies are operating successfully without sodium sulfite treatment. I n the survey previously mentioned, it was also revealed that 24 of 50 companies are feeding sulfite to the feed water system. It has also been observed that decomposition of sodium sulfite with attendant formation of sulfur dioxide or hydrogen sufide can occur in high pressure steam generating equipment, and this condition may increase corrosion rates in the steam-feed water cycle appreciably (7). Also, with any appreciable feed of sodium sulfite, boiler blowdown requirements may be increased. A recent development has been the proposed use of hydrazine hydrate in boiler feed water treatment. This chemical has the desirable properties of removing traces of dissolved oxygen and elevating the p H of the feed water with no increase in concentration of boiler water solids. Hydrazine hydrate has the disadvantages of being expensive and decomposing at temperatures above 350’ F.; its reactions with normally protective metallic oxide coatings have not been thoroughly investigated. INTERNAL TREATMENT
Treatment of boiler water for the prevention of internal boiler corrosion is quite complex because of the various types of corrosion encountered. Internal boiler corrosion may be conveniently divided into three classifications-pitting, concentrating film effects, and caustic embrittlement. The usual causative agents in pitting-type corrosion are stress, dissolved oxygen, carbon dioxide, galvanic effects due t o porous sludge and scale
May 1954
deposits, and perhaps influence of copper. Chemical treatments to prevent pitting-type corrosion are aimed at elimination of dissolved oxygen, maintenance of proper alkalinity, and elimination of scales and sludges in order t o preserve the protective boiler metal films. For dissolved oxygen control, sodium sulfite, lignin derivatives, colloidal iron, ferrous sulfate, and other reducing materials have proved successful. Scale and sludge control may be obtained in some instances by a combination treatment involving maintenance of high alkalinity with some phosphate and dispersive addition ( 1 ) . If magnesium deposits are troublesome, the addition of silicates to the boiler water as proposed by Hall ( 5 ) may solve the problem by preventing the formation of adherent magnesium phosphate. I n some instances magnetic iron oxide deposits have been excessive, and the use of sodium nitrite treatment to convert the magnetic iron oxide to ferric oxide may be of substantial benefit in minimizing this problem ( 1 4 ) . The role of copper deposits in connection with boiler corrosion has not been clearly defined. Copper deposits in boilers are commonplace, but only in isolated cases has corrosion occurred. Evans has proposed the explanation that metallic copper can catalyze Shikorr’s reaction 3Fe(OH)*6 FesOa
+ 2H,O + H,
If this occurs in a boiler, then ferrous hydroxide, produced by reaction of water on iron a t breaks in the magnetite scale, will deposit fresh magnetic iron oxide not on the metal exposed at the break but on the adjacent metallic copper As the copper becomes coated, additional copper may be depositcd and the corrosion process continues, This explanation may account for the magnetic oxide covered copper particles sometimes found in boilers. The second type of internal boiler corrosion, concentrating film effects, is fortunately not commonplace. Concentrating film corrosion occurs in boiler areas of high heat input with questionable circulation. When caustic concentration from the boiler water is the promoting corrosion factor, the problem may be successfully controlled through application of Purcell and Whirl’s coordinated phosphate-pH control (8). This method of treatment is founded on the maintenance of a specific boiler water pH for a given concentration of phosphate ion so that concentration of the boiler water ail1 not result in any appreciable increase in the concentration of hydroxyl ion. Another approach to combatting this type of attack has been proposed by Hall (6) and Straub (11) involving the addition of a neutral salt t o the boiler water in a concentration much greater than the caustic concentration so that as the over-all boiler water concentrates, the effect of the caustic is appreciably reduced by the dilution effect of the added salt. Correction of circulation deficiencies, design changes to control localized high heat input, chemical cleaning to remove accumulated deposits, and careful control of alkalinity are means which have been utilized in controlling attack by concentrating films. The third type of corrosion previously mentioned, caustic embrittlenient, is fortunately quite rare a t the present time. Much research work has been done on this problem, and the causes and remedial chemical treatments to prevent this type of metal attack have been published (10, 19). It will suffice to state that chemical treatment of water for boilers susceptible to caustic embrittlement has been based on addition of sodium nitrate or quebracho extract. These additives to boiler water have sharply reduced the incidence of failures in boilers where caustic embrittlement has been a major cause of metal failure. LITERATURE CITED
(1) Adkins, S. K., Proc. Midwest Power Conj., X (1948). (2) Archibald, F. L., and Pursell, J. W., Jr., Proc. Amerkan Power COnf., XIV, 443-58 (1952). (3) Berk, A. A., and Nigon, J., U. S.Bureau of Mines, Tech. Paper 714 (1948).
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Cerna, Wendell W.,Proc. Seventh Annual Kater Conf., Eng. SOC. of W.Pa. (Jan. 6-8, 1947). Hall, R. E., Proc. Ninth Annual Water Conf., Eng. Soc. of VI‘. Pa. (Oct. 18-20, 1948). Hall, R. E., Trans. A n . Soc. Mech. Engra., 6 6 , 457-88 (1944). Ongman, H. D., Combustion, 24 No. 8 , 40-4 (1953). Purcell, T. E., and Whirl, S.E’., Trans. Am. Soc. M e c h . Engrs., 64, 397-402 (1942). Rivers H. M., and Sonnet:, W. XI.>Combustion, 21, No. 12,41-6
(1950). Ychroeder, W. C., and Berk, A. A,, C . S. Bureau of Xlines. Hull. 443 (1941). Straub, F. G., Mech. EILQ., 61, 199-202 (1939). Thornleg, J. L., Industry and Power, 65, S o . 2, 58--G1, 1953. T ~ a n sAm. . SOC.Mech. Engrs., 64, 393-444 (1942). Ulmer, R. C., Whitney, J. H., arid U’ood, J. W., P I O CAnieilcan . Power Conf.,XIV, 459-67 (1952).
Discussion +
+
If 3Fe 4H20 --* FerOI 402 is the main reaction for good film (thin) protection, how do you account for FeaOl and Fe in boiler deposits? What is method of setting FeaOc filnitime, temperature, and alkalinity? R. F. ANDRES: The relative amounts of t,he various iron oxides in boiler deposita are controlled by the oxidizing or reducing characterist)ics and dissolved oxygen content of the water in contact with the metal surfaces. These factors vary throughout the plant cycle and change wit’h temperature, pH, and ferrous ion activity. An excellent discussion of the reactions of iron with mater is presented in a paper by Marcel Pourbaix, ,‘Some Applications of Electrochemical Thermodynamicfi,” C o n m i o n , 6 , No. 12,395404 (1950). The various factors controlling protective magnctic iron oxide film formation have not been thoroughly investigated. Much fundamental research ~ o r kon ii.on-wat,er reactions must be completed before definit,ive rules can be established for t,he hest methods of forming prot,ective iron oxide films on boiler metal surfaces. What concentrations of alkalinity are necessary for destruction of protective ironbxide film on boiler metal? R. F. AMDRES: I cannot define limits for the concentrations of alkalinity that destroy protective iron oxide film in an operating boiler. It has previously been repovted that minimum corrosion of iron results when the caustic concentration in the \rater is in the mnge 40 to 400 p.pni. corresponding to pH values of 11 to 12. Caustic concentrations as low as 5% may be aggTessive to boiler steel according to C. E. Kaufman, I-, hl. AIaroy, and W. H. Trautman, “The Behavior of Highly Concrnt,rsted Boiler Kater,” Proc. S:’zth dnnzcol TT‘aier Cor!f., pp. 23-42 f 1945).
What is the difference in the mechanism of this corrosion and the corrosion at low pH? R. F. ANDRES: The corrosion of iron at low p H values in Kater solutions results from removal of the polarizing hydrogoti film and the absence of any formation of protective oxide films. Caust,ic attack, on the other hand, is charactri.ized by removal of protective iron oxide films as soluble sodium ferrite or ferratp folloFed by reaction of the water with the metal t o replenish thcl oxide film. If the caustic concentration remains high, this film formation is prevented and corrosion continues. At what pH does the corrosion at the so-called excessive caustic concentration occur or accelerate? R. F. ANDRES: Above pH 12 the relative attack of caustic on steel increases very rapidly. I t is impract,ical to attempt to designate the pH above m-hich corrosion may occur because il small increment of pH represents a high increase in caustic, concentration in the pH range 12 to 1.1. I heard a paper recently which reported that nitrogen was added-i.e., a nitrogen atmosphere was maintained-in order to reduce corrosion due to oxygen in water. Have you any experience along these lines or could this be adapted to boilers in any way? The work was laboratory work on the corrosion of metals and alloys in which they proved that removal of hydrogen was as important as the removal of oxygen. R. F. ANDRES: I cannot visualize maintaining a nitrogen atmosphere in contact with n-at8erin an operating boiler. Piitrogen has been used following acid cleaning of boilers to prevent, excessive iron oxide formation which would result if air came in contact mit,h t,he highly reactive pickled metal surfaces. For, protection of idle boilers, a nitrogen blanket could be maint’aine.1 under pressure to exclude oxygen from the boiler. Sulfite breakdown to H,S (not SOa) seems to be associated with dry areas (faulty circulation) in the boiler, whereas SO, formation is a matter of equilibrium. The breakdown is caused apparently by the formation of nascent hydrogen and usually accompanies the formation of excessive magnetic iron oxide in the boiler. Any comments? R. F. ANDRES: The presence of sulfide in a boiler or evidence of hydrogen sulfide in steam is an indication of sulfite breakdown probably resulting from the causes mentioned. Elimination of sulfite h a t m e n t mill not correct the boiler corrosion problem but may prove beneficial in combatting possiblc corrosion from hydrogen sulfide in other portions of the natersteam cycle. The sulfur dioxide evolution with the strain from sulfite trpated boilers is n chemical equilibriii.:u. reaction arid tho quantity is dependent on temperature condit,ions arid the concentrat,ion of sulfitr h i n g maintained in tlic watei..
Foaming in Boilers W. L. DENJIAN Deurhorn Chemical Co., Chicago, Ill.
OIIZR water forming in general is a term applied loosely to cover the formation or generation of steam containing liquid water and is to be differentiated from eo-called spray carryover. In the case of foam, the water is actually present in the steam as free liquid water; whweas, in the case of spray carryover the boiler water ie dispersed in the steam as fine globules. 992
Foaming does not result from the formaticin of a distinct foam layer or blanket on the water surface; rather, it results primarily because the apparent volume of‘ the hoiler water increases dur to the presence of an extremely large number of very small stable steam bubbles within the volume of the boiler water. Small steam bubbles rise much RIOKPI. tha.ri do larger steam bubblrs
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46, No. 5
