Tackling the MIC challenge

The industry continues to explore effective solutions to the MIC challenge. However, despite the industry’s effort to mitigate the formation of MIC through proper equipment design or a water treater, pitting and crevice corrosion due to the formation of biofilms seems to be unavoidable. Refiners have resorted to using materials in heat exchanger manufacture that have a high pitting resistance equivalent (PRE) number and corrosion resistant qualities as one way to reduce and possibly avoid MIC.

Water source and composition

Once-through cooling water systems utilise untreated water that can either come from fresh water sources such as rivers or lakes or seawater. In either case, the natural composition of this untreated water brings its pollutants, deposits and microorganisms into the cooling water system, forming colonies and contaminating it uncontrollably. In the case of seawater, it is often stated that the composition of seawater is the same all over the globe, but in reality it varies significantly. The total dissolved solids fluctuate from approximately 8000 mg/l (ppm) in the Baltic Sea to up to 7.5 times that value in the bay areas of the Arabian Gulf. The composition on which artificial seawater is based is about 35 000 mg/l of total salt and is representative of most ocean waters. The pH value of artificial seawater is approximately 8. Seawater temperatures vary between only a few degrees centigrade at great depths and in the polar regions, reaching up to 30 – 35°C (86 – 95°F) at the equator.

Figure 1. Super austenitic stainless steels may be appropriate for combatting MIC in heat exchanger tubes.

Biological fouling

There are two types of biological fouling: micro and macro fouling. Micro fouling is built up by soft slimes, algae and hydroids that attach themselves to materials in seawater. Macro fouling occurs as a result of organisms including oysters, barnacles, mussels and tubeworms.

Microbiological biofilms develop on all surfaces in contact with aqueous environments. The chemical and electrochemical characteristics of a metal surface influence the formation rate and cell distribution of micro fouling films in seawater during the first hours of exposure. Water temperature, pH levels and the content of organic and inorganic ions also affect the microbiological settlement. Biofilms produce an environment at the biofilm/metal interface characteristically different from that of the bulk medium in terms of, for example, pH value, dissolved oxygen and sulfide. MIC is used to designate corrosion caused by the presence and activities within biofilms. The reactions are usually localised and can include sulfide, acid or ammonia production; metal deposition; or metal oxidation and reduction.

Differential aeration, selective leaching, under deposit corrosion and cathodic depolarisation have been reported as corrosion mechanisms for MIC. Localised bio corrosion attacks can also be accelerated by carbon dioxide (CO₂), hydrogen sulfide (H₂S) and ammonia (NH₃). Copper alloys as well as low-alloyed stainless steels such as 304 and 316 austenitic grades can suffer from MIC.

The optimal environments for the development of MIC can include stagnant conditions, organic nutrients in the water, sediment, the absence or neglect of chlorination practice, and the presence of chlorides and sulfates.

A challenge for copper alloys and standard stainless steels in once-through water cooled heat exchangers is the presence of sulfate-reducing bacteria (SRBs). These can interfere with the formation of the protective cuprous oxide film on copper-nickel materials and are even capable of removing an already existing protective oxide layer. Severe localised pitting can occur on iron-bearing 90Cu-10Ni if exposed even briefly to anaerobic seawater. The situation is similar to standard stainless steel. Some SRBs use hydrogen, depolarising the cathodic surface and accelerating attacks on a crevice site. SRB-induced corrosion is often characterised by an encrusted deposit over a deep pit with a black powdery sulfide corrosion product underneath. MIC is a general term used for different localised corrosion attacks when a biological species is present. At temperatures below approximately 40°C (104°F), a biofilm present on the stainless steel surface will raise the open circuit potential (OCP) to 300 – 400 m V/SCE. At this temperature, the potential risk for a localised attack on standard austenitic stainless steel grades increases. At temperatures above 40°C (104°F), the microorganisms in the biofilm are no longer active, resulting in a potential drop to 0 m V/SCE for the stainless steel material.

Since localised attacks such as pitting are dependent mainly on potential and temperature, the possible reduction at higher temperatures is advantageous for corrosion-resistant super austenitic high nickel alloys that can withstand temperatures above 40°C. At lower temperatures, the corrosion resistance qualities of such materials reduce the likelihood of localised MIC attacks, while at higher temperatures the chances of corrosion attacks are lower.

Chlorination

A common practice to prevent the buildup of biofilm and to remove attached biological species is to chlorinate the seawater by adding a hypochlorite solution. Chlorine is widely used to control the microscopic biofilm in condensers and heat exchangers. The chlorination can be either continuous or intermittent, when chlorine is added to the system for a short period each day. Continuous chlorination levels of approximately 0.1 – 0.4 ppm are most frequently used in seawater-cooled heat exchangers.

In continuous chlorination, both copper-based alloys and stainless steel tubes have performed well in water containing up to 2 ppm residual chlorine, but have failed in more heavily chlorinated environments, and there is a marked difference in behaviour between copper-based alloys and high-alloyed stainless steel.

With intermittent chlorination, higher doses of chlorine are used for limited time periods. The amount of chlorine added to the system varies, but levels up to several parts per million are not uncommon for heat exchanger cooling systems. This can make low-alloyed stainless steels and copper-based alloys unsuitable as tubing material for systems using this type of chlorination process.

When added continuously, the chlorination of seawater increases the corrosion potential for the material to approximately +600 m V/SCE. When adding the chlorine intermittently, the electrochemical OCP of the system will be lower than for continuous chlorination but will vary with sharp potential peaks at the time of chlorine addition. The OCP of the system is probably the single most important aspect affecting seawater corrosivity. The higher the potential, the greater the risk of localised corrosion in stainless steels.

Stress corrosion cracking

Low-alloyed stainless steel grades are prone to chloride-induced stress corrosion cracking (SCC) at temperatures above 60°C (140°F), which is a reason why these materials are inappropriate for use in many chlorinated or seawater-cooled heat exchanger systems. Duplex grades and high nickel alloys do, however, possess very good resistance to stress corrosion in these cooling systems for condensers and heat exchangers due to their dual microstructure and/or high-alloying contents.

Copper alloys do not suffer from SCC in the same way as low-alloyed stainless steels. The prime cause of SCC in these materials is the presence of ammonia and its salts in the solution. Brasses, for example, have a clear tendency towards SCC in the presence of ammonia. Some microorganisms produce ammonia, and within the biofilm sufficiently high concentrations can be generated, allowing SCC to occur.

Pitting corrosion

The primary cause of pitting is the presence of chloride ions. Pitting attacks are often initiated at precipitates around a chromium-depleted zone. Welds can be potential initiation sites for localised corrosion attacks because of their occasionally inhomogeneous structure and the presence of intermetallic precipitates in the melted or heat-affected zones adjacent to the welds.

Pitting is temperature and flow rate dependent, and chlorination of the seawater to be used for cooling heat exchanger systems increases the corrosion potential of the tube material to approximately +600 m V/SCE. At this level, pitting can occur very rapidly in low-alloyed stainless steels.

Figure 2. Pitting attacks are often initiated at precipitates around a chromium-depleted zone.

Crevice corrosion

Crevice corrosion, in the form of under deposit corrosion on the heat exchanger tubing, can occur if the chlorination procedures and continuous flow of the system are insufficient to prevent fouling or sediment adhering to the metal surface.

Much of the sediment in cooling water exits condenser and heat exchanger tubing at velocities below approximately 1 m/s and during shutdowns. Since there is a range of velocities in individual tubes above and below the nominal design velocity, the minimum practical design velocity for heat exchangers and condensers is at least 1.5 m/s. Any unit designed or operated at lower velocities could be at risk of under sediment crevice attack, MIC or both. It is of great importance to rinse the inside of the tubes during shutdowns with fresh water, irrespective of which material is used to manufacture the tubulars.

Solid salts

One form of fouling that cannot be prevented by filtration or maintaining flow rates in chlorinated seawater cooling applications is the precipitation of solid salts, which occur at approximately 60°C (140°F). Often, it is only the seawater in contact with the hot tube wall that reaches sufficient temperatures to cause such problems. However, it can result in the deposition of sulfates, carbonates and other salts on the tube wall. At temperatures approaching boiling point, chloride salts can also deposit to create very aggressive conditions. In conjunction with chlorinated seawater, such deposits can lead to a risk of crevice/under deposit corrosion shortly after their formation.

Reducing MIC risk

MIC does not involve new corrosion mechanisms. Increasing the resistance of stainless steels to this corrosion phenomenon can be achieved by raising the content of the alloying elements in the material that lead to higher PRE numbers. This can be beneficial in resisting pitting and crevice corrosion.

Standard austenitic steels, such as ASTM 304 and 316, are vulnerable to both macro and micro fouling, whereas hyper-duplex (UNS S32707), super-duplex (UNS S32750) and high-alloy austenitic stainless steel grades such as UNS N08935 can be viewed as more resistant to MIC in seawater due to their higher PRE numbers.

Conclusion

The exposure of stainless steel heat exchanger tubes to once-through water as part of the cooling process induces the development of biofilm. The activity of the microorganisms in the biofilm causes the OCP of the stainless steel to increase. This will increase the risk of crevice corrosion or pitting if the resistance levels of the steel grades are exceeded. At 35 – 40°C (95 – 104°F) the biofilm is, however, killed and the effect vanishes.

The presence of an active biofilm on the stainless steel surface may result at temperatures below 35 – 40°C (95 – 104°F) and the seawater becomes more aggressive as the corrosivity develops.

Chlorination is a common practice to reduce biofilm buildup. However, it can encourage SCC and pitting corrosion in lower-alloyed stainless steel grades. Such corrosion challenges can be overcome by using expensive high nickel alloys.

One alternative is the use of super austenitic stainless steels with high anti-corrosion properties and reduced nickel content, which have been developed as a cost-effective solution to the use of high nickel alloys for heat exchanger tubes. Such materials may be an appropriate consideration for combatting MIC in refinery heat exchanger tubes.

Written by Karen Picker, Sandvik, USA.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/09032021/tackling-the-mic-challenge/