US3001919A

US3001919A - Methods for protecting immersed metallic structures against corrosion

Info

Publication number: US3001919A
Application number: US836543A
Authority: US
Inventors: Petrocokino Denis Dimitri
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-08-27
Filing date: 1959-08-27
Publication date: 1961-09-26
Anticipated expiration: 1978-09-26

Description

This invention relates to the protection of metallic structures against corrosion by chemical agents, more particularly for use with structures immersed in heavily I corrosive media such as sea water brine. This application is a continuation-in-part of U.S. Patent application "Ser. No. 606,221, filed August 27, l956,'now.abandoned.

The problem of corrosion-protection of metal,structures has been the subject ofextensive research and investigation for many years. The simplest and most obvious method is to coat the exposed surfaces of the structure with a simple isolating coating such as coats of paint and varnish of any of various kinds. Such coatings act merely by a mechanical isolation of the structure from the corrosive medium and are Wholly inadequatein' cases where the medium is at all corrosive, since any minute "defect or gap in the coating, and such defects or gaps .are inevitable, will initiate an attack which will of necessity expand with time to an ever-increasing degree.

Aside from this primitive method mentioned here for I completeness, there have been two major approaches 'to the problem of corrosion protection of immersed structures namely passivation and cathodic protection. Th e passivation processes are those wherein the ex- .posed surfaces of the structure to be protected are coated with non-metallic compositions 'so selected that certain constituents thereof will react chemically with the metal of the structure to yield reaction products protecting the surface against corrosion. A well-known example of p'a'ssivation treatment is so-called phosphatation, wherein the coating applied to the metal surface contains phos- ..phoric acid which on reacting with the metal will yield a phosphate forming a protective coating. over the surface. Such passivating treatments are open to a number of drawbacks. Firstly the resulting coating is relatively :thin, about 20 to 40 microns as an order of magnitude, and imperfectly bonded to the underlying metal. Moreover, such coating, even though it is initially insoluble in the surrounding medium, usually an alkaline medium such as ocean brine (pH 8 to 8.5), gradually undergoes incipient hydrolysis and amphoteric salts are released whereby the surrounding pH is reduced, so'that the coat- ,ing 'grows more and more soluble with time. It has in fact been found that the 01- ions tend to destroy a conventional phosphate coating in a matter of weeks. To prevent this it has been suggested that zinc and manganese compounds be added to the passivating compositions. 'But this has not materially added to the eifective time of protection as, regardless of the chemical nature .of the coatings, the inevitable small differences and unevenness found in them will lead to a formation of small isolated anodic areas that will tend to accelerate corrosion by generating small corresponding electric voltage differentials or couples. Thus passivating treatments have not succeeded in adequately protecting immersed metal structures beyond periods of a few weeks ormonths.

The second above-mentioned class of treatments, cathode protection, involves immersing in the surrounding corrosive medium one or more metallic elements spaced from the structure but connected in circuit therewith so as to act as anodes with respect to the structure acting as a cathode. In the intervening medium, which then behaves as an electrolyte bath, an electric field is created between the anodes and the structure, and the resulting electrolysis acts to deposit alkaline and .related metals upon the surface of the structure.

ments is quite high, requiring at frequent intervals.

ice

Where the corrosive medium is sea water, having an average ion composition of 10-12 g./l.'Na+, 0.4-0.5 g. /l. Ca++,'1l30-1.45 -g./l. Mg++, the deposit mainly comprises lime and magnesia. Suchcoating is'amorphous and continuous, i.e. non-porous, in character. However, the extent of the metal structure that is actually protected in this way depends narrowly'upon the extent of the electric field between it and the adjoining anode. In other words the protected :at a relatively small distance away. To overcome this limitation it has been suggested to use additional boosting anodes, serving to extend the'range of protection afforded by the main anode.

But such an expedient complicates the protective system and increases'the amount of metal required. 7

Another shortcoming of conventional cathode protection processes is that'the protective deposit is compara- Furthermore, the rate-of destruction of the anode ele} replacement of the anodes Ithas already been suggested that both above mentioned types of protection, i.e. passivation and cathodic protection be combined to reduce the, above drawbacks ments. Such prior attempts have-yielded more or less satisfactory results in the case of underground structures but have failed completely when applied to structures immersed in brine.

. This failure can be ascribed to the fact that heretofore ,the ingredients of the protective coating were selected in a more or less empirical and haphazard fashion. One

factor that has seemingly been overlooked in this connection is that as a result of the cathodic activity of the metal structure to be protected, nascent hydrogen evolves from it. If the protective coating originally applied to the metal surface is compact and nonporous, the hydrogen bubbles will form blisters and the coating is ultimately destroyed.- Also the hydrogen tends to attack various constituents of the coating such as binder and dyes. The alkaline medium also tends to destroy the binder by saponification. If on the other hand the initially applied .coating is porous, it does not provide adequate electrical insulation on the metal surface and no substantial reduction in the requisite current density and rate of dissolution of the anode.

It is therefore an object of this invention to provide an 'rosive media'such as brine. 7

.tion system wherein cathode protection and a modified .passivating coating are so combined that they will coact An object is to provide a compound corrosion-prevenin a synergistic, mutually-enhancing manner leading to greatly improved results both in the efliciency of the protection achieved and in the economy of operation of passivating coating for metallic surfaces that willbe both compact and continuous enough to provide electric insulation for the surface whereby current density and resulting rate of anode destruction will be reduced, while being thin and porous enough to prevent the build-up of substantial osmotic pressures liable to result in a. retention of hydrogen bubbles and blistering; and to provide such a coating that will be comparatively inexpensive.

It is another object of this invention to provide such a process wherein the protective coating produced by the electrolytic activity of the cathode-protection system will be crystalline, as opposed to amorphous, innature. Another object is to provide such a method wherein the formation of the protecting coating will be initiated by ,an initiator agent such that the protection of the structure will begin after a preliminary induction period of shorter duration than heretofore.

In an important aspect of the invention, the improved corrosion protection method comprises, in a first stage, applying to the surfaces of the metallic structure to be lprotected, prior to exposure of the structure to the cor- Irosive medium, a phosphating composition comprising "as essential ingredients therein phosphoric acid, at least one phosphate of a metal of group VI of the period table, and at least one phosphate of an alkaline-earth metal, allowing said composition to reactwith'the underlying metal'of the structure to substantial neutrality, fthen exposing the coated structure to said medium, providing anode means in said medium in spaced relation to said structure and connecting said structure and anode ,means in an electrolytic circuit with said structure as 'the cathode therein.

In a preferred aspect of the invention, the method fur- :ther comprises the additional step of adding an alkaline initiator agent into the corrosive medium to accelerate 'the electrolytic action.

A preferred composition for the acidic 'phosphating coating first applied to the structure prior to exposure 'to the corrosive medium comprises, per 100 parts by weight: about 45 to 55 parts orthophosphoric acid solu- "tion containing 55% P about 5 parts chromium 'orthophosphate, about 5 parts magnesium orthophosphate, about 1 part calcium orthophosphate, about 20 parts ethanol and about 5 parts triethanolamine.

The first stage acidic coating should have a pH value "not greater than about 4.5. It may be applied by dipping or spraying techniques. When applied by dipping 'the amount of coating composition required may be "about 1 kg. per sq. m. area, when applied by spraying about 1 kg. per 5 or 6 sq. m. are necessary. After the first acidic coating has been applied, the reaction 'between itand the underlying metal surface is 'allowed to proceed until the pH has risen to substantiallythe neutral value pH 7. This may require a variable amount of time depending on conditions, primarily on the surrounding temperature, and may take e.g. from about 24 hours or less in summer to twice that and more in 'winter. The progress of thereaction is preferably monitored by applying a pH sensitive element to the coated surface. By the time the pH is found to have attained its final value of about 7 indicating that all free acid in the coating has been neutralized, coating has also dried.

The second stage comprises, basically, immersing the structure passivated as just described, and associating with it a cathodic protection system which may per se be of conventional type. According however to the preferred aspect of the invention, there is added to the immersing medium adjacent to the structure being protecied an initiator agent the function of which is to promote the formation of the desired final coating of porous crystalline character. Preferably this initiator agent has "the following composition by weight:

About 35-45 parts trisodium phosphate About 35-25 parts monohydrogen calcium phosphate About 10 parts sodium sulfate and about parts dolomite The initiator maybe added .to the medium in which the structure is immersed in a proportion of say about to' 300 g./cu. m. This agent serves to accelerate the transition of the metallic surface potential to electronegative potential values. It will be understood that where the medium is alkaline in character, as is true in the particularly important case of brine, once the metal surface potential has reached the negative range all further dissolution of the metal will be completely and positively prevented, i.e. full immunity from corrosion will be achieved.

The objects and features of the invention will be fully understood from the ensuing description which refers to exemplary embodiments of the invention which, of course, are illustrative rather than restrictive. In the accompanying drawings:

FIG. 1 is a chart in which the surface potentials in the case of an iron-base structure are plotted as a function of pH values of solution in water as measured with a hydrogen electrode; and

FIGS. 2 to 6 are highly schematic illustrations of the probable theory of operation of a system according to the invention.

Referring to the chart of FIG. 1, the areas of pH and potential in which corrosion occurs have been indicated by cross fhatching. It will be noted that there are two separate such areas, but only that corresponding to the lower pH values is of practical significance and has .bcen shown completely. To one side, i.e. above, this main corrosion area is an area, designated I, wherein corrosion does not occur owing to passivation, while below it is an area II where corrosion is absent due to immunity. The corrosion area is bounded with respect to areal by a curve C1, and with respect to area H by a curve C2. Curve C1 represents the passing of the iron into solution as ions Fe in the upper part of the curve (sub-area A), and as ions Fe in the lower part (sub-area B), from the insoluble oxides and/or hydroxides of the area I. The curve C2 represents the passing into solution as ions Fe++ of the insoluble iron metal which comprises the area II. It is therefore evident, that where the pH value of an aqueous medium is predetermined, the potential values are also known which should 'be provided over the surface of an iron structure .immersed in that medium in order to ensure protection of said surface by immunity or by passivation. In practice, the curves C1 and C2 are somewhat displaced to one or the other side from their theoretical positions by amounts depending on the quantities of iron effectively dissolved. 'lhus, if the medium contains 560 mg./l. iron in solution, the curves defining the corrosion area AB are displaced in the positions C1 and C2; while if the iron concentration in the medium is 0.056 mg./l., the pertinent curves are as shown at C1, C2". In other words the medium is more or less corrosive according as it has less or more iron dissolved in it.

As is apparent from the graph, in the case of any acidic or reasonably alkaline medium liable to be cncountered .in practice, i.e. having a pH value not greater than about 8 or 9, theoretical immunity of an iron surface from corrosion will be achieved if the surface potential of the metal is reduced below a certain negative value which is about 0.62 volt as measured with -a hydrogen electrode. The process of the invention achieves this result and in fact succeeds in lowering the surface potential to a strongly negative range of about -O.9 to

.result is accomplished by the above-specified combination of steps which comprise, in effect, acting both on the electrolyte medium by addition of the initiator agent :thereto,; and .upon the metallic :cathode surface by applying the modified phosphating coating to it, and .simultaneously associating providing a simple form of conventional cathode protection apparatus without requiring any booster anodes.- r 1 Although it is not stricted to any particular theory of operation, I propose .to disclose. the broad lines, ofwhatl now believe this theory of operation to be. Reference will be had to FIGS. 2 to 6 of the drawings.

FIG. 2 diagrammatically illustrates the process occurring in a conventional cathodic protection system. S

designates the metal surface being protected, and A an anode electrically connected therewith by a conductor 0.

An electric field is created in the medium as shown by the electrostatic flux lines L, the intensity of the field decreasing from a maximum at the point 1 directly opposite anode A to a minimum ofrero at points 2 remote from said anode. Thus a protected area is created which has an extent L1 across the surface S and in this area corrosion is prevented due to the formation of an electro- "negative-cathodic deposit of oxides and hydroxides of the metals contained in the medium. The depth of the i-deposit is a maximum at the point 1 and drops to zero -at points 2; Aspreviously indicated the length Ll-is relatively small.

--As stated above it has been attempted to extend the length'of the protected area-by theprovision of booster anodes. -Such aconventiona1 system is shown diagrammatically in FIG. 3 where two booster anodes are shown at a. Their action is to extend the length of the protective coating as far' as a certain point 3 so that the total eifective length of the. protected area becomes L2. The improvement thus achieved is only temporary and bill}? at'the caster an additional'expense in anode metal and increasing complication. Still the protective deposit provided in this conventional way does have a more um- --form depth over--a--more extensive area, so that this known system has constituted a notable improvement over the simple form'depicted in FIG.2.

FIG. 4 illustrates afmajor difiiculty that has been encountered in earlier attempts at combining a cathode protection system of'either of the forms of FIG. 2 or 3 with a passivation coating covering the metal surface. in FIG. 4 the passivating coating is shown in the form of a continuous layer 4, while in FIG. 5 the coating is assumed to be a porous or discontinuous one. In the continuous coating of FIG. 4, the nascent hydrogen that evolves at thecathode surface S due' to the action of the electric flux lines is prevented from escaping and forms bubbles such as 5 in which the gas pressure builds up sufficiently to burst through the protective layer 4=no matter how firmly thisis bonded to the underlying metal.

This eifect inevitably results in destroying the coating. I Also in the case ofa relatively porous layer such as that shown in FIG. '5, the nascent hydrogen produced by electrolysis tends to attack the constituents of the coating and leads to destruction of it. Similarly the chloride ions contained in the brine generally attack the conventional phosphatingcompositions proposed heretofore further contributing to their rapid destruction.

conventional composition with a cathode protection sys- "temj is'thatthe particles 6 of the coating permit passage of the electric flux lines L between them with a relatively "high current density, leading-to a" rapid-rate'of wear of. the anodes. The particles Got a conventional coating may be assimilatedftoajmultiplicity of minute booster anodes such asa in FIG. 3.-

' j The. process of the invention is based primarily on the .applicants extensive investigationof the factors involved in the mannerof operation of the various conventional corrosion prevention systems as just outlined, which has I led to a recognition of the reasons believed responsible for their comparatively defective operation. According- 1y, an important aspect of the invention-resides in having desired that the invention be re- Another drawback of "the" combination of a porous coating 6 of :zdevelop'ed a 'fo'rrnof coating composition that will: elii'ninate these defects and thereby enable an effective use. of

.such a coating in conjunction with conventional cathodeprotection systems to secure improved over-all operation.

The novel phosphating coating composition of the invention is ofa crystalline character such that its porosity is suflicient .to allow escape of hydrogen bubbles and thus avoid the formation of blisters as shown in FIG. 4, while .This characterof the novel phosphate coating is .a

result both of the composition of the phosphating agent used; and of the provision, in the bath in which the structure is immersed, of the so-called initiator agent.

' "Due to the chemical analysis of the composition" used in providing the phosphating coating, said composition is not attacked by brine and thus provides an optimum base-layer for the formation ofthe deposit resulting from the operation of the cathode protection system. Preferably, the phosphating composition of the invention 'contains a synthetic resin of the type resulting'from polymerization of one or more monomers containing vinyltype unsaturation, e.g. vinyl chloride, vinyl acetate, or

.the. like, or one'or more copolymers thereof. The proportion of resin is not critical, but may be of the order of 5% by weight. This resinacts somewhat in the manner of a binder, and'while binding agents for imparting compacityto a phosphating coating have already been used,generelly in the form of fatty substances or somet mes synthetic resins, such resins have not apparently heretofore been used for the purpose of providing compact yet porous layers in the neutral or alkaline pH range, suitable as base layers, for cathodic deposits on metal structures.

. ...-As already mentioned,..inthe composite system of the invention full corrosion protection is achieved a very short time after installation of the system. After application of the phosphatingcomposition, neutralization and drying thereof, and immersion of the coated structure,

containing the initiator agent of the invention; it is found that the electric potential on the surface requires much less time to drop to animmunitylevel less than -O.62

volt (see FIG. 1) than in any conventional cathode protection system; also this level is attained and maintained more reliably than in the latter systems The short initiation period is a definite further advantage or theimproved method over prior methods wherein the metal structure lay open to a substantial degree of attack during the comparatively long initial period before protection was achieved.

. j-Further the protection is, more eificient and reliable 60 because as explained above with reference to FIG. '1 the surface potential is brought down to very low values 10f ..+.90;, to, -l'. 0 volt which j represent full theoretical immunity.

-- These-j advantages are illustrated by the following examples. In all. the tests to be described a standard test piece .was used comprising a. square of steel sheet .20 mm..to a side. andZ mm. thick. The brine had'the followingaverage composition inmetal ions: Na+ 12 g./l.;

Ca++ 0.-5-g./l.; Mg++ 1-.4 g./l. pH 8. The tests were conductedin brine at a temperature of 15 C.

i This example relates to a control test wherein the structure is coated with a widely used conventional phosphating composition, sold by the firm La Framalite by the name Framanol. Its composition is:

Percent Phosphoric acid 0 Ethanol w 20 Triethanolamine 6

Alkyl sulfate

0,5 Water Balance The test piece is coated with a coating 50 microns deep of the phosphating agent, and air-dried at 20 C. for 20 hours, whereupon the surface pH is found to be 5.

The testpiece is then immersed in a bath of the standard brine as specified above, and is tested at two hour intervals for surface potential with a hydrogen electrode, pH value and aspect. The results are in Table I:

,These control tests are blank tests in that they are performed in the absence of cathodic protection and serve to show that a very short time after immersion in brine a metal surface coated with a conventional phosphating composition is practically devoid of any protection whatever.

E A LE 2 This example illustrates the protection imparted by a conventional cathode system. The system used comprises a magnesium electrode immersed in the bath and electrically connected with the test piece. The results are the following:

Table II Time (hrs) Poten)tlal pH Surface aspect 90. 25 8 Qlean bare metal. 0. 35 9 unaltered. o. 55 10 Do. 0. 75 11 130.

The table shows that protection is achieved about 5 hours after immersion. The protection will last for so long s magnesium em i s ailab e n th a od the rate of dissolution of the magnesium anode will be described later.

EXAMPLE 3 p This test relates to the process of the invention involving simultaneous use of a modified phosphating coat and conventional cathode protection.

The testpiece is coated with a 50 micron layer of the following composition:

Weight percent 50 The coated testpiece is air-dried 20 hrs. at 20 C. At this time the surface pH as measured with a moist pH paper'is 7 and its surface potential ..-0.20-volt.

The testpiece is immersed in the brine. The. test results are:

Table III Potential Time (hrs) E H2 pH Surface aspect (volts) O 0. 20 7 White cement-like. l D. 62 8 unaltered; 2 i. 0. 75 10 Do. 8L 0. 85 11 D0.

The table shows, first, that the immunity potential level is attained within a much shorter time than in the absence of the coating, and that the potential level reached is substantially lower.

EXAMPLE 4 i xa l is en a il sstta as h m at en Pa played b h ni iat r sse st lfld 1 1 h 5 medium in accordance with the preferred method of the invention.

Example 3 is repeated, but in addition the following m t s de i t the r m n a mr t e Qt s ee Trisodium phosphate- Monohydrogcn calcium phosphate nu k Sodium sulfite l0 -""TTT'T "TTf ""T The following results were noted:

Table V Potential Time (Mn) E/Hz pH Surface aspect (volts) 0. 2 7 8 DO. Ui65 0 Do. 0. 10 Deposit-coated.

1.0 ti Do.

It will be seen from this table that the time required to reach the immunity potential of 0.6 volt has been drammatically reduced by the presence of the initiator agent of the invention, from 1' hour in Example 3 to l s han 2. min te EXAMPLE 5 This example serves to compare the rates of wear or dissolution of the anodes in the conventional cathode protection process and in the improved process of the invention.

The above Examples 2, 3 and 4 are repeated. In each case the initial weight of the magnesium anode is 5 grams. The anode is weighed at intervals and the results obtained are shown in Table V, where the columns A, B and C refer respectively to the'above Examples 2, 3 and 4:

Anode weight (g.)

on (PFSJ The table shows that at the end of 300 hours immersion the anode sustained a loss of weight of 17.8%.with the conventional process, 1.8% in the simplified version of the invention and 1.4% in the preferred form.

9 EXAMPLE 6 This example will illustrate the dilierence in efficiencies between the conventional method and the methods of the invention when applied to the protection of seagoing vessels, specifically hydrocarbon tankers. Tanker ships are frequently required to carry a ballast of seawater on their return trip after having delivered their cargo of crude or processed oil to destination. The corrosive action of this ballast brine upon the internal steel structure of the ship is very severe especially because of its intermittent character, the periods of brine alternating with periods of hydrocarbon.

In a first ship, the Chenonceau, a 30,000 ton tanker chartered by the shipping firm M.P., a cathode protection system comprising 300 anodes each containing 20 kg. magnesium was installed on April 1, 1955.

In a second ship, the Porthas, a 30,000 ton tanker chartered by Mobil Oil, a protection system according to the invention was installed in November 1955 including a 300-anode system similar to that aboard the Chenonceau. Moreover the ships hold was coated with a phosphating composition according to the invention.

In a third 30,000 ton tanker, the Isanda, chartered by Shell Oil, another system according to the invention was installed.

The Chenonceaus electrodes has to be replaced in May 1957, after 25 months service. The Porthos electrodes were replaced in October 1958, 35 months service; and the Isandas electrodes were still satisfactory as of July 15, 1959, 41 months after they were installed. Examination of the hold walls of the three ships when empty further revealed that in the Chenoncezm the steel surface was reddish in color and granular; in both the Porthos and Isanda the surfaces were a sound whitish pink and smooth.

Thus tests under actual service conditions have fully 10 confirmed the laboratory results of the previous examples.

What I claim is:

1. A process of protecting a metallic structure immersed in a liquid from corrosion, said process comprising the step of applying to the structure a protective aqueous coating containing per 100 parts by weight substantially 45 to parts of orthophosphoric acid containing about 55% P 0 substantially 5 parts by weight of magnesium oithophosphate, substantially 1 part by Weight of calcium orthophosphate, substantially 20 parts by weight of ethanol and substantially 5 parts by weight of triethanol amine, the step of drying the surface of the structure after the protective coating has reacted to neutrality, and the step of immersing the structure and a cathodic protective anode in the corrosive bath.

2. A process as claimed in claim 1 including the further step of placing in the corrosive liquid an initiator comprising a mixture containing per parts by weight substantially 35 to 45 parts of trisodic phosphate, substantially 35 to 25 parts of mono acid dicalcic phosphate, substantially 10 parts of sodium sulfite, and substantially 20 parts of dolomite.

3. A process as claimed in claim 1 wherein the protective aqueous coating is applied in the amount of about 1 kilogram to 5 to 10 square meters of surface.

4. A process as claimed in claim 2 wherein the quantity of the initiator is about 100 to 300 grams per cubic meter of the corrosive liquid.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A PROCESS OF PROTECTING A METALLIC STRUCTURE IMMERSED IN A LIQUID FROM CORROSION, SAID PROCESS COMPRISING THE STEP OF APPLYING TO THE STRUCTURE A PROTECTIVE AQUEOUS COATING CONTAINING PER 100 PARTS BY WEIGHT SUBSTANTIALLY 45 TO 55 PARTS OF ORTHOPHOSPHORIC ACID CONTAINING ABOUT 55% P2O5, SUBSTANTIALLY 5 PARTS BY WEIGHT OF MAGNESIUM ORTHOPHOSPHATE, SUBSTANTIALLY 1 PART BY WEIGHT OF CALCIUM ORTHOPHOSPHATE, SUBSTANTIALLY 20 PARTS BY WEIGHT OF ETHANOL AND SUBSTANTIALLY 5 PARTS BY WEIGHT OF TRIETHANOL AMINE, THE STEP OF DRYING THE SURFACE OF THE STRUCTURE AFTER THE PROTECTIVE COATING HAS REACTED TO NEUTRALITY, AND THE STEP OF IMMERSING THE STRUCTURE AND A CATHODIC PROTECTIE ANODE IN THE CORROSIVE BATH.