Propulsion

Diesel Engine General

Flexible couplings

With a medium speed system an number of engines and other devices are connected together. Flexible couplings are required to account for the slight misalignment which can exist. The claw type coupling as used with steam turbines allows for this misalignment and provides for a large area of contact which keeps he stress limited.



The diesel engine drive is pulsating and normal face to face contact cannot be allowed. Rubber blocks are therefore fitted between the claws. When the dive takes place the leading blocks are compressed allowing clearance at the trailing blocks which are hammered because of the pulsating drive. This results in wear. Blocks must there fore be pre compressed so that trailing blocks can expand and maintain normal contact with the drive


Crankcase explosions

Under normal conditions the atmosphere in the crankcase when the engine is running contains a large amount of relatively large oil droplets (200 micron) in warm air. Because of the droplets small surface area to volume ratio, the possibility of ignition by a heat source is very low.


Should overheating occur in the crankcase, say by failure of a bearing, then a hot spot is formed (typically over 400'C although experiments have shown two separate temperature ranges, the other between 270 - 300'C>. Here lub oil falling on to the surface is vaporised ( in addition some is broken down to flammable gasses such as Hydrogen and acetylene), the vapour can then travel away from the hotspot where it will condense. The condensed droplets, in the form of a dense white mist, are very much smaller (6 to 10 microns) than the original and so have a high surface area to volume ratio. Ignition by a hot spot (generally of the flammable gasses which in turn ignite the fine droplets in the mist), which may be the same on that cause the original vaporisation, is now a possibility.


Oil mists are formed at temperatures of around 350oC

Ignition occurs at under 500oC


The white mist will increase in size and density until the lower flammability limit is exceeded (about 50mg/l is generally found in real situations ), the resultant explosion can vary from relatively mild with explosion speeds of a few inches per second and little heat and pressure rise. To severe with shock wave and detonation velocities of 1.5 to 2 miles per second and pressures of 30 atmospheres produced. This is the extreme case with pressures of 1.5 to 3.0 bar more normal raising to a maximum of 7.0 bar.



It can be seen that following the initial explosion there is a drop in pressure, if the initial explosion is not safely dealt with and damage to the crankcase closure occurs, it is possible that air can be drawn in so creating the environment for a second and possible larger explosion. The limiting factors for an explosion is the supply of fuel and the supply of oxygen, the air as shown can be drawn in by the slight vacuum created by the primary explosion. The supply of fuel may be created by the passage of the shockwave shattering the larger oil droplets into the small size that can readily combust.


By regulation,non returning relief doors must be fitted to the crankcase in order to relief the pressure of the initial wave but prevent a rapid ingress of air



Vapour extraction fans

These generally take the form of a small electrically driven fan. They are fitted with flame traps on the exhaust side.

Although the fans keep the crankcase at a slight negative pressure thereby increasing the risk of air being drawn in, this is seen to be more than compensated by the removal of flammable vapours and the reduction in oil leakage.


Crankcase doors

These when properly designed are made of about 3mm thick steel with a dished aspect and are capable of withstanding 12 bar pressure. They are securely dogged with a rubber seal arrangement.


Crankcase relief door (setting 1/15bar)

Due to the heavy force of momentum the gas shockwave is not easily deflected. Thus any safety device must allow for a gradual change in direction, and be of the non-return type to prevent air being drawn back into the crankcase

The original design was of cardboard discs which provided no protection against the ingress of air after the initial explosion, in addition it was known for these discs to fail to rupture in the event of an explosion




The valve disc is made of aluminium to reduce inertia. The oil wetted gauze provides a very effective flame trap This reduces the flame temperature from 1500'C to 250'C in 0.5 m. The ideal location for this trap is within the crankcase where wetness can be ensured. The gas passing from the trap is not normally ignitable. The gauze is generally 0.3mm with 40% excess clear areas over the valve.


Specifically the regulations are;

Non-return doors must be fitted to engines with a bore greater than 300mm, at each cylinder with a total area of 115sq.cm/m3 of gross crankcase volume. The outlets of these must be guard to protect personnel from flame. For engines between 150 to 300mm relief doors need only be fitted at either end. Below this bore there is no requirement. The total clear area through the relief valve should not normally be less than 9.13cm2/m3 of gross crankcase volume

Lub Oil drain pipes to the sump must extend below the surface and multi engine installations should have no connections between the sumps

Large engines, of more than 6 cylinders are recommended to have a diaphragm at mid-length and consideration should be given to detection of overheating (say by temperature measuring probes or thermal cameras) and the injection of inert gas.

Engines with a bore less than 300mm and a crankcase of robust construction may have an explosion door at either end

Means of detection of oil mist fitted.


Continuous extraction by exhauster fan may be used but this tends to be costly, flame gauzes must be fitted to all vents. Similarly a continuous supply of air can be used to reduce gas mist levels.



Crankcase oil mist detector (Obscuration)(set point 2.5% L.E.L)



Oil mists can be readily detected at concentrations well below that required for explosions, therefore automated detection of these oil mists can be an effective method of preventing explosions

Shown above is the Graviner oil mist detector. This is in common use in slow speed and high speed engines. The disadvantage of this type if system is that there is a lag due to the time taken for the sample to be drawn from the unit and for the rotory valve to reach that sample point. For this reason this type of oil mist detector is not commonly used on higher speed engines.

Modern detectors often have the detection head mounted in the probe, the probe is able to determine the condition of the crankcase and output an electrical signal accordingly


The assembly consists of;
Extraction fan-draws the sample from the sample points through the reference and measuring tubes via non-return valves.

Rotary valve-This valve is externally accessible and is so marked so as to indicate which sample point is on line. In the event on exceeding the set point , the valve automatically locks onto that point so giving a clear indication of the locality of the fault condition.

Reference tube-measures the average density of the mist within the crankcase, as there will always be some mechanically generated mist.

Measuring tube- measures the opacity of the sample by means of a photoelectric cell as with the measuring cell. To exclude variables in lamps a single unit is used with beams directed down the tube by mirrors.


The photoelectric cell gives an output voltage proportional to the light falling on it. In this way the opacity of the sample is measured, the voltages generated in the cell in the measuring and reference tubes are compared in an electronic circuit. The difference is compared to a potentiometer varied setpoint which if exceed initiates an alarm circuit. The alarm circuit, dependant on installation, will generally declutch the drive to the rotary valve, give an output signal to the engineroom alarm monitoring system and an output to the engine protection system causing it to slowdown.


The rotary valve also has a position marked 'O' at which air is supplied to both tubes, and zero automatically (and manually if necessary) adjusted at each cycle. In addition at position 'L' an average sample of the crankcase is compared to air.



Crankcase oil mist detector (light scatter)

The disadvantage of obscuration types is that they are generally slow to operate and suffer from inaccuracies and false alarms caused by such things as a dirty lens.
Light scatter do not suffer from these problems, are faster reacting and do not need to set zero during engine operations.


The relationship between the light landing on the sensor is nearly proportional to the oil mist density therefore the unit can be calibrated in mg/l.


It is possible to have the sensor and a LED emitter in a single unit which may be mounted on the crankcase. Several of these can be placed on the engine each with a unique address poled by a central control unit. The results of which may be displayed on the control room


having these heads mounted on the engine removes the need for long sample tubes which add to the delay of mist detection.This makes the system much more suitable for use with medium and high speed engines were otherwise detection would be impossible.


Crankcase doors (non relieving)

The older type consisted of doors lightly held by retaining clamps or clips. With doors of this type a pressure of 0.5psi would give a permanent set of about 25mm, the doors would be completely blown off by pressures of 2 to 3 psi Modern large slow speed engines have two types of crankcase door, a large securely held heavy mild steel square door which allows good access for heavy maintenance.

A second smaller round dished aluminium door at around x-head height which allows entry for inspection. Due to the curved design the door is able to withstand pressures well above the setpoint for the relief doors.


Actions in the event of Oil Mist detection

The consequences of a crankcase explosion are extremely serious and the greatest possible caution in the actions taken should be exercised.


Should the oil mist detector activate an alarm condition, then personnel should take steps to ascertain if the fault is real. They should initially assumed that it is, the bridge should be informed and the engines slowed if the oil mist detector has not already done so. Should the bridge require manoeuvrability, and it is essential that the engine be operated then consideration of evacuation of the engineroom should be made. Otherwise the engine should be stopped and turned on gear until cooled.

The Graviner Oil Mist detector indicates via markings on the rotary valve which sample point has the high readings. By inspection of the graviner, and by viewing crankcase (or thrust, gearcase) bearing readings it is possible to ascertain whether a fault condition exists.


Under no circumstances should any aperture be opened until the engine has sufficiently cooled, this is taken as normal operating temperatures as an explosion cannot occur when no part has a temperature above 270'C (Cool flame temperature)

Once cooled the engine can be opened and ventilated (the crankcase is an enclosed space).


An inspection should be made to locate the hotspot, the engine should not be run until the fault has been rectified.



Crankcase safety fitting

For the purpose of this Section, starting air compressors are to be treated as auxiliary engines



Relief valves


Reversing a slow speed engine

For an engine to be reversed consideration must be given to the functioning of the fuel pump, air distributor and exhaust valves. That is, there commencement and completion of operation in respect to the crankshaft position.


Distributor

Due to the differing requirements for the change in angle between the distributor and the Fuel and exhaust cams, two camshafts are fitted although this adds to the cost of installation. The small distributor camshafts has seperate ahead and astern cams adjacent to each other. Reversing is by pulling and pushing the camshaft axially.

During normal engine operation the pistons of the distributor are held off the cams, this simplifies the changeover of the camshaft. When starting air is required the pistons are first forced onto the cams. Starting air is emitted during the low points and stop and the majority high points.

Reversing methods


There are five solutions to reversal of the engine timing are


Timed cylinder lubrication

Cylinder lubrication should be injected in carefully metered amounts. The injection points should be spaced around the periphery in such a way as to ensure adequate coverage when the piston passes the feed points. The best timing for injection is suggested as being between the first and second rings. The difficulties in achieving this are great, but injecting at TDC and to a lesser extent BDC assists

Lubrication is of the total loss system i.e. the oil is expected to be completely combusted without residue. The oil is injected through quills which pass through the liner wall.


Cylinder lub oil properties

The type of cyl l.o. required will depend upon the cylinder conditions and the engine design e.g crosshead or trunk piston. However, the property requirements are basically the same but will vary in degree depending upon the fuel and operating conditions.

Normal properties required are;

adequate viscosity at working temperature so that the oil spreads over the liner surface to provide a tough film which resists the scrapper action of the piston rings

the oil must provide an effective seal between the rings and liner

only a soft deposit must be formed when the oil burns

alkalinity level (total base number or TBN) must match the acidity of the oil being burnt

detergent and dispersant properties are required in order to hold deposits in suspension and thus keep surfaces clean


Behaviour depends upon the temperature of the liner, piston crown and piston rings. TBN and detergency are closely linked. This can have an adverse effect when running on lighter fuels with lower sulphur content for any period of time. Coke deposits are can increase.

Consequences of under and over lubricating

Over lubrication will lead to excessive deposit build up generally in the form of carbon deposits. This can lead to sticking of rings causing blowpast and loss of performance, build up in the underpiston spaces leading to scavenge fires, blockage and loss of performance of Turboblowers as well as other plant further up the flue such as waste heat recovery unit and power turbines.

Under lubrication can lead to metal to metal contact between liners causing microseizure or scuffing. Excessive liner and piston wear as well as a form of wear not only associated with under lubrication but also with inadequate lubrication called cloverleafing




Causes


The fine adjustment operates in such away that by screwing it in the stroke of each pump may be accurately metered. Additionally it may be pushed into give a stroke enabling each p/p to be tested. The eccentric stroke adjuster acts as a coarse adjustment for all the pumps in the block. Additionally it may be rotated to operate all the pumps, as is the case when the engine is pre-lubricated before starting. Correct operation of the injection pumps whilst the engine is running can be carried out by observing the movement of the ball




Electronic cylinder lubrication



Exact injection timing of cylinder lube oil is essential for efficiency. A move to electronics for the control of this has been made by some large slow speed engine manufacturers.


The system is based on an injector which injects a specific volume of oil into each cylinder on each ( though more normally alternate) revolution of the engine. Oil is supplied to the injector via a pump or pumps. A computer, which is synchronised to the engine at TDC each revolution, finitely controls the timing . Generally most efficient period for lubrication is taken at the point when the top rings are adjacent to the injection points.


The injection period is governed by the opening of a return or 'dump' solenoid which relieves system pressure.


Quantity can be adjusted by manually limiting the stroke of the main lubricator piston, by altering the injection period or by the use of multiple mini-injections per revolution.


The high degree of accuracy with this system allows for lower oil consumption rates.


Shown is the injector unit fitted to modern camshaftless slow speed engines. The motive force is via a dedicated or common hydraulic system. The hydraulic piston acts on multiple plungers one for each quill. At the dedicated time the electric solenoid valve energises an allows hydraulic oil to act on the piston commencing oil injection. One or two pumps per unit may be fitted dependent on cylinder diameter and oil flow requirements.


Precise control of the timing of injection allows oil to be delivered into the ring pack, something which has proved impossible with mechanical means. This has reduced oil consumption by as much as 50%.


Pre- lubrication for starting may be built into the bridge remote control system or carried out manually



Cylinder lubricator quill


Crankshaft Deflections

To see why crankshaft deflections are taken it is first necessary to look at one section i.e. two crank webs, a crank pin  and two journals.#


If a straight length of shafting I supported at either end is subjected to a central load the effect is for the shaft to sag with the upper material in compression and the lower in tension


This effect is applicable to the section of crankshaft described above with the bearings supporting the assembly at the journals and the point loading being effect by the weight of the piston and conrod assembly ( ignoring other loads found operational conditions such as combustion and centrifugal ).


Effect on Crankshaft



It can be seen that the effect is to increase the distance between the webs at top dead centre (TDC) and reduce the distance at bottom dead centre (BDC). This deflection is normally found in all crankshafts although for smaller engines with very rigid cranks this may be very small.


A set of measurements taken from an engine will reveal this deflection which should be constant through each  crank/piston unit. The caveat to this is that increase deflection is seen at the fly wheel and cam chain gear wheel sections due to the increased loading.


Finding faults

After initial installation and alignment a set of deflections are taken. These then form the datum line to which all other recordings are measured against.


it should be noted that changes in circumstances will effect the deflections are not indicative of faults. These include;


these effects are well known and an experienced engineer will take into account these factors when looking at a set of recordings



If a situation now occurs where a bearing becomes more worn than an adjacent one the effects will be shown as a change in the pattern of deflections. When the cranks is turned from BDC to TDC the weight of the running gear causes the crank webs and crankpins to bend in such a manner that the distance between the webs decreases, and continues to decrease until the bearing is no longer in contact with the journal<br> The deflection when the crankshaft is approaching TDC will then go from its normal positive reading to zero and then to negative readings at which point the assembly is supporting the weight without the assistance from the lowered main bearing.





Thus, any changes from natural deflections can be related to main bearing misalignement and is proportional to the differences in height of the bearings



Taking Measurements

These are generally taken using a spring loaded dial gauge. The crank webs ar pock marked to ensure that the readings are taken in the same place each time. Five measuring points are taken- TDC, 90' either side of TDC and 30' either side of BDC. The latter two measurements are required as it is not possible to measure at BDC due to the Con rod.


The measurements are always taken starting at the same starting point. In this case we will say Port side near BDC. The gauge is fitted and zeroed. The engine is rotated continuously and the readings read off during rotation. After the final reading the egine is rotated back to the start point. If the reading is not zero then it indicates that the gaige is moved and the readings are re-taken.



Example

These readings were taken from a B&W 6K76EF (I bet you haven't sailed with one of them, it's the one with the rocker arms and the self adjusting tappets that make you crap yourself when they fail)





Crank Position

No1 Cyl

No2 Cyl

No3 Cyl

No4 Cyl

No5 Cyl

No6 Cyl

Port near BDC  (X)

0

0

0

0

0

0

Port Horizontal  (P)

6

1

7

-9

-4

4

TDC  (T)

12

3

13

-16

-12

5

Stbd Horizontal (S)

6

3

6

-7

-8

3

Stbd near BDC  (Y)

-1

2

-2

2

1

4

corrected BDC  (X+Y/2=B)

0

1

-1

1

0

2

Vertical Alignement




These figures may now be used to draw a misaligement curve similar to the one below and may be analysed to see which bearings are in need of adjustment.


the assistance from the lowered main bearing.



Crank Position

No1 Cyl

No2 Cyl

No3 Cyl

No4 Cyl

No5 Cyl

No6 Cyl

Vertical alignement     [T-B=V]

12

2

14

-17

-12

7

Horizontal Alignement


<table class = "list">

<tr><TD>Crank Position<TD>No1 Cyl<TD>No2 Cyl<TD>No3 Cyl<TD>No4 Cyl<TD>No5 Cyl<TD>No6 Cyl

<tr><td>Horizontal alignement <b>[P-S=H]</b><td>0<td>-2<td>1<td>-2<td>4<td>1


</table>




Gauge reading Check



C & D should be practically the same, hence the readings from No6 Cyl may be suspect

Crank Position

No1 Cyl

No2 Cyl

No3 Cyl

No4 Cyl

No5 Cyl

No6 Cyl

Horizontal  alignment  [P-S=H]

0

-2

1

-2

4

1

Crank Position

No1 Cyl

No2 Cyl

No3 Cyl

No4 Cyl

No5 Cyl

No6 Cyl

T+B=C

12

4

12

-15

-12

3

P+S=D

12

4

13

-16

-12

7

Jacket Water System


Shown above is a typical cooling water circuit for a slow speed engine.

Water is pump via one of two centrifugal pumps. One is normally in use with the other stand-by. The water passes through to the distributing manifold on the engine side.


Jacket Water Heater In the line is a steam jacket water heater. When the engine is shut down steam heating maintains the engine in a state of readiness reducing the time needed for starting. Attempting to start the engine without heating can lead to poor combustion, poor lubrication and thermal shocking. A modern variation on this is the "blend" water from the stand-by auxiliary alternator engines into the main engine circuit increasing plant efficiency


The water enters and leaves the engine via a series of cylinder isolating valves. In this way each cylinder may be individually drained to prevent excessive water and chemical loss. In addition dual level drains may be fitted which allow either full draining or draining of the head only. A portion of the water is diverted for cooling of the turbocharger.


Deaerator Was an essential part of engines incorporating water cooled pistons were air was deliberately introduced in to the system to aid the "cocktail shaker" cooling action. Air or gas entering the system can lead to unstable and even total loss of cooling water pressure as the gas expands in the suction eye of the circulating pumps. In the event of gas leakage via the head or cracked liner rapid loss of jacket water pressure can occur. The deaerator is a method to try to slow this process sufficiently to allow the vessel to be placed in a safe position for maintenance. This system also allows the vessel to operate with minor gas leakage.


Jacket Water Cooler The hot water leaving the engine passes to a temperature control valve were a portion is diverted to a cooler. Temperature is controlled using both a feedback signal (temperature measured after the cooler) and a feed forward signal (temperature measured at outlet from the engine). In this way the system reacts more quickly to engine load variations.


Evaporator Increases plant efficiency by utilising heat in jacket water to produce fresh water. Modern systems sometimes rely on the evaporator to supplement a reduced size main cooler.

 

Expansion or header tank Maintains a constant head on the circulation pumps reducing cavitation at elevated temperatures. Allows the volume of water in the system to vary without need for dumping. Acts as a reserve in the event of leakage


HT/LT systems


Scaling of Jacket Water System

Scale and deposit formation

In areas of deposit formation, dissolved solids, specifically Calcium and magnesium hardness constituents can precipitate from cooling water as the temperature increases. Deposits accumulate on the heat transfer surfaces as sulphates and carbonates, the magnitude of which is dependent on the water hardness, the dissolved solid content, local temperatures and local flow characteristics. Temperature solubility curves for CaSO4


Scales can reduce heat transfer rates and lead to loss of mechanical strength of component parts, this can be exacerbated by the presence of oils and metal oxides.

The degree and type of scaling in a cooling water circuit are determined by;

Calcium Carbonate

Appears as a pale cream, yellow deposit formed by the thermal decomposition of calcium bi-carbonate

Ca(HCO3)2 + Heat becomes CaCO3 + H2O + CO2

Magnessium Silicate

A rought textured off white deposit found where sufficient amounts of Magnesium are present in conjunction with adequate amounts of silicate ions with a deficiency onh OH alkalinity

Mg2+ + OH- becomes MgOH+

H2SiO3 becomes H+ + HSiO3-

MgOH+ + HSiO3- becomes MgSiO3 + H2SO4

Silicate deposit is a particular problem for systems which utilise silicate additives for corrosion protection. Thi sis typical of systems with aluminium metal in teh cooling system. The silicate forms a protective barrier on the metal surface. A high pH (9.5 - 10.5) is required to keep the silicate in solution. In the event of sea water contaimination or some other mechanism that reduces the pH the silicate is rapidly precipitated and gross fouling can occur.

Iron Oxides- Hematite (Fe2O3)


Is a loose red /brown deposit and is indicative fo active corrosion within a system


CopperThe prescence of copper within a cooling system is very serious ast it can lead to agressive corrosion through galvanic action. Specific corrosion inhibitors are contained with cooling water system corrosion inhibitors.

Effects of scale deposition

The effects of scale deposition can be both direct or indirect,typically but not specifically

Insulates cooling surfaces leading to;

Indirectly;


Corrosion inhibitors used in Jacket Water System

Jacket Cooling water system

In order to maintain mechanical strength the components surrounding the combustion zone must be cooled. The most convenient cooling medium is water, the use of which could lead to possible problems of corrosion and scaling if not properly treated.

Within the jacket water system a number of corrosion cells are available but the two most common and most damaging are due to dissimilar metals and differential aeration. In both types of cell there exists an anode and a cathode, the metals which form part of the jacket system, and an electrolyte which is the cooling water. The rate at which corrosion takes place is dependent upon the relative areas of the cathode and the anode and the strength of the electrolyte. It is the anode that wastes away. Corrosion due to temperature differences is avoidable only by the use of suitable treatments. Dissimilar metals-a galvanic cell is set up where two different metals and a suitable liquid are connected together in some way. All metals may be placed in an electro-chemical series with the more noble at the top . Those metals at the top are cathodic to those lower down. The relative positions between two metals in the table determined the direction and strength of electrical current that flows between them and hence, the rate at which the less noble will corrode

Galvanic Action

Corrosion within cooling systems can occur if the coolant, i.e. water, has not been properly treated. The corrosion can take the form of acid attack with resultant loss of metal from a large area of the exposed surface, or by Oxygen attack characterised by pitting. A primary motive force for this corrosion is Galvanic action

The Galvanic Series.

Or Electromotive series for metals

Cathode  

Gold and Platinum  

Titanium  

Silver  

Silver solder  

Chromium-Nickel-Iron (Passive)  

Chromium-Iron (Passive)  

Stainless Steel (Passive)  

Copper  

Monel  

70/30 Cupro-Nickel  

67-33 Nickel-Copper  

Hydrogen  

lead  

Tin  

2-1 Tin lead Solder  

Bronzes  

Brasses  

Nickel  

Stainless-Steel 18-8 (Active)  

Stainless Steel 18-8-3 (Active)  

Chromium Iron (Active)  

Chromium-Nickel-Iron (Active)  

Cadmium  

Iron  

Steel  

Cast Iron  

Chromium  

Zinc  

Aluminium  

Aluminium Alloys  

Magnesium  

Anode  

The metals closer to the anodic end of the list corrode with preference to the metals towards the cathode end.

A galvanic cell can occur within an apparently Homogeneous material due to several processes on of which is differential aeration where one area is exposed to more oxygen than another. The area with less oxygen becomes anodic and will corrode.


Galvanic action within metal


Galvanic action due to temperature gradient



This situation can exist in cooling water systems with complex layout of heat exchangers and passage ways within the diesel engine. Systems containing readily corrodible metals such as zinc, tin and lead alloys can complicate and intensify problems by causing deposit formations.

Differential Aeration

-Where only a single metal exists within a system corrosion can still take place if the oxygen content of the electrolyte is not homogenous. Such a situation can occur readily in a jacket water system as regions of stagnant flow soon have the oxygen level reduced by the oxidation of local metal. The metal adjacent to water with reduced levels of oxygen become anodic to metals with higher oxygen content electrolyte in contact with it.. Generally, the anodic metal is small in comparison the cathode i.e. the area of stagnant flow is small compared to the area of normal flow of electrolyte, and high rates of corrosion can exist. One clear case of this is the generation of deep pits below rust scabs.



Solutions

Water treatment

To remove the risk of corrosion it is necessary to isolate the metal surface form the electrolyte. One method would be by painting, but this is impractical for engine cooling water passages. A better solution would be a system which not only searched out bare metal coating it with a protective barrier, but also repaired any damage to the barrier.

Corrosion Inhibitors

Common Corrosion Inhibitors

Anodic Inhibitors

Nitrite (NO2- )- These are the most commonly used form of treatment and operate by oxidising mild steel surfaces with a thin, tenacious layer of corrosion product (magnetite Fe3O4). Relatively high volumes of treatment chemical are required so this method is only viable on closed systems


Sodium Nitrite- (sometimes with Borate added)-effective with low dosage, concentration non-critical. It is non toxic, compatible with anti freezes and closed system cooling materials. It does not polymerise or breakdown. However protection for non-ferrous materials is low. An organic inhibitor is thus required. Although will not cause skin disease it will harm eyes and skin. Approved for use with domestic fresh water systems.


Sodium Nitrite is a Passivator, a passivator will act chemically to produce an insulating layer on the metal surface. Whenever corrosion takes place the corrosion products including bubbles of gas, are released from the metal surfaces. Passivating chemicals act on the corrosion products preventing release from the metal surface and thus stifling further corrosion. If the insulating layer becomes damaged, corrosion begins a gain and the passivator acts on the new products to repair the layer.


Chromate's-the first passivator product was Sodium Chromate which was an excellent inhibitor. Inexpensive, effective and concentration easily tested. Corrosion may increase by incorrectly dosing, dangerous to handle, poisonous and can cause skin disease. Not allowed where domestic water production is in use (Jacket water heated evaporators). Unfortunately it was also highly toxic, a severe pollutant and staining agent, was incompatible with antifreeezes, and will attack zinc and soft solder slightly. Due to its toxicity is sometimes used as a biocide in such places as brine in large Reefer plants.


Silicates- react with dissolved metal ions at the anode. The resultant ion/silicate complex forms a gel that deposits on anodic sites. This gel forms a thin, adherent layer that is relatively unaffected by pH in comparison to other inhibitors. The inhibiting properties increase with temperature and pH, normal pH levels are 9.5 to 10.5.

Care should be taken with the use of silicates, which are often used for the protection of system containing aluminium. In the event of boiling increased concentrations and lead to aggressive corrosion due to the high pH.

Orthophosphate Forms an insoluble complex with dissolved ferric ions that deposit at the anodic site. It is more adherent and less pH sensitive than other anodic inhibitors. The film forms in pH of 6.5 to 7.0. Dosage is typically 10ppm in neutral water

Cathodic Inhibitors


Polyphosphate- Forms complexes with Calcium, Zinc and other divalent ions, this creates positively charged colloidal particles. These will migrate to the cathodic site and precipitate to form a corrosion inhibiting film. The presence of calcium is required at a typical minimum concentration of 50ppm.



Extreme variations in pH can upset the film and a reversion to orthophosphate will occur with time and temperature.


Positively charged zinc ions migrate to the cathodic site and react with the free hydroxyl ions to form a zinc hydroxide stable film at pH 7.4 to 8.2. If the water is too acidic the film will dissolve and not reform. If it is too alkaline the zinc hydroxide will precipitate in bulk and not at the cathodic site.

Phosphonates. Initially introduced as scale inhibitors to replace polyphosphates, they exhibit absorption at the metal surface especially in alkaline hard water. Generally used with other inhibitor types



Both Anodic and Cathodic Inhibitors

Benzotriazole and Triazole Specific corrosion inhibitor for copper. They break the electrochemical circuit by absorbing into the copper surface.
They are generally added to standard treatments.


Soluble and dispersible oils. Petroleum industry recognised that emulsifying cutting oils (erroneously called soluble oils) were able to reduce corrosion on metals by coating the surface. There were disadvantages though, if the coating became too thick then it could retard the heat transfer rate. Adherent deposits form as organic constituents polymerise or form break down products which can accumulate and disrupt flow. MAN-B&W recommend it not to be used.


It is effective in low dosages, safe to handle and safe with domestic water production. Effectiveness is reduced by contamination with carbon, rust, scale etc. Difficult to check concentration, overdosing can lead to overheating of parts
Oils are classed as a barrier layer type inhibitor. The surfaces being coated in a thin layer of oil.



Modern treatment

Nitrite-Borate treatment is most effective with a high quality water base. This treatment has no scale prevention properties and its effectiveness is reduced by high quantities of dissolved solids.


A modern treatment will be a Nitrite -Borate base, with a complex blend of organic and inorganic scale and corrosion inhibitors plus surfactants, alkali adjusters, dispersants and foam suppressers. A high quality water supply is still strongly recommended.



The Use of Sacrificial Anodes

-Electrolytic protection for the whole system by the use of sacrificial anodes is impractical. Parameters such as water temperature, relative surface area of anode and cathode, activity of metals in system and relative positions in galvanic series come into play. Anodic protection has become out of favour for cooling water systems as it can lead to local attack, causes deposits leading to flow disturbance and it has no scale protection



Preparation for cooling water treatment

-All anodes should be removed and the system inspected. No galvanised piping is to be used (old piping can be assumed to have had the Galvanising removed). High quality water should be used and chemicals measured and added as required. A history log should be kept



Microbiological Fouling

Under certain conditions bacteria found in cooling water systems can adapt to feed on the nitrite treatment.This can lead to rapid growth, formation of bio-films, fouling and blockages.
Typical evidence is a loss of nitrite reserve but a stable or rising conductivity level as the nitrate formed still contributes to the conductivity,
Problems of this sort are rare due to the elevated temerpatures and pH levels. Should it occur treatment with a suitable biocide is required.