SHIP MATERIALS

STRESS CORR ❖ Combined effect of tensile str metals and a particular co environment. Only when together and only on certain m • E.g. stainless steels and ch ions, brass and ammonia. ❖ Small cracks form and pro perpendicular to tensile stress, is brittle - no or little deformation. ❖ Stress may be external OR in residual stress. ❖ To avoid check combinations of and environment, keep stress and if necessary, stress-relieve.

ROSION CRACKING ress on Photomicrograph showing orrosive intergranular stress corrosion cracking in brass. both metals. chloride opagate failure plastic nternal f metal ses low .

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CATH ❖ Used to protect steel ship hulls from ❖ eg. a common manganese bronze p hull, and is an example of bi- immersed in corrosive electrolyte, hull would corrode very fast and i had cathodic protection. a) Bi-metallic couplings connected plates attached to the hull along b) The zinc plates placed close electrolytic seawater. c) Cathodic protection of the steel The zinc slabs corrode at a faster they protect. The zinc plates can the service life of both the prope ❖ For marine structures aluminium a anodes because their higher pote metallic structure being protected where the metal with the lower becomes the cathode, offering cat corrode at a faster rate.

HODIC PROTECTION m such reactivity and corrosion. propeller is usually very close to the steel -metallic coupling of differing metals, seawater. Under this condition the steel its service life would be smaller than if it d electrically by zinc cathodes. (thick zinc its length. to the propeller and immersed in the hull as the zinc assumes an anodic role. r rate than the steel hull or propeller that n be replaced on a regular basis extending eller and the steel hull. and zinc are commonly used as sacrificial ential is more reactive than the marine d. This sets up an electrochemical cell r potential, being the marine structure thodic protection while the zinc anodes

IMPRESSED ❖ The impressed currents are provided by number of inert anodes to the area to be p discussed. ❖ Impressed current systems can be used fo protection for hot water tanks, cooling metallic networks and antifouling mechan 1. Microprocessor control. 2. Controlled adjustable protection. 3. Long lasting protection of vital comp 4. Long - term reliability. 5. Lower maintenance and repair costs. 6. Increased service life. 7. Combined antifouling protection. ❖ When corrosion does occur, it occurs in enabling pre-emptive action to be taken. ❖ The impressed current protection system 1. Impressed current systems force a pr 2. It enables the service life of the prod 3. The zinc plates used as sacrificial ano 4. Preventative maintenance schedules of certainty and reliability. 5. It allows for corrective measures to complete failure eventuates

D CURRENT SYSTEM y a power-generating source through a small protected in much the same way as previously or many other shipboard applications such as pipe systems, boilers, power plant and other nisms giving the following benefits: ponents. . n a galvanic cell, uniformly and predictably, has the following benefits: rotective current to flow. duct to be estimated. odes can be replaced on a regular basis. to be planned and programmed with a degree be implemented, before severe conditions, or

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❖ Corrosion takes place in a galv in contact with a corrosive el reduction in corrosion occurs ❖ The anode end will be reactive experience a reduction react intact by the application of cat

GALVANIC CELL vanic cell comprising of dissimilar metals lectrolyte at the anode (+) end, while a at the cathode (-) end. e and corrode, while the cathode end will tion, remaining protected and relatively thodic current.



GALVANIC CELL

PROTEC ❖ Because of the high zinc con manganese bronze, (fasteners or “should not” be used for exposed submersion in seawater, as they w ❖ Instead, aluminium bronze, sil should be used. However, preca they are not, in contact with d corrosion protection strategies a protection, impressed current sy as epoxy paint have been used. ❖ Austenitic stainless steel faste aluminium components such as climatic conditions and seawater with aluminium, stainless steel, g ❖ Copper, copper alloys, brass a aluminium otherwise severe galv

CTION STRATEGIES ntent of brass, navel bronze, and r fittings made from these materials) d to water applications and complete will corrode at a rapid rate. licon bronze, or phosphor bronze autions must be taken to ensure that dissimilar materials, unless adequate are in place such as, zinc cathodic ystems, or preventative coatings such eners should be used to secure masts, out-drives, fittings exposed to r as long as they are only in contact galvanised steel. and bronze must not be joined to vanic corrosion will occur.

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PR ❖ Metallic coatings- are applied to depos nickel, copper, cadmium and zinc, to th protect from corrosion such as steel, ferr the prevailing climatic conditions. - T resistant layer protecting the part at the ❖ Some of the methods used are the app hot-dip galvanising and flame spray coa ❖ Electroplating - an electrical current solution containing the part to be pl electrochemical cell that forms. The part into contact with plating the surface wit ❖ A variety of metals can be used for bronze, cadmium, copper, chromium, i and silver. ❖ Efficient electroplating needs the followi 1. purity of the bath solution; 2. temperature; 3. length of time the part stays in soluti 4. amperage; 5. voltage.

ROTECTIVE COATING sit thin layers of metal such as, chromium, he base metal of the marine structure to be rous alloy often caused by the seawater and The coating provides a durable, corrosion e expense of the coating medium. plication of zinc rich paints, electroplating, ating. t is passed through a dissolved metal ion lated, which becomes the cathode in the t derives its ions from the solution it comes h a thin barrier coating. electroplating such as aluminium, brass, iron, lead, nickel, tin, zinc, gold, platinum ing parameters must be controlled. ion;

PROTEC ❖ An Electroless nickel (EN) p reduction process and a metal p ❖ The nickel is placed in an aqu that causes the nickel ions to flo ❖ The solution of nickel ions is a a combined mixture of disso ferrous metal alloy is placed in the part being coated in the for thickness of the plating is uni faces and parts. It is a measure ❖ For galvanic protection Coppe nickel because of its higher pos ❖ EN plated objects are expensive those that have been nickel - pl

CTION STRATEGIES plating process uses a chemical catalytic plating medium such as nickel in solution. ueous solution containing a reduction agent ow through the aqueous solution. agitated thoroughly to ensure uniformity of olved nickel ions and solution. When the nto this mixture the disposition adheres onto rm of a consistent plating barrier coat. The iform to all internal and external features, of its corrosion resistant qualities. er alloys are sometimes coated with (EN) sition on the galvanic series. e but generally superior when compared to lated using the electroplating process

HO ❖ Hot-dip galvanising is a pr coating to cleaned, oxide fre molten zinc bath, leaving a metal to form an alloy of the ❖ The zinc forms a galvanic co it from corrosion for long depending on the thickness a environment involved. ❖ Over time where damage disturbed, the zinc will sacri waste away. Due to galvani form. And, if left untreated will be consumed in the corro ❖ Hot –dip galvanising is used ferrous metals are used suc sheet metal parts, piping, cl decking and many other app and rust resistant barrier coat

OT DIP GALVANISING rocess of metallurgically bonding a zinc ee, ferrous metal parts, by immersion in a zinc coating that is bonded to the ferrous base metal. ouple with the ferrous metal, and it protects ng extended periods and often decades, and quality of galvanising and the corrosive occurs or the zinc galvanising coat is ifice itself to protect the ferrous metal and ic corrosion, rust will eventually begin to for an extended period, the ferrous metal osion process. for many shipbuilding applications where ch as fabricated components, cargo holds, ladding, exposed tubes, rails, balustrading, plications that require a durable corrosion ting.

HOT • The cleaning process is very acid bathing • Acid bathing of the parts e rinsing and drying.

T DIP GALVANISING y important in preparing the part for ensures scale and rust removal before

HOT

T DIP GALVANISING • For chemical cleaning the parts are immersed in a hot flux solution of zinc ammonium chloride • Parts are galvanised by immersion in molten zinc hot-dip galvanising bath. • After galvanising the parts are chilled to form a bonded zinc coating.

HOT ❖ The galvanising - protects th environment by giving Cathodic ❖ An irregular, complicated shap piping and tubes are equally sui ❖ Its products have a high resist anti-corrosive properties in a th base metal, eliminating mainten ❖ After careful surface preparatio as etch primers, secondary coat and other such protective coatin for extra durability or decorative

T DIP GALVANISING he ferrous metal from the corrosive c and sacrificial protection. pes, including internal surfaces such as itable for hot-dipped galvanising. tance to mechanical damage as well as hick zinc coating seal alloying with the nance requirements for long periods. on and special adhesion treatments such tings can be applied. Epoxy based paints ngs can be applied over the galvanising e requirements.

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ACK CEMENTATION

❖ The thermal spray coating process is used to and extend its service life. ❖ There are three methods of thermal spray coa 1. Flame spray using combustion torch. 2. High-velocity oxy-fuel (HVOF) and detona 3. Electric wire arc and plasma arc spray. ❖ The process involves the use of a high-ener (HVOF) together with powdered particles of heated to high temperatures until they reach ❖ A stream of compressed air or gas is used t prepared substrate to which they adhere an striking the substrate the powdered particles layers that need sealing before use. ❖ As well as powdered materials, rod and wir thermal spray coating include metal, ceramic piston rings, gas turbine blades, steering sh rotational vanes and many parts exposed to sa ❖ The steps in the thermal spray coating proces 1. Substrate preparation. 2. Masking. 3. Fixturing. 4. Coating. 5. Finishing. 6. Inspection

THERMAL SPRAY apply a metallic coating to the surface of the part ating. ation torch. rgy spray system, such as a high velocity oxy-fuel f metal, ceramic or carbide compounds that are a plastic state. to spray the semi-molten metal particles onto the nd form a permanent durable tough coating. On flatten and form thin adhered overlapping porous re can be used in this process. Materials used for c or carbides. Examples of use include brake discs, hafts, combustion cylinders, injector nozzles and alt atmospheres requiring corrosion resistance. ss are:

O ❖ Various paints and resins s vinyls, rubbers, coal tar ep aliphatic polyurethane, latex rich products can be used to increases the corrosion resis against corrosion and environ ❖ Surface preparation includin important to ensure adeq longevity of coating protection ❖ Prior to use obtain (MSDS) M each of the products to be u storage and use instruction must be followed carefully preparation and use.

ORGANIC COATING such as alkyds, epoxies, esters, poxies, aromatic polyurethane, x, acrylics, phenolics, and zinc- o provide a protective film that stance of the part and protects nmental damage. ng thorough cleansing is very quate bonding adhesion and n. Material Safety Data Sheets for used and follow safe handling, ns. Manufacturers instructions y in all aspects of product

❖ A barrier coating can be applied t process of treatment called anod phosphoric acid solutions, to form metals surface. ❖ The process is an electrochemical pr and an electrolyte, usually sulphuric a ❖ The aluminium itself is the anode. ❖ The electric current passing through surface leaving a aluminium oxide p corrosion resistance, and anodic environment. ❖ Anodising is not considered a stand and is usually accompanied by a seco powder coating or other protective or corrosive, alkaline or acidic situations

ANODISING to aluminium alloys using an electrolytic dising that uses sulphuric, chromic, or a thin oxide scale protective film, on the rocess involving an anode, electric current acid. the electrolyte coats the anodic aluminium protective film that has increased abrasion, c protection in a corrosive seafaring alone process in shipbuilding applications ondary application of finishing by painting, rganic coating medium especially in highly s

SURFACE P 1. The surface must be thoroughly clean Before grit blasting the surface sh removed using only recommended c on the surface that may inhibit prop protection. 2. Organic solvents, high-pressure ste spray or grit blasting equipment, ma dried, prepared and coated immedia 3. For maximum benefit, coating adh must free of dirt, grease, dust, rust prior to the application of the f vacuumed prior to application and b 4. Galvanised materials should be adeq or phosphate solution, rinsed and dr 5. Surface oxides must be removed f chloride and muriatic acid solution drying and final application of the co

PREPARATION ned to aid adhesion and mechanical bonding. hould be degreased. Contaminants must be cleaners that don’t leave cleaning compounds per coating adhesion and reduced durability eam, caustic solutions, detergents, water jet ay be used providing the parts are thoroughly ately after cleaning. hesion and corrosion protection the surface t, mill scale, old paint or protective coating, final coating medium. Surfaces should be be moisture free. quately etched in a suitable chemical etching ried thoroughly prior to coating. from copper or brass alloys using a ferric n. This must be rinsed off thoroughly before oating medium.

PROTEC ❖ Inhibitors offer protection to t reaction that forms a prot corrosion process thereby d Inhibitors are chemical in natu added to offer protection by de higher the concentration g efficiency of the inhibitor. ❖ Vapour phase inhibitors are confined spaces by volatilisatio this by neutralizing acidic carb pH values, from acid and corro ❖ On contact with the metal condenses and is hydrolysed b ions and acting inhibition for protection.

CTION STRATEGIES the exposed surfaces by chemical tective seal slowing down the decreasing the corrosion rate. ure and small concentrations are ecreasing the corrosion rate. The generally results in increased compounds used in closed or on to prevent corrosion. They do bon dioxide, or by modifying the osive, to less acid and corrosive. surface, the vapour inhibitor by moisture to release protective r extended periods of enduring

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