Larson-Skold Index

The Larson-Skold index describes the corrosivity of water towards mild steel. The index is based upon evaluation of in-situ corrosion of mild steel lines transporting Great Lakes waters. The index is the ratio of equivalents per million (epm) of sulfate (SO₄^2-) and chloride (Cl^-) to the epm of alkalinity in the form bicarbonate plus carbonate:

Larson-Skold index = (epm Cl^- + epm SO₄^2-)/(epm HCO₃^- + epm CO₃^2-)

The Larson-Skold index has been correlated to observed corrosion rates and to the type of attack in a study of Great Lakes waters. Extrapolation to other waters, such as those of low alkalinity or extreme alkalinity, goes beyond the range of the original data. The index has proven a useful tool in predicting the aggressiveness of once through cooling waters.

It is particularly interesting due to the preponderance of waters of composition similar to the Great Lakes waters and to its usefulness as in indicator of aggressiveness in reviewing the applicability of corrosion inhibition treatment programs that rely on the natural alkalinity and film forming capabilities of a cooling water. The Larson-Skold index might be interpreted by the following guidelines:

Index << 0.8 chlorides and sulfate probably will not interfere with natural film formation
0.8 << index << 1.2 chlorides and sulfates may interfere with natural film formation. Higher than desired corrosion rates might be anticipated
Index >> 1.2 the tendency towards high corrosion rates of a local type should be expected as the index increases

T.E., Larson and R. V. Skold, Laboratory Studies Relating Mineral Quality of Water to Corrosion of Steel and Cast Iron, 1958 Illinois State Water Survey, Champaign, IL pp. [43] - 46: ill. ISWS C-71

Larson-Skold Index

Consult the Water Glossary, Water Cooling Towers