SHIP MATERIALS

CONTENT ❖ Introduction of corrosion ❖ Types of corrosion ➢ Galvanic corrosion, stre ❖ Electro chemical series ❖ Common corrosion protectio ➢ Cathodic protection – zin ➢ Impress current system ➢ Coating (organic and met

ess corrosion, pitting corrosion. on strategies nc anode. tallic coating) – flowchart

INTRODUCTION ❖ Corrosion is a gradual process of de types of metals used in ship constru water and air. ❖ Definition : A chemical action which metal by oxidation, electrolysis, or chem ❖ All metals will corrode on contact with ❖ Reduction or minimization of corrosio life span and suitability of materials for

eterioration or degradation affecting some uction when placed in direct contact with cause gradual destruction of a surface of a mical contamination h water and air. on important for determining the potential outfitting, structural and shipbuilding

EFFECTS OF CORRO  Losses are economic and safe • Reduced Strength • Downtime of equipment • Escape of fluids • Lost surface properties • Reduced value of goods  The consequences of corrosion effects of these on the safe, re operation of equipment or st than the simple loss of a mass  Failures of various kinds and th replacements may occur even t destroyed is quite small.

OSION ety: on are many and varied and the eliable and efficient tructures are often more serious of metal. he need for expensive though the amount of metal



Corrosion in ships Pipelines are the lifeline of ships. As most of the pipelines are made of ferrous material, pipe surfaces are required to be protected against corrosion. Painting of the pipelines is necessary to protect the pipelines. Merely painting the pipe surface without surface preparation is of no use.



Corrosion influenced by flow-1 The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped. The high velocities in the pump accentuated the corrosion damage.



Galvanic corrosion This rainwater guttering is made of aluminium and would normally resist corrosion well. Someone tied a copper aerial wire around it, and the localised bimetallic cell led to a “knife-cut” effect.



Corroded ship surface

TYPES OF CORROS Uniform corrosion • Corrosion over the entire expos e.g..Atmospheric corrosion. Maximum metal loss by this form be measured in the laboratory. • Oxidation & reduction occur uni Pitting corrosion • Downward propagation of small extremely localized attack causin • It affects a relatively small surface into thickness of the material Crevice Corrosion • Crevice corrosion is a localized f influence of \"crevice geometries“ linings/coatings, fasteners, lap join

SION sed surface at a uniform rate. m. Not dangerous, rate can iformly over surface. pits & holes, or a form of ng holes in the metal e area but extends deeply form of corrosion, under the “ (flanges, gaskets, disbonded nts and surface deposits)

TYPES OF CORROS Galvanic Corrosion •Dissimilar metals are physically joined. The m 1 2 “dissimilar metal corrosion” •1 Aluminium - less noble 2 Copper - most noble Stress Corrosion Cracking •Caused by cumulative exposure to a tensile st environment and may due to service loads, fab •The cracks begin at an area of high stress conc bulk of the metal, causing complete failure of t Erosion-Corrosion •Erosion corrosion is a degradation of material often by impinging liquid, abrasion by particles bubbles or droplets, cavitation, etc Intergranular Corrosion •Corrosion along grain boundaries, ofte

SION more anodic one corrodes. Also called STEEL AND BRASS tress in the presence of a corrosive brication or assembly technique centration and then penetrate into the the component. surface due to mechanical action, suspended in fast flowing liquid or gas, Corroded brass water pump en where special phases exist.

Erosion-Corro

osion

CORROSION PRO The corrosion process is an electrochem process. 1. Metal reacts to its environment 2. Its atoms are exposed to the moistu filled atmosphere 3. Giving up electrons 4. Releasing positive charged ions int moisture filled solution or electroly 5. Electrolyte becomes conductive and forms a cell. 6. The point of corrosion is called an Anode

OCESS mical ure to the yte d n

CORROSION PRO ❖ The electrolyte acts as the corros passing through it, releases positiv the same time releasing electron cathode. ❖ The cathode - metal surface in contact with the part of the same metal. - accepts the release of electrons electrons, flowing through the forming. ❖ The anode and cathode form a par ❖ The metal anode component of would normally in an electrolyte, cathode, corrodes slower than nor

OCESS sive medium, where the metal anode vely charged ions into the solution, at ns to another electrode site called a e anode and electrolyte, different or s and sets up a cathodic reaction of metal trying to inhibit corrosion rtnership known as a ‘couple’. the couple corrodes faster than it and the other component, being the rmal.

ANODIC & CATH ❖ The three components form a comprising an anode, cathode electrolyte. ❖ These are known as anodic and cath cells, or potential sites for bime electrolytic or galvanic corrosion o metals. ❖ Metallic corrosion occurs when following essential requirements met: ➢ A potential difference between dissimilar metals; ➢ The presence of an effe electrolyte or electrically condu solution; ➢ An electrical connection bet the two metals.

HODIC CELL cell, and hodic etallic, of the n the s are n two ective ucting tween

CORROSION PRO ❖ When these 3 conditions are met solution, metal will discharge pos solution and remain as negative c ❖ This is called a metal’s negative p is an indication of the metal’s cor ❖ Electrode potential A metal’s electrode potential i chemical series table. The hig higher the position towards the greater potential tendency to co ❖ Electro – Chemical Series The corrosion resistance of a electro-chemical series when metals. Its position is arrange potential.

OCESS t, the metal in contact with the liquid sitive charged particle ions into the charge. potential, or, electrode potential, and rrosion resistance. is graded, and shown in an electro- gher the electrode potential grade, the anodic end of the table, because of its orrode. metal is shown by its position in the compared to other commonly used ed according to its relative electrode

GALVA

ANIC SERIES

GALVA

ANIC SERIES

TYPES OF CORROS

SION

GALV ❖ Galvanic corrosion is a comm immersed in seawater in a connection is made. ❖ Any metal that is above the o at a faster rate than the other ❖ One will corrode while the ot ❖ The farther the two metals greater the potential differenc ❖ The least noble metal is anod at the expense of the most n reactive and protected.

VANIC CORROSION mon problem when dissimilar metals are marine environment, and an electrical other, in the galvanic series, will corrode r. ther is protected. s are apart on the galvanic series, the ce between them. dic. It will be more reactive and corrode noble metal, which is cathodic and less

Uniform / General Attack Uniform chemical reaction acros surface. Some areas anodic some these change with time giving u corrosion. Usually produces a scal - General rusting of steel - Tarnishing of silver,.

UNIFORM ATTACK ss entire metal e cathodic but uniform overall le or deposit.

CR ❖ Concentration cells form due to ion concentration in the electro piece(s). ❖ Metal in contact with more con metal in contact with more dilut ❖ Electrons flow from the low-ox anode to the high-oxygen area o

REVICE CORROSION o differences in metal (or dissolved gases) olyte between two regions of (same) metal ncentrated electrolyte becomes “cathode”, te solution becomes anode and corrodes. xygen area on the metal which acts as the on the metal which acts as the cathode.

CR ❖ Eg. Localized electrochemica solutions exist. (Cracks, crevic porous deposits.) • Liquid gets into crevice but d less) stagnant. ❖ Deposits such as rust or wate the metal underneath is anodic ❖ Occurs in many alloys: stain alloys. ❖ e.g. s/s in seawater. Salt incr corrosion.

REVICE CORROSION al attack in crevices etc. where stagnant ces, under paint, under gaskets, rivets, bolts, does not flow in/out. (i.e. gaps of ~ mm's or er droplets shield the metal from oxygen so c and corrodes. nless steels, cu-alloys, titanium, aluminium reases conductivity of solution so increases FIGURE On this plate, which was immersed in seawater, crevice corrosion has occurred at the regions that were covered by washers.

PIT ❖ Localized attack which forms s penetrate through sheet witho crevice corrosion. ➢ Pits usually grow downwards ➢ Initiation may be at surface sc ➢ Polishing helps reduce pitting ➢ Stainless steels are suscepti greatly increases resistance. The pitting of a 304 stainless steel plate b acid-chloride solution.

TTING CORROSION small holes or pits. Can be very deep and out much warning/indication. Similar to s due to gravity. cratches, defects etc. g. ible but alloying with 2% molybdenum by an

❖ Localized attack at/near to gr disintegrate along grain bounda ❖ Sometimes precipitates form in/ prone to attack. ❖ Intergranular Corrosion e.g.: in 304 (18/8) stainless ste may form if heated at 500- 800 (sensitisation). ❖ Cr → Cr23C6 (ppts) (Cr norm corrosion of Cr depleted zones-

INTERGRANULAR rain boundaries of alloys. Makes specimen aries. Very common in some stainless steels. /near GB's which make GB very sensitive or eel, (0.08C, 19Cr, 9Ni, 2.0Mn), Cr carbides 0oC for some time mally protects Fe from corrosion) so now get - grains fall out or cracks run down GB's.

❖ Can occur during welding of st ❖ Protect by: 1. proper heat treatment, (red 2. carbon content (to <0.03w 3. add Nb, (347) Ti (321) to carbides.

INTERGRANULAR tainless steels – known as weld decay. dissolve carbides at high T), wt%C - 304L) so minimal carbides form o form stable carbides instead of chromium Weld decay in a stainless steel. The regions along which the grooves have formed were sensitized as the weld cooled.

❖ Combined effect of chemica mechanical abrasion (slurry). ❖ All metals are affected; -Very bad for metals that protected by passive layer – s aluminum. Erosion removes p exposing metal. - Soft metals also more susceptib alloys. ❖ More prevalent in piping, elbo when flow changes direction turbulent. ❖ Cavitations and bubbles can problems. ❖ Reduce impingement, turbule particulates.

EROSION al attack and are normally Impingement failure of an stainless steels, elbow that was part of a protective film steam condensate line. ble - Cu and Al ows, bends etc n or becomes n also cause ence and any