Metal components in industrial fluid systems often have to operate in harsh conditions . If left unchecked , adverse conditions can trigger and aggravate the corrosion of these components and cause physical and financial harm to an operation . In metal tubing , for example , the National Association of Corrosion Engineers ( NACE ) estimates the profit loss to offshore and nearshore operations from corrosion is more than $ 1 billion per year .

Recognizing corrosion early and addressing the root causes can help mitigate the worst damage , before repair and replacement costs accumulate . It is imperative that facilities ensure their technicians can identify the most common types of corrosion quickly , and understand what actions to take . Technicians can often take simple actions to remediate the damage if they know what to look for .

Two common types of corrosion are pitting and crevice , and they are normally responsible for the most expensive damage over time . Before determining how to fix the problem , it is important to understand why corrosion occurs , how pitting and crevice corrosion differ , and how to protect components from corrosion , to help avoid premature failures and costly replacements .

Corrosion is a set of electrochemical reactions that involve oxidation ( loss of electrons ) at an anode and a reduction ( gaining of electrons ) at a cathode ( Figure 1 ). For example , iron in tubing may oxidize , yielding two electrons and dissolving into water as a Fe 2 + ( positive ) ion . Simultaneously , the electrons from oxidizing iron may participate in a reduction reaction that uses O 2 dissolved in H 2 O to form OH-

( negative ) ions .

Anodic Oxidation : Fe ‣ Fe 2 + + 2e -

Cathodic Reduction : ½ O 2

+ H 2 O + 2e- ‣ 2 ( OH- )

Overall Corrosion Reaction : Fe + ½ O 2

+ H 2 O ‣ Fe 2 + + 2OH -

The most common applications for metal tubing are analytical and process instrumentation , hydraulic lines , and control and utility . These tubes are often made from stainless steel with more than 10 % chromium ( Cr ), which builds a passive oxide layer on the surface and inhibits corrosion ( Figure 2 ). Stainless steel corrosion does occur , however , when environmental conditions or mechanical damage causes that layer to break down . If the protective oxide cannot reform in a given solution ,

corrosion reactions may progress rapidly . In both pitting and crevice corrosion , the local breakdown of the passive oxide layer creates a region where corrosion damage is greatly accelerated .

How Pitting Corrosion and Crevice Corrosion Differ

The types of corrosion possible on oil and gas installations are myriad and depend on what component materials are used , the operating environment , and what fluids the components transport . However , two forms of corrosion are most common : pitting corrosion and crevice corrosion .

Pitting Corrosion - This type of corrosion occurs when the protective chromium-rich oxide layer on the surface of stainless steel breaks down , allowing the bare metal underneath to become susceptible to continued attack in a corrosive solution ( Figure 3 ). Cavities form as the stainless steel corrodes and creates pits .

Although the entry point of a pit may be detectable via thorough visual inspection , there may be a deep network of lost material lurking below the surface . Without appropriate remediation , pits continue to expand and can sometimes create holes in tubing walls . If process fluids leak out of these holes , it costs money in lost fluid , creates environmental and safety hazards , and can be expensive to fix . Additionally , pitting corrosion can cause cracks in already strained components . Environments with higher chloride ( Cl- ) concentrations , including those created by evaporation from deposited saltwater droplets , are likely to cause pitting corrosion – especially at high temperatures .

is reddish-brown iron oxide deposits and pits on the metal surface . Special attention ought to be paid to upward-facing surfaces where chlorine-bearing water ( e . g ., seawater ) may pool and evaporate , or downward-facing regions where hanging droplets dry . As water evaporates in these regions , the chlorine concentrations in the remaining water will increase and become more corrosive , resulting in pitting corrosion .

Crevice Corrosion - This type of corrosion is similar to pitting corrosion ; the protective oxide film on components deteriorates as it is exposed to corrosive fluids . The particularly insidious nature of crevice corrosion is that it frequently occurs out of sight , in crevices under clamps or similar tight spaces , making it hard to identify ( Figure 4 ). After crevice corrosion has started , wide and shallow pits form , weakening the component material and increasing the chances of mechanical failure .

In a typical fluid system , crevices exist between tubing and tube supports or clamps , between adjacent tubing runs , and underneath dirt and deposits that may have accumulated on surfaces . No matter how well the system is designed , crevices are practically inevitable , and the tighter they are , the harder they are to service . Catching crevice corrosion is far trickier than seeing pitting corrosion on the surface and , because it happens out of sight , it can quickly devastate systems .

In coastal or offshore applications , crevice corrosion often occurs when seawater diffuses into a crevice , leading to a chemically aggressive environment out of which corrosion-causing ions cannot readily dif-

The most obvious sign of pitting corrosion

Figure 1 . Corrosion occurs when a metal atom is oxidized by a corrosive fluid , which leads to a loss of material in the metal surface . It may appear in the form of general corrosion or rust on carbon steel , as well as pitting or crevice corrosion on stainless steel .

Why Stainless Steel Corrodes

All metals are liable to corrode when exposed to certain operating conditions , but proper planning and preventive maintenance can keep the damage to a minimum . Taking simple steps to halt corrosion before it spreads requires that technicians possess knowledge of how different types of corrosion occur , and which surfaces should be most frequently examined . Careful observation and remediation efforts can save time and money in the long run .

Figure 2 . In ambient air , stainless steels automatically form a passive , chromium-rich oxide layer on the surface that protects the material from corrosion ( top ). If that layer is damaged ( middle ), it will reform automatically ( bottom ). If the environmental conditions are severe , localized damage to the passive oxide layer can render the metal below susceptible to pitting or crevice corrosion .

Figure 3 . With localized pitting corrosion , electrochemical reactions initiate the formation of small cavities within the surface of stainless steel that can eventually grow deep enough to perforate a tube wall , or serve as stress concentration points where fatigue or stress-assisted corrosion can be accelerated .

Figure 4 . Localized crevice corrosion forms between a washer and plate under a bolt ( top ), or in tubing and tubing supports ( bottom ), with accelerated reactions taking place within the confines of the crevice .