The following discussion thread reflects the opinions of a few corrosion specialists on the subject. The Internet discussion was held between July 24 and 27 in the year 2000.
The question was:
How does one relate such the Langelier Index to corrosion rates with some degree of comfort? Or, how many of you are comfortable using such an index?
The responses were:
There is no correlation between the LSI (or RSI) and corrosion of steel in waters. They are simply indications as to the deposition or dissolution of calcareous deposits. The Stiff-Davis Index (SDI) applies to brines. A first approximation of a Corrosion Index for steel is
CI = (me Chlorides + me Sulfates)/ me Methyl Orange Alkalinity
Of course, temperature, DO and DS are also involved. See MTI Publication No. 43, "Materials of Construction for Once-Through Waters" (1995).
What effects are ascribed to suspended solids ?
One does not relate corrosion to any of the indices. Precipitation of sparingly soluble salts (which the indices indicate) has no relation to corrosion. Suspended solids are not included in the scale indices. However, they are sites for crystallites to adhere to and can also adsorb scale control agents (inhibitors) reducing overall effectiveness of the chemical treatment program.
Dr. Dillon just gave me a lesson on "never say never" last week. I am now more inclined to think "sometimes" or "it depends" are very important terms that must be used regularly in this craft. Quoting page 70 of "The Corrosion Engineer's Reference Book"..."If the index is a minus quantity, calcium carbonate does not precipitate, and the probability of corrosion (if dissolved oxygen is present) will increase with an increase in the negative value of the index."
I agree with Paul that these indexes will only indicate to you the tendency of given waters to deposit scales on metal substrates. Generally speaking, scales precipitated onto metal surfaces can provide protection of the substrate from general corrosion. On the other hand, if the scales are defective and contain voids and cracks etc. they could lead to localized corrosion. For applications such as fluid flow and heat transfer the build-up of scales can create problems. I hope this helps.
R. P. (Mo) Anantatmula
The Langelier saturation index (LSI) gives the corrosion tendency. If LSI is: >0 the water is oversaturated with respect to calcium carbonate =0 the water is stable, without tendency to scaling nor to dissolve calcium carbonate <0 the water have tendency to dissolve calcium carbonate and a possible tendency to corrosion.
When scales inhibitors as polyphosphates, phosphonates, acrylates, are present, (as they affect to calcium carbonate solubility) the LSI does not give information about the scaling tendency.
The LSI does not offer complete information about the corrosivity without to consider other factors, for example, chlorides, sulphates, water velocity, oxygen dissolved, buffer capacity, solid particles (suspended),....
The LSI does not offer information about the tendency to form scales of calcium sulphate, magnesium hydroxide, silicates, and calcium phosphates for example.
A better index is the Singley index. The Singley index considers chlorides, sulphates, alkalinity, oxygen dissolved, buffer capacity, calcium, LSI, exposure time. Singley obtain an expression of corrosion rate for mild steel. (Reference: Pisigan, R. A., & Singley, J. E. (1984). Evaluation of Water Corrosivity Using The Langelier Index and Relative Corrosion Rate Models. Paper presented at the meeting of the National Association of Corrosion Engineers, New Orleans, LA)
Another index is the Ryznar index (RI) if RI is: <6 the water have a tendency to scale >6 the water have a tendency to dissolve CaCO3
This index is empirical.
This index considers the pH of water and pH of saturated water.
This index offers a quantitative measurement of the amount of scales.
Another index is Larson index (LI). Larson considered chlorides, sulphates and total alkalinity. If LI is <0.2 the water is relatively not corrosive.
Another index is Riddick index (RI). The Riddick index offers information about the corrosivity of waters from northeast of United States of America. Riddick considered hardness, alkalinity, chlorides, nitrates, SiO2, oxygen dissolved, oxygen dissolved saturated( at water temperature).
If RI is: <5 the water is too much not corrosive between 6 and 25 the water is not corrosive between 25 and 50 the water is corrosive (light) between 51 and 75 the water is corrosive (moderate) between 76 and 100 the water is corrosive >100 the water is very corrosive.
There are others indexes as Driving force index (FDI), momentary excess index (ME), aggressive index (AI), which considers the calcium carbonate solubility. On the other hand, there are two formulas to calculate (using the Ryznar index) the pit coefficient and the depth pit for cool water and hot water (<=57ºC or <=135F) for mild steel and galvanized steel pipes.
But that relation is not exact for all cases, however gives an approximate corrosion rate for these materials. There are also two formulas for copper pipes and for cool water and hot water. This expression is valid only for certain concentration intervals of pH, oxygen dissolved, carbon dioxide, chlorides and sulphates.
The LSI parameter is related to scaling only, not corrosivity. The factors of scaling and corrosivity are not inversely related. There are, or can be causal relations between corrosivity and scaling for a given water, or even in general. But it is also true that there can be waters which are very non-scaling but also non-corrosive. For example, deionized water that is purged of oxygen, i.e. boiler feedwater.
Also, waters may contain natural or man-made inhibitors that render the water non-corrosive even at negative LSI's. For example a water at pH 6 with LSI =-1.5 with 150 ppm of chromate. It is important to keep in mind that scaling relates only to the tendency of sparingly soluble materials to precipitate on metallic surfaces, and that corrosivity relates only to the tendency to support reactions that make up the corrosion process (anodic and cathodic). While there may be a casual link between the two conditions, there is no causal link.
I agree with Orin, Paul Dillon, and others who have pointed out that the LSI only relates to the solubility of calcium carbonate. The assumption that water which is below saturation with respect to calcium carbonate is corrosive, while occasionally correct, is not reliable. For those who are interested, there are a number of references in the literature reporting on studies of water corrosiveness and attempts to relate the corrosiveness determined by the methods used in each study to water chemistry.
To my knowledge, none of the attempts have been proven to be reliable for general use. The Larson-Skold Index, noted by Dillon, which is [(Cl + SO4)/alkalinity], all in epm, is as good as any. This index was published in 1958. It was based on USA Great Lakes waters, which are not deficient in either alkalinity or buffering capacity. Rob Ferguson includes this index in his Water Cycle computer program, but he cautions against attempting to extrapolate its use to other waters. My opinion is that if you want to know how corrosive the water is, you must collect the empirical data.
I agree. If you all want to know the how corrosive the water is, you should collect empirical (experimental) data. However, the indexes offer (at least), qualitative information about the scaling tendencies (mainly) and corrosivity.
In oilfield produced water all bets are off! The water composition in two different wells may be essentially identical, with the same acid gases and organic acids present, and one well may eat up a pump in a few weeks, and the other produce for years with no corrosion failures. Tubing may perforate in one well in a few weeks, and never in the other.
There have been many papers on predicting corrosion in oilfield waters, several computer programs written and marketed, and still a well will corrode when it is predicted to be non corrosive. In addition, the various scaling indices are of very little use in oilfield brines. I have had access to a proprietary program written by a major oil company research lab and the results were scaling tendencies, which in many cases were not at all indicative of the actual scaling of the well. There are several good prediction programs commercially available, and a lot of work has been done on these for best predictions.
There has been enough discussion about scaling indices to fill a small library. But what often seems to be unappreciated is that an index is not a phenomenological parameter. An index is a measure of a tendency by taking a number of related factors into account. As such, it cannot predict specific incidences. For LSI, one can have a strongly positive LSI and still not experience scaling.
Among their reasons are kinetics, substrate, or unknown or unconsidered constituents that may inhibit scaling. On the other hand, supersaturation ratio is an absolute predictor of precipitation. As long as all relevant substances are considered it will positively predict whether scaling will take place in a given time frame if the kinetics are properly accounted for. Not only will it predict scaling, but it can also predict the amount of scale that can form.
It is a phenomenological parameter. LSI is convenient as a guide because it is relatively simple to calculate (one needs very few analytical inputs), and often gives good agreement with observations. But this should not obscure the fact that it is imprecise, and is still only an index, not a parameter.
I've been reading all these cogent replies to the problem of scale/corrosion prediction and want to point out one important factor: the water analysis itself. My experience has been that many of the water analyses used for the index calculation are poorly collected and analyzed.
It is necessary to run carbon dioxide , pH and alkalinity determinations on site at the time of collection because these components will change rapidly on standing .The presence of organic acids will be reported as alkalinity which will of course badly skew the index by reporting too high a bicarbonate content. Careful examination of the data is necessary to detect errors and anomalous values which can affect the calculation.
See also: Calcium carbonate, Carbon dioxide, Chlorination, Dissolved oxygen, Langelier calculation, Langelier index, Larson-Skold index, Oddo-Tomson index, pH, Puckorius index, Ryznar index, Scaling Indices, Stiff-Davis index, Total dissolved solids, Water corrosivity