Corrosion of Mixed Metal Fire Sprinkler Systems

This paper is in response to the many questions that are asked regarding automatic fire sprinkler systems using copper tube and fittings in conjunction with conventional steel pipe.

The capability of mixed metals specifically to resist galvanic corrosion, is sometimes questioned. The concern expressed focuses primarily on composite systems where copper tube branch lines and cross mains are used in connection with steel pipe feed mains, risers and standpipes. Other areas of concern are the use of steel band or ring hangers to support copper tube, the practice of threading copper tube through steel sleeves for wall penetrations and the potential for galvanic corrosion between copper tube and metal building studs in interior wall partitions. Frequently, the erroneous assumption is made that copper components corrode preferentially. This is not accurate. Should galvanic corrosion occur, it is the steel or cast iron components which are normally attacked. The protection of the ferrous materials via dielectric separation, protective insulating tape, sleeves, or grommets must be evaluated. The beneficial results achieved versus the attendant increased costs of such protective measures must also be evaluated. The following analysis is provided for your information.

A standard galvanic series (a practical simplification of the electrochemical series of the elements) provides a first approximation of galvanic corrosion potential and is a starting point for understanding the problem. In a standard galvanic series, the common metals are ranked from the most active (anodic) to the least active (cathodic). Generally, when two dissimilar metals are coupled in the presence of an electrolyte (such as water), the potential for accelerated corrosion of the more active metal in the couple increases in proportion to the position of the metals in the standard galvanic series.

For example, using the standard galvanic series for common metals in sea water ( Table 1), aluminum which is ranked second in activity to magnesium would exhibit a greater potential for galvanic corrosion when in direct contact with copper, which is ranked in the lower third of the series, than iron (steel) which is ranked in the middle third of the series.

The amount or severity of galvanic corrosion, however, cannot be predicted simply on the basis of the relative ranking of the two metals forming the couple in a standard galvanic series. The rate and extent of corrosion potential are also dependent upon:

• The electrical resistance of the joint between the metals
• The conductivity of the electrolyte
• The relative areas or masses of the anodic metal with respect to the cathodic metal
• The polarization of the anodic metal through the buildup of adherent surface films

Considering each of these factors in the context of the automatic fire sprinkler system, we find that:

• Black steel pipe is anodic with respect to copper tube. However, the close proximity of the two metals in the standard galvanic series indicates that the corrosion potential is only moderate.
  
• Where the transition from copper tube to steel pipe employs a standard gasketed flanged fitting, the electrical resistance of the joint becomes quite high with the potential for corrosion becoming reduced.
  
  The use of threaded transitional fittings of either wrought or cast copper alloy are also appropriate when joining copper tube and steel pipe conductors. The pipe dope or tape sealant, which is normally applied to the threads further tends to increase the joint's electrical resistance, further decreasing the corrosion potential.
  
• In a wet pipe sprinkler system, the standing water condition tends to reduce the aggressive character of the electrolyte (conductivity of the water) as the corrosive elements in the water react with the pipe or tube to form superficial surface films. This is unlike the situation encountered in domestic and process water distribution systems where intermittent flow tends to replenish the strength of the electrolyte.
  
• In dry pipe sprinkler systems, particularly where positive drainage is not assured, the potential for galvanic corrosion may increase slightly in those portions of the system where water collects in the presence of a copper-steel couple which acts as a catch basin.
  
• The relative masses of the metals in contact have a significant impact on the galvanic corrosion potential. When the mass of the copper is small in comparison to the mass of the steel, the corrosion potential is relatively low. For example, no special precautions are taken when installing bronze sprinkler heads in either cast iron or malleable iron fittings. Bronze bodied valves are frequently installed in steel pipe sprinkler systems without exercising special protective measures. In large iron-bodied valves, bronze seats, wedges and stems are commonly employed.
  
  Since economics tend to dictate the use of smaller diameter, lighter weight copper tube branch lines and cross mains in conjunction with steel pipe feed mains and risers, the mass or area of the steel pipe portions of the system tend to be relatively large in comparison to the copper tube portion of the system, thus reducing the potential for significant corrosion.
  
• Polarization of the anodic metal through the buildup of adherent surface films is probably not a factor.

In essence, when all factors are weighed, the potential for significant corrosion of steel pipe in a composite copper/steel pipe fire sprinkler system is relatively low and the requirement for dielectric separation usually is not warranted, unless the mass of the steel is small in comparison to the mass of the copper and intermittent water flow is permitted within the system thus replenishing the electrolyte. Routine inspection testing and flushing, or other infrequent maintenance that introduces fresh electrolyte into the system is not a significant factor. The inherent corrosion resistant properties of the copper metals indicate that system flushing of all-copper systems need not be completed on the same frequency as is normally required for the flushing of corrosive scale and rust from all-steel systems.

Copper fire sprinkler systems are frequently concealed within the interior stud wall partitions, When metal building studs are employed, the copper tube is normally threaded through an opening in the web of the metal stud, A common field practice permits the tube to rest on the edge of the web opening, thereby eliminating the need for additional hangers, Since the water within the fire sprinkler system is stagnant, it will assume the ambient temperature found within the wall partition with the result that condensation will not collect on the outside surface of the copper tube. Without the presence of moisture at the point of contact between the copper tube and the metal building stud, galvanic corrosion should not occur. The requirement for protective insulating sleeves or grommets, or the application of tape or other insulating material at the point of contact, may not be warranted in this instance. Care should be exercised, however, to insure that the copper tube is not abraded when it is threaded through punched or drilled openings in the metal building stud during installation of the fire sprinkler system.

Similarly, the use of steel band or ring hangers, either plain or cadmium plated, for the support of copper tube and steel sleeves (for wall penetrations) which are in contact with copper tube will normally prove satisfactory except in those instances where contact surfaces are frequently or continuously moist. Laundries, dye houses, piers and wharves are a few examples of locations where plain steel hangers should not be employed in conjunction with copper tube. Threading copper tube through steel sleeves for the penetration of foundation walls, or any other wall where the presence of moisture could be expected to occur, are examples of locations where plain steel sleeves should not be employed in conjunction with copper tube unless a suitable insulating material is installed to minimize or preclude the corrosion of the steel component.

Economics and modern building practices and techniques, have spurred the rapid growth of copper tube and fitting use in fire sprinkler system installations where copper tube and steel pipe are either joined, or the two metals are placed in direct contact. There have been no reported cases of galvanic corrosion in these systems, nor are any anticipated where all factors have been considered and rational judgments made.

While it is acknowledged that a significant galvanic corrosion potential will not occur simply by the contact or joining of copper and steel materials, reasonable care must be exercised in determining the desirability of isolating the contact surfaces for each individual application. A blanket requirement to insulate the contact surfaces would place unnecessary additional costs onto the fire sprinkler system installation and in most typical applications would afford negligible additional protection.