US2954332A - Alkaline hydroxides in cathodic protection of metals in seawater and brines - Google Patents

Description

ALKALINE HYDRXEES iN EATHODIC PRGTEC- THUN GF METALSIN SEAWATER-ANDBRINES Oliver sborn, Lake heksen, and John lloseph Newport Hl, Freeport, rf'e'x.,'assignorsto'he Dow Chemical Company, Midland, Mich., a'coiporation of- Dela- Ware v Filed'May 23, 1951,.Ser."N0.`227,906

s claims. (ci: 2er-s6) On the other hand, Vwith larger currents,A e.g. over 0.03 2

ampere per square foot, a calcareous coating is produced and thoroughly covers thes metal within afday or two, or even much less time, if a high enough' current is used (cf. U.S. Patent 2,200,469). This coating appears to consist mainly of the carbonates and hydroxides of calcium and magnesium.l Although slightly soluble tn seawater, it will remain in place and afford considerable protection to the metal for a long time after all flow of current is ended. Further, the coating may be maintained intact at much lower current densities than are required to produce it; Optimum cathodic protection of metal in seawater, then, calls for intentional rapid production of the calcareous coating at high current, followed bythe application of a lower maintenance current, continuously. This latter procedure, though eifective, has been expensive 2,954,332 Patented Sept. 2'?, 1960 of these latter are thej diluted seawater' found in many ports, and the inlandbrines of Michigan and Ohio, which are probably concentrates of the water of ancient seas.

The water-soluble alkaline hydroxides used in the invention shouldbe strongly basic. Sodiumhydroxide'appears to be most satisfactory and usually also the cheapest. However, other soluble basic hydroxides, especially calcium hydroxide, aqueous ammonia, and the hydroxides of potassium, barium, and strontium maybe used where available. The hydroxide may be added in solid form or in aqueous solution, either concentrated o'r dilute. Sincev the purity of the hydroxide is of no great moment, it is usually most economical, where' possible, to use waste alkaline hydroxide-containing process liquors, which may be quite dilute, such aseuentcatholyteffrom electrolytic brine cells, causticsodafevaporator tail water, spent caustic liquor from organic chemical reactions, and the like.

The proportion of alkaline` hydroxide accordingto the invention' falls in the approximate range' of 2 to 300 parts per million. At even as little as 2 p`arts per million, very substantial savings in current canbe realized in forming a calcareous coating on steel seawater. At much over 300 parts per million, alkali is consumed to no purpose. Preferred'concentrations, in the Yoase of -sodium hydroxide, are in the range of 25 to 50 parts per million'when' a' cathodic coating is to be formed rapidly, and from 2'to` l0 parts perlmillion when it is to be produced more slowly, or when a coating already formed is to be maintained.` l'

The currentfdensities required for coating formation according to the invention depend" in some measure on the concentration of alkali added to the yseawater or other brines, and also on the salinity of the brine. In allcases, however, they are lower thanthose required inthe absence of the alkali. For instance, Vin seawaterY to which4 25 to 50 parts per million of sodiumhydroxide is-added,

` a current density notl over V0.02 ampere per square foot because of the high current requirements while the coating is being formed. If suicient permanent capacity is inL stalled to supply the initial current, mostl of thatl capacity remains unused for the duration of the protection period, often for years. On the other hand, the cost of acquiring and movingV a portable high-current low-voltage generator, such as might be used asa booster in the initial stages of protection, has also proven excessive.

The present invention seeks to eliminatev this source of expense by providing -al method of forming the protective cathodic calcareous coatingson metals` withoutv supplying the high initial currents heretofore required'. It also provides a methodV of forming and` maintaining such coatings at far lower-current densities than have been possible previously.

These results are realized according tol the invention by adding to the seawater or other like brine in contact with the' metal to be protected a water-soluble alkaline hydroxide in an approximate proportion of from 2 parts to 300 parts'per million. With the water thus alkalized,

is adequate for formation of' the protective calcareous coating in a reasonably short time, usually less than `a month, and often with a few days or less; Ifrapidity of coating' formation isnot important, adequate cathodic protection can be realized at asodium hydroxide concentration of 2 to l0 parts per millionV andat a current density of from 0.002 to: 0.005 ampere per square foot. A coating Vmay form slowly over a period of several months. These latter conditions willv also suflce tomain-- tain an existing cathodic calcareous coating.

The process of the invention is extremely useful in connection with steel and other metal vessels for storing seawater and brines, and the similarV tanks aboard ltanker formation of a protective calcareous coating occurs at current densities as little as a tenthY of those'v required when the alkalinity isV absent, The current* density needed to maintain' the coating is also reduced, though not quite as much. v

So far as known, the new process is' applicable in the cathodicprofe'ctionT n'ot only of iron and steel, but also of all other' corrodible metals no more anodic than zinc. Itis eifctivein' pevntingcorrosion both by seawater andby other saline Brines containing both calcium andmagne'sium salts" in substantial proportion;V Typical ships, where seawater is sometimes used for ballast. In such cases, only a ysingle addition of sodium hydroxide is required for each filling of the tank. Another use is in the cathodic protection of flumes, pipelines, condenser jackets and other conduits through which seawater is made to Vllow. 'In these instances, the alkali must be` continuously added to the seawater, preferably at or ahead of its point of entry into the conduit.

in the invention,` the requisite current forv rendering cathodic the metal to be protected may be generated in any of the usual ways. However, the use ofsacrihcial anodes, particularly those made of magnesium or magnesiurmbase alloys (hereinafter collectively termed magnesium meta-l) is eminently satisfactory. Magnesium has the advantage that its sel-f-generated potential is sufr-cientV to insure adequate cathodicproteetion but is not'sucient to cause excessive electrolysis. Hence, with magnesium anodes, there is automatically supplied a current which isonly slightly in excess of' the minimum required for adequate protection.

The invention, in one preferred embodiment, may be explained with reference to the accompanying schematic drawing. In the system there shown, a large bare steel pipeline 1 is being used for carrying tiowing seawater, only a small portion of the line being shown. A covered manhole 2 is provided for access to the interior of the pipe 1. Within the pipe, sacricial magnesium anodes 3 are placed at intervals, being suspended on electrically conducting cables 4 which are welded to the pipe at one end and at the other are iirmly connected to the magnesium. Near the inlet of the pipe 1 is 'a tank 5 for In the system shown in the drawing, the inside surface of a 30inch bare steel pipeline was protected against corrosion in seawater by sacrificial magnesium lanodes each weighing 17 pounds, the anodes being spaced at 5-foot' intervals. Recording ammeters were used to follow the current demands of the l-ine. The rate of seawater iiow Was 100 gallons per minute.

For experimental purposes, weak sodium hydroxide solution was injected continuously into the pipeline part way along its length. In this manner, the rst portion of the line was cathodically protected in ordinary seawater while the latter half was protected in alkalized seawater. The proportion of sodium hydroxide was regulated at 250 parts per million of seawater. Over a period of 26 days, the rst part of the line drew current at a rate declining gradually yfrom' 0.045 to 0.033 ampere per square foot. No appreciable calcareous coating formed. The last part of the line drew current at a rate declining gradually from 0.020 to 0.010 ampere per square foot. A signincant calcareous coating was formed.

As the experiment showed, the effect of the sodium hydroxide addition was to lower the current requirement-of the line to about one-third of that called for in the absence .of the hydroxide, and also to increase the rate at which a calcareous coating formed.

Example 2 The wallsof a large iiume carrying seawater at a velocity o'r a few feet per second are constructed of bare corrugated sheet-steel piling. A direct-current source was installed to supply current to the piling as cathode in an effort to protect it against corrosion. Over a period of several months, current was supplied at the maximum capacity of the source, corresponding to about 0.035 to 0.04 ampere per square foot of piling exposed to the seawater. Corrosion was reduced but com-plete protection was not achieved, and the structure never became fully polarized.

With the source still connected, aqueous sodium hydroxide was fed continuously into the ume near its midpoint longitudinally. The 4rate of addition corresponded to 2.5 parts per million of seawater. The portion upstream from the addition continued to draw current as before, with only partial protection being realized. However, downstream from the point of addition, it was found that complete protection and polarization could be realized with -an ultimate current density of 0.002 ampere per square foot. Gradual formation of a calcareous coating took place over a periodof several months.

The `example shows that, with the addition off only 2.5 parts per million of sodium hydroxide, the current demand was reduced more than twenty-fold, at the Same time providing better protection.

Example 3 In a laboratory demonstration, identical steel coupons having a surface area of 18.7 square inches were weighed and then subjected to corrosion testing. One group of coupons were immersed in a vessel of seawater and current was supplied to the coupons as cathodes -at a current density of 0.005 ampere per square foot for a period of 17 days. At the end of this time, the coupons were removed, cleaned of corrosion, and reweighed. Average weight loss was about 1.15 gram.

In a parallel test, in which the seawater contained 50 parts per million of sodium hydroxide, after 17 days at 0.005 ampere per square foot, the average weight loss was about 0.29 gram. The addition of the sodium hydroxide to the seawater reduced the corrosion to one-fourth.

The electrochemical phenomena on which the invention is based arenot `fully understood. It is believed that the -addition of the basic hydroxide to seawater or like brine induces precipitation of calcium carbonate in colloidal form. This precipitate may then become transferred by electrophoresis to the cathode being protected. There must be other factors at work, however, since the addition of sodium carbonate, for instance, will induce precipitation of calcium carbonate but will not enhance coating formation.

It is realized that in the conventional cathodic protection of steel in seawater, some small proportion of alkali may be generated at the steel surface by the electrolysis. However, the formation in situ of such hydroxide does not produce the results according to the invention, which requires the intentional addition of pre-existing basic hydroxide from an external source.

Wht is claimed is:

1. 1n a process wherein a corrodible metal no more anodic than zinc in contact with seawater is protected against corrosion by applying electric current to the metal as cathode to produce a calcareous coating thereon, the method of etfecting such protection at minimum expenditure of electric energy which comprises the steps of dispersing a pre-existing water-soluble alkaline hydroxide throughout the seawater in an approximate proportion of from 2 parts to 300 parts per million and supplying current to the'metal at an effective current density not exceeding 0.02 ampere per square foot and less than that required to produce a calcareous coating in the absence of added hydroxide.

2. A process according4 to claim 1 wherein the metal is steel, and the hydroxide is sodium hydroxide supplied in a proportion of 25 to 50 parts per million.

3. VIn a process wherein a steel conduit containing owing seawater is protected against corrosion by applying electric current to the steel as cathode to produce a calcareous coating theeron, the method of eiecting such protection at minimum expenditure of electric energy which comprises the steps of dispersing a pre-existing strongly basic hydroxide throughout the flowing seawater in a proportion between 2 parts and 300 parts per million prior to passage thereof through the conduit and supplying current to the steel at a current density not exceeding 0.02 ampere per square foot and less than that required to produce a calcareous coating in the absence of added hydroxide.

4. A method according to claim 3 wherein the hydroxide is sodium hydroxide.

5. A methodV according to claim 4 in which the hydroxide is added in an approximate proportion to 2 to 10 parts per million and the current is supplied at an approximate current density of from 0.002 to 0.005 ampere per square foot.

6. In a process wherein steel in contact with seawater is protected against corrosion by applying electric current to the steel as cathode to polarizel the same, the method of effecting such protection at minimum expenditure of electric energy which comprises the steps of dispersing r D pre-existing sodium hydroxide throughout the seawater in FOREIGN PATENTS an approximate proportion of 2 to 10 parts per million 3 388 Great Britain of 1903 while supplying current to the steel at a current density of from 0.002 to 0.005 ampere per square foot, such OTHER REFERENCES density being less than that required to produce polariza- 5 tion in the absence of added hydroxide Water Works and Sewerage, v01. 89 (1942), pages 285-291, article by OBrien.

References Cited in the le of this patent 8;C0r1`0`i0sv 11 (October 1946), Pages 175 thru 1 artic e y u ra in. UNITED STATES PATENTS 10 corrosion v01. 4 (July 194s), pages 35s thru 37o, 2,444,174 Tal'r et al 111116 29, 1948 article by Humble. 2,534,234 COX DSC. 19, 1950

Claims (1)

1. IN A PROCESS WHEREIN A CORRODIBLE METAL NO MORE ANODIC THAN ZINC IN CONTACT WITH SEAWATER IS PROTECTED AGAINST CORROSION BY APPLYING ELECTRIC CURRENT TO THE METAL AS CATHODE TO PRODUCE A CALCAREOUS COATING THEREON, THE METHOD OF EFFECTING SUCH PROTECTION AT MINIMUM EXPENDITURE OF ELECTRIC ENERGY WHICH COMPRISES THE STEPS OF DISPERSING A PRE-EXISTING WATER-SOLUBLE ALKALINE HYDROXIDE THROUGHOUT THE SEAWATER IN AN APPROXIMATE PROPORTION OF FROM 21 PARTS TO 300 PARTS PER MILLION AND SUPPLYING CURRENT TO THE METAL AT AN EFFECTIVE CURRENT DENSITY NOT EXCEEDING 0.02 AMPERE PER SQUARE FOOT AND LESS THAN THAT REQUIRED TO PRODUCT A CALCAREOUS COATING IN THE ABSENCE OF ADDED HYDROXIDE.

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