Cold corrosion is an electrochemical reaction that occurs on a metallic surface that is at a lower temperature compared to the dew point of the corrosive gas mixture to which it is exposed. Equipment such as aeronautical engines, vehicular engines, engines used in ships, air pre-heaters and boiler economizers are particularly susceptible to cold corrosion. Induced draft blower fans used in boiler plants face this problem occasionally.
The deposit of cold corrosion products on the surfaces of component parts can cause deterioration of equipment productivity. For example, cold corrosion can reduce boiler productivity and efficiency, reduce engine power, and also adversely affect heat transfer system capability. Cold corrosion is a serious problem in most cases, and one that requires a systematic approach for early detection and preventive care. This article examines the cold corrosion problems of internal combustion (IC) engines and boiler plant air pre-heaters.
Cold Corrosion Internal Combustion Engines
In the case of internal combustion engines, cold corrosion affects engine parts during very light duty operation and during idle or shutdown conditions and cold starting in winter season. The prevalence of cold corrosion depends on:
- The age of the engine components
- Weather condition
- Geographic location
- Frequency of use and duty cycle
- Fluctuations in load and temperatures
Among aircraft engines, those that are idle for 30 days and longer are susceptible to severe cold corrosion. Engines that are used regularly are less susceptible. (For more on this topic, see Aviation Coatings for Corrosion Prevention.)
The Economics of Cold Corrosion
The economics of cold corrosion may be understood by citing a practical case. If an engine overhaul is due after 2,000 hours of operation at a cost of $60,000, it works out to $30 per hour. Unfortunately, if an overhaul is needed every 500 hours due to cold corrosion damage, then cost per hour would be four times higher—at $120 per hour. More overhauls also create additional costs in the form of more downtime and lower reliability, as well as higher insurance costs.
If a brand-new engine is not commissioned within six months, an anti-corrosive preservative coating may be applied. (Related reading: Temporary Corrosion Protection During Storage, Transportation and Handling.) During running in, mineral oil rather than a synthetic dispersant fluid should be used for better corrosion protection. New engines must be operated more frequently to eliminate moisture condensation on the components. As the engine components have rough surfaces initially, cold corrosion due to infrequent use can cause deposition of metallic salts, resulting in severe wear at random high spots and, eventually, premature failure. Coastal areas and humid zones are more corrosive than dry regions. When the engine is cold, its piston rings, cylinders, valve guides, valves, camshafts and bearings become more vulnerable to corrosion due to sulfur and water vapor reacting on the component surfaces.
New Generation Engines
The newer generation of engines, which are designed for compliance with the latest nitrogen oxide regulations and the energy efficiency design index guidelines, appear to be prone to higher corrosion rates, as they create more moisture condensation on cylinder liners. The new design of cylinders enforce higher pressures and lower operating temperatures, causing (below dew point temperature) conditions conducive for rapid condensation of water vapor and sulfuric acid droplets.
Preventing Engine Cold Corrosion: Condition Monitoring
A planned condition monitoring program using scrape-down oils for wear debris analysis can give early warning of corrosion damage. Large fleet owners use ferrography and spectrometric oil analysis to monitor engine wear and incipient failure modes on metallic surfaces. Lube oil suppliers often provide corrosion and wear monitoring services by testing total base number (TBN) and metallic wear particle size distribution in the engine lube oil. When the scrape down oil sample indicates a significant rise in corrosion products such as iron salts, the onset of cold corrosion is confirmed. Some oil companies offer proprietary oils and additives to minimize the impact of cold corrosion damage. Gadgets are also available that can monitor abnormal wear particle count and oil condition.
Engine designers also suggest that each aero engine must operate at least once a week in order to minimize cold corrosion. Otherwise, engine storage procedures recommended by the manufacturers may be followed for corrosion protection. Most warranty conditions of aero engines stipulate that an engine must be used for at least 20 hours each month.
In order to expel the condensed moisture in the oil, engine oils should be maintained around 180°F (82°C) during operation. Lube oil changes based on TBN (alkaline additive remaining), emulsified oil, moisture content and oxidation products in oil are also recommended by some engine oil experts. Emulsion of oxidized oil accelerates the corrosion of piston rings and cylinders. As a golden rule, an oil change should be done once every six months, or after 50 hours of engine operation. It may be wiser to invest in timely oil changes than to allow the engine to get damaged.
Preventing Engine Cold Corrosion: Fuel Dilution
Fuel dilution of lube oil leads to engine corrosion and wear and tear by abruptly reducing the viscosity of lube oil. (Learn more about viscosity in the article 6 Ways to Measure Fluid Viscosity.) In order to reduce this form of damage, the fuel injectors must be checked regularly and the control system must be fine-tuned. Excessive sudden acceleration of the engine often causes unburned fuel oil to remain in the combustion zone, which will migrate to the lube oil sump, thus causing oil to thin. Thinner oil can increase the wear rate and corrosion rate very aggressively.
Cold-End Corrosion in Boiler Plants
In the boiler plant, cold-end corrosion is related to air pre-heater tubes and condensers. Factors responsible for high corrosion rates are the fuel sulfur content, high dew point of the acidic constituents of flue gases, and moisture content of the fuel and air used for combustion.
The emission of acid particulates in the stack, which condense on the colder metallic surfaces of a boiler’s air pre-heaters, is the main source of a recurring cold-end problem. The bigger particulates (100 mesh or larger) in the emission get attached to metallic surfaces nearer the stack. Because these particles are acidic in nature, they can initiate a corrosive electrochemical reaction on the solid surfaces. Engines and boilers using fuels containing sulfur generally face this problem.
The sulfur reacts with oxygen and first gets converted into sulfur dioxide (SO2), which can further lead to the formation of sulfur trioxide (SO3) in the next stage. The oxidation reaction is as follows:
(SO2) + O2 = 2(SO3)
Because sulfur trioxide gas is highly reactive, it readily combines with water at temperatures below its dew point and forms sulfuric acid.
Parameters Affecting Acid Formation
The rate of the formation of sulfur trioxide is determined by factors such as:
- Temperature range
- Catalysts available on cold-end surfaces
- Air fuel ratio of combustion
- Concentration of SO2
- Residence duration of contents of flue gas near the cold-end surfaces
The iron oxide or vanadium compounds on surfaces act as catalysts and accelerate the creation of sulfuric acid. Surface catalytic action depends on the area covered by the catalyst. By improving boiler cleanliness, it is possible to minimize the availability of the catalyst for corrosion. The cold-end surface temperatures, as well as the flue gas SO3 concentration, are also continuously monitored. A higher moisture percentage in the flue gases increases the dew point of the acidic gas particles, and acid particle condensation increases rapidly.
The corrosive potential of boiler fuel oils is lower when compared with gases such as refinery gas or natural gas for given sulfur content. The hydrogen content of petro gases readily combines with oxygen to generate heat and moisture. As a result, the dew point is automatically increased. When the flue gas temperature is less than 140°F (60°C), the probability of corrosion increases dramatically.
In case of boilers using coal, the corrosive reactions are very rare because the alkaline coal ash neutralizes the sulfuric acid particles. The dew point of the gas mixture is lower on account of a lower percentage of acid in the flue gases, which are also laden with alkaline ash particles.
Fouling Problems of Boiler Air Pre-Heaters
Air pre-heaters in the boiler houses can fail due to the accumulation of cold corrosion products within the hot air tubes. The pre-heater tube metal produces sulfates of iron due to its reaction with sulfuric acid, which reduces heating efficiency as well as the flow rate of the air supply. A high corrosion rate results in frequent system outages, seriously affecting the plant availability and need for servicing.
More recently designed regenerative pre-heaters, which ensure that heat transfer surface temperatures remain above the dew point of the acidic particles, have the advantage of built-in corrosion resistance. They can replace the existing recuperative type of pre-heaters, adding to overall plant economy. Newer designs provide for multiple air pre-heaters, each facilitated with built-in soot blowing and water washing systems for ease of tube cleaning. One of the pre-heaters can be disconnected for servicing, while the boiler continues to get the hot air supply from remaining pre-heaters for uninterrupted steam generation.
The size and design of the pre-heater and the metals chosen for construction are determined by the average temperature at the air pre-heater's cold end and the space constraints. Apart from maintaining and monitoring cold-end temperatures, the choice of corrosion-resistant metals and metallic coatings can also contribute to the minimization of cold corrosion of air pre-heater tubes. (Metallic coatings are discussed in the article How Metallic Coatings Protect Metals from Corrosion.)
Reducing Cold Corrosion in Boiler Plants: Steam Coil Heaters
In some boiler plants, additional air heater coils are provided between the forced draft fan outlet and the air pre-heater inlet point to compensate for the seasonal changes occurring in the incoming ambient air temperature. These temperature compensators are designed as steam coil air heaters. They contribute to temperature stability for the cold-end of the air pre-heater, which is monitored and maintained well above the acid particle dew point. The steam coils also help us to maintain a steady combustion air temperature, irrespective of fluctuations in ambient temperature due to varying weather conditions.
Whenever the boiler is modified (from gaseous fuel or coal) to petroleum oil firing, the cold-end temperature required is higher as compared to the original set up. Therefore, an additional steam coil heater becomes necessary. These additional steam coil heaters also contribute to higher steam production by combating cold corrosion of air repeaters.
Reducing Cold Corrosion in Boiler Plants: Air Ingress Control
When the boiler is operated with around a 5% air shortage, sulfur trioxide content is substantially reduced. On the other hand, any additional air infiltration into the boiler flame zones accelerates generation of sulfur trioxide and consequent corrosion attack. In order to minimize air infiltration in the flame zone, regular preventive inspection and maintenance practices can be helpful.
Reducing Cold Corrosion in Boiler Plants: Moisture Control
The dew point of flue gas acid particles in the boiler is determined by the extent of the presence of particles of sulfuric acid as well as moisture. The moisture percentage in flue gases is the sum of the percent of moisture in the fuel and percent moisture in the air supplied into the flame zone, as well as the percent of water vapor produced due to combustion.
The moisture percentage of coal supplies can be controlled by testing the coal at source and following appropriate handling precautions, as well as storage preservation systems. Even in case of petro-fuel oils and gases, the storage and handling systems must ensure that the moisture is well controlled.
Factors responsible for higher moisture content in flue gases are:
- Leakages and breakages in boiler tubes or in steam coil pre-heaters
- Damages due to excessive soot blowing
- Water wash system leaks and nozzle leaks
Whenever a boiler is fired simultaneously by more than one fuel, the dew point is influenced by the ratio of the fuel quantity used. Hence, the ratio needs to be controlled very precisely. If one of the fuels has a higher hydrogen percentage, then the moisture content in flue gases may go up.
Reducing Cold Corrosion in Boiler Plants: Additive Treatments
Some chemical additives have been successfully used for preventing cold corrosion as well as deposition at cold ends. Some of these are deployed as cold-end surface coating additives, whereas others are added as additives in the fuel during combustion. Surface additives for the cold end are injected into the backside of the boiler past the hot steam-producing parts so that they form a coating only on the cold end tube surfaces.
Reducing Cold Corrosion in Boiler Plants: Chemical Additives for Fuels
Chemical additives based on alkaline substances such as magnesium or aluminum-magnesium neutralizes sulfur compounds such as sulfur trioxide present in the flue gases, thus reducing the formation of the sulfuric acids. Catalytic action of vanadium as well as iron is reduced by these additives. Deposits formed on surfaces will have a higher pH value and hence corrosion will not occur. These are generally used for petroleum-based fuel oils used for boilers. These are injected into the superheated flame zone of the boiler. Due to the reaction of the magnesium, the magnesium sulfate will get deposited in the air pre-heater tubes, which can be cleaned by using water flushing nozzles fitted with pre-heaters. This reaction product is non-corrosive and hence the fouling of pre-heater air is substantially reduced. The amount of magnesium compound injected would depend on the sulfur content of the fuel.
Reducing Cold Corrosion in Boiler Plants: Surface Chemicals for Cold Ends
Certain surface chemicals can control and prevent cold corrosion very effectively. The chemicals injected for deposition on the surfaces include corrosion inhibitors and acid neutralizers. Corrosion inhibitors injected at the cold end do not neutralize the sulfur trioxide. Instead, they protect the surfaces from corrosive reaction. Reaction is effectively retarded and the surface is physically separated and deactivated. The production of sulfate deposits is significantly reduced, thus enhancing the availability of air pre-heaters.
Systematic preventive maintenance and monitoring procedures can alleviate equipment damages due to cold corrosion in IC engines as well as boiler plants.