Corrosions in the naval constructions

C

Corrosiôns

in

Fhe

naval

onsruciiOns

by Dott. M. GIANNOTTI by order

of.

Gen. Ispet'tore I.. BATTIGELLI

Jaruary 1964

Tech nfsche Hogesehool

Dclii

ITALIAN NAVY RESEARCH CENTER

-. SHIPBUILDING DEPARTMENT

ITALIAN NAVY RESEARCH CENTER

SHIPBUILDING DEPARTMENT La Spezia

Corrosions

in

ihe

naval construclions

by Dott. M. GIANNOTTI by order of

Gen. Ispettore I. BATTIGELLI

Acknowledgements

The author

.sh to acaowledge the

en-couragenient, guidance and. helfu].

sug-gestions of Generale Ispettore Genio

Navale Italo BATTIGELLI President of

"Comitato Progetti Navi" of Italian

Navy.

SHIPBUILDING DEPARTMENT

CORROSIONS IN THENAVAL CONSTRUCTIONS

Dott. Mario GIANNOTTi

(Italian Navy Research Center)

SUMARY

After a short account of the corrosive phenomena in the :flaval con.struotions,the author describes the methods adopted.

to reduce said; phenómèna in the. construction phase and the O1lowing protection methods judged most satisfactory both in:the comp].étion and *orking. phases.

Lastly are stated the results of experiences performed with densified electrolytes containing colorimetrlc reactives to reveal zone at nodic and cathodic proceeding on metallic

sfaces,

1. NOTES ABOUT THE CORROSION PHENOMENA IN THE FIELD OP THE

NAVAL CONSTRUCTIONS

1.1 - It is by this time known that the first cause of corrosion

in the steel

constructions is due to the presence of sheeting oxydes film (calamine) on the plates and on the sections..

When sea water, following to the inevitable ageing of the prptective paint fIlm,, can reach a solution of cOntinuity in the layer ofc1amiÜe(orhemimorphitO) microbatteries are originated into which calamine acts as a cathode and

the bare steel as an anode! :

'..

The final result consists in the pitting of

the plates in

the anodic points and in the eparations. both of the cala ne and of the paint in the cathodic zones, following the

developing of hydrogen.

The fact is tht once formed the pi'ttings following the.

generation of microbatteries, these last contiue to start working ever

time

they come to contact sea water cOnsecluents

1.

the reached permeability of the successive ;eriodical pain-tings.

1.2 - A second and important cause of corrosion is due to works of electric welding on a floating hull when special

precau-tions are not taken.

In fact, during the welding operations outputs and imputs of stray currents can be generated when long sections of the hull are used as return conductor.

In the points in which happens the outpat of the stray cur rent from the hull it is observed anodical proceeding (+). with consequent pitting while in the hull input zones., that is at cathodic proceeding (-) it is ascertained an attack to the paint either for saponification and subsequent solu.bi iization, or for detachment following the development of hydrogen: see Pig. 1 - 2 - ₃ enclosed.

Now1 if a bottom on which said Electrochimic phenomena have taken place Is painted again without first eliminating the residuating acids and basics by means of an accurate sand-blasting and successive phosphating, we shall have an unsati

sfaetry protection and the same phenomena will take place again within a more or less short space of time following the type and quality of the paints used and the Lnlber of

layers applied. Moreover, as soon as the total film of paint will become permeable and the sea water can reach the paint where the electrochimic processes were temporarily stopped, they will resume their damaging action spreadin more and more deep and serious when there is presence of calamine on

the plates of the bottom.

1.3 -.

Another source of corrosion is that the Galvanic type cur-rents either for coupling of metals of different nature or forthe arising of localized p.d. The first case is well known though sometimes It is forgotten, but not so well the second one, so that many corrosion phenomena apparently unaccounta-ble are currently attributed to stray currents of the electric

plants and equipments on board. Today, on the cóntrary,it.. --possible to observe for ijistance that two steel plates ap-. parently from.the point both analitic and micrográphlc, once

coupled by riveting or welding, may assume by contact with fresh water, and worse with sea water,one anodie'progres,sing and the other cathOdic progressing.

Or it is possibe to ascertain the establishment of p.d. ioôãlized on structure.s undergoing a strain also if the

mi-crographic examinatiàn. doe. not show a marked sensibility to interoristalline corrosin; We shall go back ).ater tO this

subject.

1.4 - Last cause of corrosion for a working:ship is that of strays of the electric plants and equipment on board, parti cularly from the side of "earth" bondings of the electronic equipment if not .provieed out of the hull through a su.ita-ble system.

2. 1rETHODS TO PREVENT CORROSION DURING THE COMPLETING WORKS

-WITH FLOATING. ±iUJL

After. sorne years of experiences both in th 'laboratory and in the practical field, the following provisions have been established:

2.1 Preliminary treatment of plates and sections. 2.2 - Rules for electric welding operations.

2.3 Cathodic protection (CP) of the hull üring the com-pleting works.

2.1 -. Preliminary treatment of plate and sections. 2.1.1 - Steel

Steel plates and sections are sanded,cold.phosphatized and protected with unlnflam.able paint, of 'the zinc chromium type fit both for steel and for light alloys.

The same paint is used for the. proper touch ups in the

shop works and on board. . .

On the bottom plates, before the launching, are applied two layers of anti-rust grey paint composed with pigments resistent to alkali and acid aggression due to electro

chemical phenomena.

At any rate, the definitive treatment of the bottom is schedule4 at the end of the completing works (See 3.2). 2.1.2 - In light alloy (L.A.)

For the L.A. plates and sections it is previewed a treat ment of alodizing (ho± or cold system, following the sizes) which leaves on the surfaces a thin protective film of

cromate.

Successively a layer of Antic.L.A. is applied, executing touch-ups after the working in the shop and after and

during the installation on board.

Lastly and in the phase of progressed works after washing and degreasing, it is proceeded to the definitive painting with uninflamable type paints, both for internal and

external zones.

2.1.3 - Coupling between steel and L.A. structures

For said couplings, special precautions are taken.Very good results were obtained interposing a layer of Cold

Catalyzable Elastometres (of Neoprene type) applying it by brush before rivetings, and protecting later the ri-veting zones and the sorrounding ones by a layer of the

same product having a thickness of 1 + 2 rn/rn.

Successively it is gradually proceeded to the definitive painting like on the metallic surfaces.

2.2 - Rules for the electric welding works

In order to avoid the arising of stray currents during the welding works on a floating hull, it will be advisable to use single circuit weldini machines with bipolar cable.

The "earth plate" shall be near to the welding zone (See Fig. 4b enclosed).

5

2.3 - Cathodic protection (OP) of the hull during the completing period

Together with the rules set forth for the electric welding works on board, on a floating hull, the OP is applied by means of anode chains in magnesium alloy hung on the sides

of the hull and joined to the main deck by means of variable resistences which alloy the maintaining of the wanted poten-tial following the necessities of protectIon.

In Intermediate positions among the chains of anodes as well as at the bow and stern, at intervals are effected de-terminations of the hull potential in respect to a reference

electrode Cu/CuSO4: by properly adjusting the resistences of the anode chains values ranging between -0,8 and -0,9V. must be maintained. The scheme of the plant is referred in Fig.

4 enclosed.

Under these conditions the hull is struck by a protective current of about 30 mA/mg..

It must not be believed that once applied the "OP" on the hull it be possible to avoid to follow scrupolously the

rules established for the Welding works. Said rules must be followed; in fact, in some instances in which welding was effebted without precautions, pittings were ascertained in the plates, though the hull was under "CI"'.

Said ascertainment results clear when one thinks that an incidental output of stray current from a hull is of the minimwii entity of some hundredth of ampere, that is much

superior to the values of the protective current. The conse-quence is that for the whole duration of the output of cur-rent from the hull, there is localized corrosion of the

plates. The benefit of maintaining the hull under "CP" lies in the fact- that as soon as the stray current ceases, the zone pitted by the anodie current is at once protected by the cathodic current.

3.1 - Outboard, superstructureS and inside spaces

The protection is effected by applying

paints of non

in-flainable type in a number of layers such as to

guarantee a

very high degree of

impermeability of the total film.

The use of chlorine caoutchouc

uninflainable paints, as a

safety precaution, implies a rigid

selection of the products

to be employed. It is well known that the advantages of

uninflamability of the dry film implies a

minor protective

value in time in respect to the inflainablè paints of

alkyd. type.

3.2 - Bottom

The bottom protection must be very

careful as this is the

part of the ship most subject to

corrosions.

To eliminate from the bottom the

alkali imbibed paints and

the calcium magnesian calcareous

residual due to "CP" it is

necessary to proceed to the

following operations:

- wet sandblasting and washing with pressurized

fresch water;

- cold phosphatizing;

- painting..

For the final painting of the bottom

at the end of the

completing operations, two working

schedules are,

hat is:

3.2.1. - Grease ocle

It provides a primer preparation by

applying

three larers

of a special

anti-rust

light-grey colored, composed by

chlorine caoutchouc and syntetic resins

pigmented with

alkali and acid.-resistent products.

As finish, the cycle schedules the

application of

subma-rine paint of the

raseous traditional type that

is of tae

first layer of anti-rust and of the

second layer of

anti-fouling.

It provides a primer preparation by applying

three

layers of a special light-gray anti-rust, having similar characte ristics to that of the grease

cycle,

but reciped so as to be totally compatible with the finishing vinyl submarine paints, that is the Grey Vinyl Primer, with acid and alkali resistent primer, and. vinylic antifouling.

This painting cycle offers good results only for long careening intervals.

4. CATHODIC PROTECTION WITH WORKING SHIP

The OP on

a working hull can be obtained by two systems that is:

4.1 - Impressed current with automatized plant

4.2 - Sacrifical anodes made with different alloys. The impressed current method,

while

easily applied to

moored hulls, was found unfit for working ships as many dif-ficulties were found in the efficiency of the current di-spersives and in the reference electrodes applied to the bot

torn.

For active

ships

the sacrifical anodes method was found to be the easily applicable one, diseerding the electrolitic zinc as easily passirable, and the magnesium alloys as they give origin to saponification of the paints or to their

frilling by the formation of hydrogen: the zinc-mercury alloy is, at the moment the most useful because it is not

pas-sivable and is self adjustable in respect to the protective action.

The zinc-mercury alloy anodes are secured to the hull fol-lowing the method described in Fig. 5 enclosed, and their number is established so to keep the hull potential between

-0,85 and. -0,9V in reference to the Copper/Copper Sulphate electrode.

Together with OP is necessary a proper painting, and if it is applied to fast hulls it necessary a particular prepa

The first experiences carried on with epossidic 1i.:iings se'em to give good results.

EIEERIENCES ABOUT CATHODIC AND ANTIPOtJLING PROTECTION BY MEANS OP SPECIAL LIIUNGS

It was tested an inorganic lining based on zinc powder and toxic salts, including mercury salts.

Said lining is made, on sanded plates, by app] ng: a

primer and a finishing layer, the whole gives a good OP due to the presence of iperpure metallic zinc, and a good anti-fouling action due to the presence of toxic salts and of toxic substances electroeheinically originated.

The first results on a hull were satisfactory, still it was observed that in the zones of the hull gathering a high

quantity of protective current, as the stern for the presen-ce of bronze propellers and. the sea water inlets of the va-rious internal circuits, it is advisable to place some zinc-mercury alloy anodes, not to weaken quickly the antifouling

finishing layer.

CONSIDERATIONS ABOUT CORROSION OF ANCILLARY EQUIPMENT The following are notes about corrosion phenomena met during the service of ships.

6.1 - Shaftings

On the shaftings in the zones between the Stainless Steel and on said Seats (lining of the strain zone) corrosions of a coisiderable entity are often verified.

The corrosions in the zones between the seats both in the part inside the hull and in external one, can be avoided with proper linings like Neoprene, Epoxic Resin and Polisti-rolic Resins, with intermediate lapping of fiberglass or with cold zinc paints and lapping with self adhesive tape of syntetic type having high dielectric rigidity, which Is

furtherly protected with

grey anti-rust for bottoms and sub-marine paints.

Prom experiences made in .1963 it was possible to observe that the corrosions on the stainless steel may take place also independea.tly from the electric leaks on board and from the galvanic currents deriving from contact of metals of different nature.

By means of thickened electrolytes containing colorim.etric detectors of the metallic ions in the zones having aiodic processing, and by means of detetors of very light alkaline concentrations in the cathodic zones, it was observed that the stainless steel, of the type commonly used for the lining of the strain zones (seats), shows anodic points and cathodic zones because of localized p.d. In.the

anodic points,

specially when the shaft in motionless, the contact with sea water fa c.iliates the formation of small pittings. See photo 1

enclosed

Besides, performing the experiment with the same electro-lyte but coupling a small sample of the same steel with the bronze of the staves supporting structure, it was observed on the steel examined separately when (as it normally happens) the bronze is slightly cathodic in respect to the seat

stainless

steel. See photo 2 enclosed.

On the contrary

performing

the test by coupling the same sample of steel with a sample of white metal of the type normally used for the construction of anti-friction tiles, it was observed that the anodic points - already seen in the first test over described - disappear, because being the white metal anodio in respect to the stainless steel, the first one exerts on the second a cathodic protection. This explains

bow, when white

metal tiles were used, pittings on the seats were very rare, and due only to electric stray on

board. See photo .3 enclosed.

support is anodic in respect to the sta±ñless steel of the

seats.

These two last experiences set

clearly forth

the

ixaportan-ce of applying a safe "CP" to the hull during the ship's working, by means of proper anodes made with not passivable alloy.

The pittings on the seats could be 3riginated only in the case of stray currents from board, and only in the case in which these are of an entity higher than that of the pro-tective current supplied by the sacrifical anodes applied on the hull.

6.2 - Propellers

Corrosions on propellers made of bronze or special brass are normally attributed to cavitation effects.

Or in the cases in which were observed phenomena of super-ficial dszincification and apparition of isolated spots

copper, it was spoken of anodic

behavior of

the propeller in

respect to

the shaft, particularly when this was made of stainless steel, or monel, as for instance of fast hulls.

Having observed, during the experiments made with revealer electrolyte, that the special iron containing bronze for propellers, shows many superficial inicrobatteries, having dispersion of iron ioni i:'. in the anodic zones and slight surfacing of copper in the cathodic zones, and having also

observed

that nikel, copper,

monel atid brass sheets

became anodic along their rims, due to the mechanical effect of a shear or in zones precedently subjected to slight bendings, it was

thought of

carrying on the following experiences:

a - Select two test pieces of special propeller bronze, which tested into the electrolytic revealer should result free from active microbatteries on their surface.

b - Keep quiet one piece and subject the second o alternate bendings with vertix deflection of +

2,5 iom. and + mxn.1O

0,1 and 0,2 Kg/minq..

c - Observe again both test pieces, by iinnierging them for 48 hours into the electrolyte revealer.

d - Wash both pieces, thenrepete the strain on the formerly stressed piece, and observe again into the electrolytic revealer.

Since the first test the strained metal showed slight

PDs on its surface. The

phenomenon wae

gradually and sen-sibly accentuating by the repetition of the strain tests.

Photo n. 4 enclosed shows the difference in behavior of the two test pieces at the examination into the densified electro lyte; the upper piece is the inactive one while the lower one had been subjected complexively to 7 stresses of 1000 cycles each with a deflection of +2,5 mm. and to 1000 cycles with a ± 10mm. deflection.

Photo

n. 5 enclosed shows the appearance of the same

pieces the lower one having been subjected to 10.000 cycles with deflection f + 10 mm.

Photo n. 6 enclosed shows the appearance of the same

pieces washed after the test shown in photo n.5: on the piece subjected to stress it is visible in the most strained zone a pink spot due to copper coming to surface aorrounded by micropittings noticeable

only

under high magnification.

The result obtained

in

these first test may let suppose that on the propeller blades, subjected to stress of about 7

Kg/Inxaq., can take place similar corrosive phenomena of

electrochemical origin independently from the erosive effect of the real cavitation.

It must be supposed that said. electrochemical phenomena can be practically avoided by applying a proper cathodic protection, as it was ascertained by the following test: the piece after being subjected to the aforesaid stresses, was immerged into the electrolyte after having applied through one of the bores a small pin of zinc-mercury alloy. The mi-crobatteries did not manifest and the whole surface of the

test piece became red colored by cathodic behavior. It is now under programmation a new set of tests,

subjecting the pieces to stress possibly of the same

anipli-tude of those to which are the blades of propellers for

high

speed hulls. 6.3 - Sea water circuits

In the sea water circulation for uses on board (hygenical, fire stations, etc.) corrosions take place caused by galvanic currents due to the different metals existing in the circuits (pipings, valves, pumps, etc.) and by anodic behavior due to vortex in the section of the highly bent pipings.

The only system which resulted fit to avoid corrosions Is that of providing a "OP" by zinc-mercury alloy anodes not passivable secured into proper boxes spread along the

cir-cults.

Among the anodes, the anode containers, and all flanges

preserr

in

the circuit it resulted necessary to constitute a safe electric connection applying a copper wire between the bolt head on one side, and the stop nut on the other side, suôh connection is

sufficient

when it is effected at least on two bolts of each structure.

As to pumps, also if of the limnersion type which are

subject to fast corrosion, due to the presence of composition metals having different structures,,it resulted satisfactory

a "CP" niade

With

zinc-mercury anodes placed forward and afterward the pump, into suitable casings.

The setting of the anodes must be done in easily reachable positions for an easy substitution as soon a it is necessary the securing system of the anodes is shown in fig. _{n. 5}

CORROSIONS DUE TO S!RAY CURRENTS ARISING FRI ELECTRIC WELDING

Example of welding carried in an irrational systea:

--i

C _{stray currents}

single generator -"

"ifsingle generator

dock

The earth plate(s)

is

very far from the

welding zone (p):

this

may originate stray currents on the quickwork.

In the output zones (1) with anodie ihavior,by the passing into

solution

of Peioni, is originated plate pitting;in the

input

zo-nsa (2) having cathodic behavior there is atraction of

Mg

and

H4ioni,with deriving phenonena of saponification and

solnbklization of

the greaae type paints,or detaching by develop ment of H2 in the case of unsaponificable linings. See partioular

2a.-Detail of fig. n. 1

Eleotrochemical ph.nomen* on the quickwork plate.

hull plate

Fi. 2

M

blister

re-exietin welding seam ++

In the point (1) snodio,it is verified detacheinent of F. iont and for * nation of F.C12,while in point (2) cathodio,besidee to alterations of the paint it can be verified a deposit of calcium-magnasiac calcareous real -duatee.

With the ceasing of the stray current,00rresiOfl will continue in point (1) because microbattery effects due to differential s.reation:on the bottoa of the pit (+) F.Cl2 is formed while F.(OH)2 precipitates on the edges (-).

In point (2) on the contvary corrosive phenomena of wide superfi-cial type can be originated when the plate is decalamined before painting. If the quickwork plates have not been sanded,and so the caianine layer is still present,the localiz?d corrosion phenomena will be more evident with a tendency to the formation of deep pittings.

Fig. paint calamine '

F

- - -. .

. aw

- A. . . 4. . . .

A

-4g seam In fact in point (1) besides the differential aereation corrosive phenomena it will be had anodloityin the steel and cathodicity in the calamire with quicker deeanlng of the pit originated by the

stray cur

rent: in point

(2) where calamine has been cracked by develomerit of H2

more microbatteries will start acting with the formation of new

pittinge.

When the hull is kept under CP'' said secondary phenomena do not show when

stray

current disappears.

CORBOSIONS DUE TO STRAY CURRENTS ARISING FR( ELECTRIC WELDING

Exmple

of stray currents produced by tultiple

welding machine:

Pig. 3

----m1t1ple welding

maohtns

With a circuit having nultiple

working atations

and a

single

ta.arthplatet

(a) when

the workers are

welding in places (p) distant froli the point (a)

stray currents nay be originated on quiokwork.

In the output and imput zones the phenoiziena

already illuatrated in

originated.-CATHODIC PROTECTI9N 'CP'' BY bIEANS OF CHAINS IIADE OF bIAG1ESItM ALLOY

Fig. 4a

Rvariable resistences inserted between ehi and anodes

1-2-3-4-5-6-7-8 positiOns of the chains with anodes

AyB-C-D-E-F-G-H- positions for the tneasureent of potential with reference anode

RULES' FOR ELECTRIC WELDING

Fig.4b

vvvv _', v,'V

adivisable connection

with single workin

unadvi9able

nne

with bipola

cable.

place,

Togetbar with the ''CP'' it is necessary to use single welders having bipolar cable and to

weld onlywith

''earthplate'' (-) in

CATHODIC PROTECTION ON A WORKING SHIP

METHOD OP PASTENING THE ZINC ALLOY ANODES Pig. 5

__4Course of stray currents with. sea output only through the zinc alloy anode

The rubber sheet 4 + 5 mm. thick under the anode is used to avoid water seepages: if these should happen they would cause a reaction around the pin with consequent reduction of operating power,and

sornetimes,loss of the anode during the navigation.

Once tightened the nut,it is applied by a small brush a layer of insulating paint to cover the pin head,the nut and the uncovered part of the washer. The electric contact bet"n the pin and the anode takes place through the washer and the stop nut;Before apply ing the zinc alloy threaded plug,it is necessary to check that the threads are well clean and without covered parts.

avJ.ng set the anode,it will be checked the electric continuity between it and the structure to be protected; should the checking give a negative result,it will be necessary to detect and eliminate the cause.

The anode so secured to the hull,beside.s to exert the "CP" Is used also as output from the hull of possible stray currents without

damaging either the pin or the stop nut,as. on the contrary happens applying the anode by the old system of bare pins and bolts.

When it be interesting to have a lower protective current it is sufficient to substitute the steel washer with a lead one of proper thickness which acts as a resistence.

The use of pre-fabbricated anodes with iron bars inoorporated,to be welded to the hull after having pu..t between a rubber sheet is unadvisablë for the following three reasons:flrst,tO avoid

weake-ning of the structures because it is necessary to wad everytime the anode is substituted;seCofld,because in case of output of stray cur.-rents from the hull it could be possible to have anodic corrosion of the stop bar and possible loss of the anode;third,becaUse in ca-se of permanence in a damp store reactions can happen between bar and anode with development of hydrogen,the danger of which is well

known.-readed plug

LO

the

same

II zinc alloy anode

fly safetey...., lower nut

/

_ØJmpiNp

washer i rubber sheet hull j aint - c /

_______ ___________

Photo 2 - Coupling of the same type steel with bronze of the staple bearer support.The steel results anodic and thence the

phenornena in photo I are

magnified

In the opposite side,that is when bronze is anodie

in respect

o steel,this last

is cathodically protec-ted by bronze;consequently the localized p.d. put forth by test i photo 1 disappear.

ged free into the revealer electrolyte it shows

localyzed pd., due probably to the geating effect during the axle lirrg

Photo 3 - Couplir.g of steel of the same type with antifriction

white metal for tiles.

The white metal,beir.g anodic in

respect to

steel,

exerts a protective action on this

last and

consequen-tly on it disappears the

localyzed p.d. put in

eviden-ce in test shown in

photo

if'.

Photo 4 -

Brass test pieces for .propelle's.

On the lower

piece are visible localyzed p.d. due to stress effect.

after further stress of 10.000 cycles

of 10 mm. on the lower piece.

rote the increasing of localyzed p.d.

Photo 6 - Brass test pieces for propellers.View of the two test pieces after test indicated in photo 5,drawri

out froci the revealer electrolyte and washed with water and alcohol.