lime scale removal

Dave · **Posted:** Fri Oct 07, 2016 4:28 pm

It's been several years since this topic was discussed - wondering if anybody has used a chemical scale remover on your boiler while it was in service to remove lime scale? If not, a safe chemical method for softening it before doing a boiler wash?

Overmod · **Joined:** Thu May 24, 2012 1:37 pm **Posts:** 2492

I was expecting the feedwater moral equivalent of one of our 'tribology nerds' to chime in here. Surely there are people interested in this who don't need to keep the chemistry or action proprietary?

I'm assuming (for purposes of this discussion) that there is no practical way to remove the calcium material from the feedwater, or to dose the tender water to preclude formation of a hard scale in the first place.

I'm also presuming that the boiler pH is being maintained at some degree of proper alkalinity, around pH 11, and there is some kind of additional treatment being dosed to control potential corrosion (e.g. at incompletely-caulked seams) in the boiler. That would (to me, at least) rule out most of the acid-based solvents for lime scaling, although I don't know if there are buffers or inhibitants that would work with something like dilute sulfamic acid to preserve the "deliming" action. In this scenario it MIGHT be possible to pump out the boiler water, neutralize the shell and inject water with a proper inhibited-acid and EDTA (or whatever) content just long enough to accomplish maintenance deliming. Ideally the CaCO3 or whatever you're using to achieve the alkaline pH should do the passivation and neutralization of any remaining acid solution at the (immediately) subsequent refilling, without the usual 'boiler cleaning' double flush afterward...

I'd be at least tempted to use some variant of Porta-McMahon treatment at general small-engine working pressure (where the polyamide antifoams will have a tolerable working life). I would try at least a high tannin content, some EDTA, and some sort of ethoxylated 4-nonylphenol surfactant in the boiler water, with periodic on-engine testing and proper dosing and injection 'as appropriate' into the feedwater.

That should at least get the ball rolling for more discussion...

Dave · **Posted:** Sat Oct 08, 2016 6:32 pm

Assume the locomotive spent many working years in a limestone industry and the inside was saturated. Over the past decade or so the lime has been leaching out and scaling the water side of the firebox and tubes. Minimal water treatment was done up until 3 years ago when some premature tube failures happened - since then it has been kept up to some extent. Figure about 20 operating days per year. A retubing is scheduled within the next few months, and it is a very small locomotive so there's limited access to the boiler interior to blast, so hoping we can find a way to soften the scale during its last several in service days so boiler washing with high pressure water will remove a lot of it. I'm not interested in very acidic measures - they can do a lot of damage. I think if we can get it clean, maintaining a good treatment program will be much easier.

I do notice there's a breakout session on water treatment at November's convention in savannah - but I'm concerned that the presenter is affiliated with a water treatment system and after a lot of trouble trusting salespeople to advise on treating which always resulted in buying more of their product whether it worked or not, so looking for mess self interested advice.

Overmod · **Joined:** Thu May 24, 2012 1:37 pm **Posts:** 2492

I think enough is known about general chemistry to allow you to avoid either the boiler-cleaning or the running-water-treatment nostrum peddlers. But we need to have some of the folks familiar with the open-source chemistry in practice chime in here.

The first question is: Do you have actual samples of the scaling inside the boiler, and have you done actual physical analysis of what is there? The "treatment" (including any 'softening up' of the baked scale) is going to depend materially on what you have in there. If it is 'hard scale' there isn't anything much you can do other than remove it (with the usual sort of 'chemical cleaning' using one of the inhibited acids, or polyammoniated EDTA, or the general approach mentioned in Shaun McMahon's paper on the experimental development of the Porta-McMahon treatment which produced all the loose scaling in the legs that had to be broken up with rods to flush out). I'd suspect that ammoniated citric acid may be the 'likeliest' treatment to undertake, and inhibited HF of course the unlikeliest! Note that one popular "boiler wash treatment" is sulfamic acid (as in some OTC household deliming/rust-removal products) combined with polyethoxylated 4-nonylphenol (with the slightly daunting alternative name poly-ethylene glycol (or poly-propylene glycol) nonyl phenyl ether). I don't know if you can do a partial acid cleaning in a riveted-seam boiler (e.g. not dissolve all the scaling in one "pass") and achieve full effective passivation for all the little areas under the scale and in the cracks where the acid might stay in high effective concentration -- my conservative principle says 'hell, no, that would be unwise'. So you might be stuck with going ahead with all the cleaning steps (up to the last hot "rinse" with full-operating-alkaline-pH water) just before your 'known' maintenance exercise with the tubes -- get it done and done right, and then take appropriate care not to let the problem recur. One gentle advantage of the full cleaning and passivation is that you could go ahead and coat or treat the waterside of the shell if that can be put in the budget.

Where ARE all the kibitz-prone boiler maintenance nerds (now that 844 is back in service...)?

Dave · **Posted:** Sun Oct 09, 2016 6:24 pm

Bob, this was just brought to my attention last week and I don't think there will be a washout until it's time to retube. Might see if somebody can reach in to a washout plug hole and scrape something off for a check at the lab. I miss those guys when I need them!

rcw7585 · **Joined:** Sun Nov 13, 2005 11:25 am **Posts:** 85

Add soda ash (or softened water) to the tender water every day until you get the alkalinity up. After you get close to a pH around 11 you can stop adding the soft water or soda ash. You won't have much scale at all.

I found that a lot of existing scale would come off with the treatment program I was running. Arch tubes looked brand new on every boiler wash, even after years of service.

One issue with soft water you will most likely have, however is a propensity for the boiler to prime. Engineers would complain to me about "foaming", but it wasn't foaming that was the issue, it was because they would rip the throttle open while carrying too much water in the glass. When the bottom of the sight glass is 11" above the highest part of the crownsheet, there is NO reason to fill it past the top of the glass, unless you are bottling the boiler up for the night, or planning on blowing down 1/4 of a glass.

Oh, yeah, plan on a LOT of blowdowns with softened water.

Of course, your water source will vary from what I had.

R.C. Whitehead

Overmod · **Joined:** Thu May 24, 2012 1:37 pm **Posts:** 2492

Quote:

"Add soda ash (or softened water) to the tender water every day until you get the alkalinity up. After you get close to a pH around 11 you can stop adding the soft water or soda ash. You won't have much scale at all."

But wouldn't he be running with a pH near 11 already? I thought that was pretty well understood as being the 'right' pH net of all chemical effects for boiler treatment in staybolted-firebox firetube boilers. Do there have to be allowances when there is a combination of existing hard scaling and older riveted construction?

J.David · **Posted:** Mon Oct 10, 2016 12:01 pm

Hi Dave:
I have sand blasted inside smaller boilers than that yours.
In my opinion, there is no substitute for sand blasting to "near white metal" and promptly painting with Apexior No.1.
We recently did our 2nd 1472 on our No.40, there was no adherant scale on the portions painted with Apexior, indeed, you could see all of the brush marks.
Good luck!
J.David
PS: I did have one experience with a locomotive which had been acid cleaned: former Commonwealth Edison No. 5 at the Illinois Railway Museum. The locomotive's owner had contracted out the cleaning before it was moved to the museum. For many boiler washes thereafter, we had enormous amounts of scale to remove, it was packed over a foot deep in the water legs of the firebox. We finally got the boiler barrel clean after we removed a number of the bottom tubes. Luckily, they lined up with the firebox door hole so we were able to pull them out through the rear tube sheet. JDC

rcw7585 · **Joined:** Sun Nov 13, 2005 11:25 am **Posts:** 85

Overmod wrote:

Quote:

"Add soda ash (or softened water) to the tender water every day until you get the alkalinity up. After you get close to a pH around 11 you can stop adding the soft water or soda ash. You won't have much scale at all."

But wouldn't he be running with a pH near 11 already? I thought that was pretty well understood as being the 'right' pH net of all chemical effects for boiler treatment in staybolted-firebox firetube boilers. Do there have to be allowances when there is a combination of existing hard scaling and older riveted construction?

Hadn't read through his follow up post, was just responding to the OP.

Every boiler wash, of course, you have a clean slate and are back to a pH of 7. I was assuming that's where he was.

It would take me about a week and a half (IIRC) to bring the alkalinity back up. My treatment program was completely non-commercial, developed by a volunteer that was a research chemist for FMC. I would get all the bulk chemicals that I needed from Textile chemicals.

I was doing titrations and TDS testing on boiler water every day to track where everything was at so I could modify the tender treatment accordingly. Chemicals I was using/tracking were Sodium Sulfite, Sodium Hexametaphosphate, Soda Ash, and Tannin. Pretty much standard stuff for steam locomotives. I think we were trying to run with a slight trace of sodium nitrate in the boiler for caustic embrittlement protection as well.

As far as descaling a heavily scaled up boiler... it would take a while with softened water, so it would not be "all fixed" in 2 weeks, no way, no how. The phosphate helped keep the scale as sludge rather than nucleated scale.

I actually would completely stop treating with softwater/soda ash after the 2nd to last boilerwash of the season just to establish a light coat of protective scale on things, before draining/washing/blowing out the boiler/superheaters for the winter.

Something really heavily scaled up... deal with it, mechanically, when you retube. It's called a scaling gun for a reason! Heh

RCD · **Posted:** Mon Oct 10, 2016 12:46 pm

For any kind of limescale or rust removal I always turn to my trusted friend HCl but I have never worked on a boilers so I can't say if that is a good idea.

Overmod · **Joined:** Thu May 24, 2012 1:37 pm **Posts:** 2492

Just remember that your trusted friend has to be INHIBITED to avoid many predictable problems (reminiscent of RFNA, now that I think of it... ;-O)

To provide a little background on the general subject of inhibited HCl, here are some notes from a review article on the subject from a couple of years ago. I have cleaned up the prose a bit for easier reading, and removed the actual footnoting to article references, but they are easily available (the paper itself is at [url]http%3A%2F%2Fwww.jmaterenvironsci.com%2FDocument%2Fvol6%2Fvol6_N1%2F28-JMES-564-2014-Ambrish.pdf[/url]

Quote:

[Historically,] A. J. Saukaitis and G. S. Gardner [5] in the year 1956 claimed a derivative of rosin amine in which rosin amine and a ketone having a reactive hydrogen atom adjacent to the carbon were reacted with formaldehyde in the presence of hydrochloric acid. The derivatives were effective as inhibitors up to temperatures as high as 302 F. Mansfield et al. in 1959 claimed a HCl inhibitor made from the reaction product of ammonia or methylamine, formaldehyde and an olefin such as vinyl toluene, alpha-methylstyrene or alpha–paradimethylstyrene. In 1963, Monroe et. al. claimed a major accomplishment: the reaction of an amine or amide (having an active hydrogen atom) in excess formaldehyde with a ketone (having an active hydrogen alpha to the carbonyl) in the presence of hydrochloric acid catalyst and an excess of tall oil fatty acid produced a good inhibitor base. This when blended with a surfactant and an acetylenic alcohol was claimed to produce a hydrochloric acid corrosion inhibitor equal or superior to arsenic (see also antimony trioxide with stannous chloride) in performance at 250 F.

Two major advances were made in 1964 and 1965 by Cizek and Karlitz. The first includes a crude quinoline benzyl chloride quaternary with an oily substance like kerosene, an alkyl naphthalene, etc. for oil-wetting properties. The second involves formic acid as an intensifier or inhibitor aid. When such an intensifier is included in the acid wash together with appropriate corrosion inhibition, the performance properties of the corrosion inhibitor are enhanced. The temperature range claimed in the first case was about 200 to 220 F and in the second 220 to 350 F at an appropriate overpressure. Beale and Oakes obtained a patent in 1966 describing synergistic combinations of acetylenic alcohols acting as corrosion inhibitors for aqueous solutions of non- oxidizing acids.

Lee in 1969 evaluated the inhibiting effect of 7-nitroso-8-hydroxy-quinoline for HCl cleaning of mild steel. The efficiency was more than 90%.

Keeney and Johnson found in 1973 that most inhibitors used with hydrochloric acid have enhanced effect when cuprous iodide is combined with the acid inhibitor over a temperature range of 150 to 450 F. This intensification was claimed to be effective over a temperature range of 150 to 450 F and relatively low pressure. McDougall et al. in the same year developed an inhibitory derivative composed of acetylenic alcohol, amine and/or amine quarternary surfactant, organic solvent, and formic acid derivatives for use in 15 and 28% hydrochloric acid at elevated temperatures. The next year, Griffin and Dollarhide described the use of an ionized iodine compound with a corrosion inhibitor consisting of a heterocyclic amine, acetylenic alcohol, a hydrocarbon solvent, and mixture of appropriate surfactants. Without the iodide salt as intensifier this inhibitor was effective only in acid without alcohols and/ or mutual solvents.

In 1977, Sullivan et al. proposed the use of quaternary chloromethyl naphthalene with acetylenic alcohol, surfactants and a formic acid derivative. Dill and Keeney added to this in 1978 by showing that mixtures of formic acid and hydrochloric acid may have particular application as stimulation fluids in very high temperature environments, and that a hydrochloric-formic acid mixture will be less corrosive than straight hydrochloric acid. Such a mixture with 10% hydrochloric acid and 'sufficient' formic acid was described as comparable to 15% hydrochloric acid in effectiveness.

Anderson et al. in 1984 found that the inhibitive effectiveness of 4,7-diphenyl-1,10-phenanthroline in acid medium is due to the presence of available electrons in the ring. Frenier et al. in 1985 developed a mixture of iodide salt, a formic acid compound, an amine or a quaternary nitrogen compound, and/ or an oxygen-containing compound such as phenyl ketone, a phenyl aldehyde, or an alkynol.

Granese and Rosales in 1987 elucidated the mechanism of corrosion inhibition of iron and steel in HCl media: effective HCl inhibitors for iron and steel mainly belong to the group of nitrogen-containing compounds such as alkyl and aryl amines, saturated and unsaturated N-ring compounds, condensation products of amines with aldehydes and ketones and carboxylates, amines, nitriles, aldoximes, keto-oximes. [sic] They observed reduced corrosion by N-containing organic compounds like acridine, hexamethylene, quarternary ammonium sulphate etc. at 85 C, and noted that urotropin (a nitrogen-containing inhibitor) is widely useful in HCl.

Growcock and Frenier developed (in 1988) what they considered an environmentally-friendly inhibitor. This consisted of cinnamaldehyde and a surfactant, and could be used at relatively low temperature. Chan et al. evaluated a macrocyclic compound by potentiostatic and impedance methods, which gave 82% efficiency at 25 C in an acid chloride environment. Hashi and Zucchi found acetylenic alcohols effective, and Trabanelli et al. examined some heterocyclic compounds containing more than one nitrogen atom, to see if they could establish correlation between molecular structure and the inhibition efficiency of the various compounds they examined. 2,2-biquinoline, quinoxaline, quinozoline and 2-mercaptopyrimidine showed good inhibiting efficiencies (80%-90%) at temperature 25 to 60 C. (The influence of some of the substances on hydrogen penetration into and consequent embrittlement of steel was also studied by these authors.)

Fonda et al. determined in 1990 that the inhibitory effect of some substituted phenyl-N-phenyl carbamates on corrosion of iron in 2N HCl occurred by way of adsorption
through the oxygen atom and the adjacent -NH nitrogen atom of a phenoxy group. Effective inhibition depends upon the concentration of the inhibitor as well as its chemical composition.

In 1991, mercury salts were claimed by Cizek as an effective inhibition intensifier. The
tested inhibitor here was said to be composed of an acetylenic alcohol, a quaternary ammonium compound, an aromatic hydrocarbon, and surfactant.

Granese et al. studied inhibition by various heterocyclic N-compounds such as N-hexadecyl derivatives of pyridine, quinoline, and acridine in HCl (by electrochemical and surface analysis) and concluded that acridine had the strongest interaction with iron and steel surface while pyridine had the least. According to them the efficiency of these compounds would therefore increase with number of aromatic systems and electrons available in the molecule. Stupnisek et al., also in 1992,looked at the inhibiting action of various substituted N-aryl pyrroles on corrosion of steel in strong acid solution. They found that inhibition efficiency of pyrroles was significantly influenced by the type and position of the functional groups. Thus an N-pyrrole bearing fluorine at ortho position gave better performance than other pyrrole derivatives.

Quraishi et al. [one of the co-authors of the review from which this is taken] studied some new triazole derivatives, namely 4-aminobenzylidene 3-propyl-5-mercapto-1,2,4-triazole, 4-aminosalicylidene-3-propyl-5-mercapto-1,2,4,triazole (ACPMT), 4-
aminovanillidene-3-propyl-5-mercapto-1,2,4,-triazole, and 4-aminodimethylamino-benzylidene-3-propyl-5-mercapto-1,2,4-triazole (ADPMT). With the exception of ADPMT, all the triazole derivatives showed an inhibitor efficiency of 98%. In particular, ACPMT and ASPMT showed an inhibition efficiency of 99%, comparable with that of propargyl alcohol.Quraishi also synthesized selected condensation products of aromatic aldehydes and p-phenylenediamine. Of these, 2,4-dicinnamyledene aminophenylene (DCAP) was found to be the best corrosion inhibitor: this exhibited 99.75% inhibition efficiency for mild steel and 99.12% for N-80 steel at 5000 ppm inhibitor concentration. While all the investigated compounds were mixed type inhibitors, 500 ppm DCAP on N-80 behaved predominantly as an anodic inhibitor.
DCAP
Quraishi et al. [30] in the year 2000 synthesized selected hydrazides and thiosemicarbazides of fatty acids with 11, 12 and 18 carbon atoms. All these investigated compounds were of mixed type, and adsorption studies showed that they all followed Temkin’s adsorption isotherm. In the same year Quraishi et al. synthesized a novel N- and S-containing heterocyclic compound (CAHMT)which is a mixed-type inhibitor where propargyl alcohol is primarily cathodic. In further work 1-cinnamaldehyde thiosemicarbazone (CTS) was found to be a particularly good corrosion inhibitor, exhibiting 99.67% inhibition efficiency for mild steel and 97.26% for N-80 steel at 5000 ppm concentration.

The next year, Quraishi et al. studied dicinnamylidene acetone (DCA), disalicylidene acetone (DSA) and divanillidene acetone (DVA). DSA exhibited the best performance, giving an inhibition efficiency of 98.7%.

4-salicylideneamino-3-hydrazino-5-mercapto-1,2,4-triazole (SAHMT)is a mixed type and it
inhibits steel corrosion by blocking active sites in the metal.
SAHMT
J. Mater. Environ. Sci. 6 (1) (2015) 224-235 Ambrish et al.
1-cinnamaldehyde thiosemicarbazone (CTS), 1-benzaldehyde thiosemicarbazone (BTS), and 2-4,dicinnamyldene aminophenylene (DCAP) were compared with a "standard commercial corrosion inhibitor." and were found to have 'higher corrosion protection values'.

Cizek produced a good review on the literature of HCl inhibitors in 1994. This had special reference to the history and development of oil well acidizing and acid corrosion inhibitors used in the petroleum industry [but the results should be valuable for our purposes here]. As deeper and hotter wells were drilled, stimulation acids were exposed to hotter conditions and this is the situation under which organic inhibitors required intensifiers. Corrosion inhibitors inherently suited to high-temperature conditions are also discussed.

Qian-ding et al. reported on Mannich reaction processes in 2008. These involve the presence of primary amine, formaldehyde, and acetophenone to which acetone has been added. By controlling reaction conditions, two hydrogen atoms in the amino group of the primary amine engage in a Mannich reaction with acetophenone, acetone, and formaldehyde respectively and thus a new kind of Mannich base can be obtained. Xiong et al. then further studied Mannich base derivation from benzylamine, acetophenone,
and formaldehyde. Their optimum synthesis conditions were determined by the orthogonal test, where n(acetophenone) n(formaldehyde) n(benzylamine) = 1:1.5:1.5, reaction time was 10 hours, pH of the reaction mixture held at 2-3, and reaction temperature = 90 ℃. [These researches and methodology may be notable because the Mannich reaction is said to be the basis of one of the Rodine proprietary inhibitor formulas]

Yadav et al. [43] investigated the effect of synthesized amino acids, acetamidoleucine
(AAL) and benzamidoleucine (BAL). Their inhibition efficiency was significantly increased with increasing concentration, and their adsorption at the surface of N80 steel was found to follow the Langmuir isotherm.

Frenier et al. studied two model compounds, n-dodecylpyridinium bromide (n-DDPB) and 1-octyn-3-ol. They were able to show that the pyridinium forms a weak bond with the chloride-covered surface and is sensitive to temperature and HCl concentration, but octynol chemisorbs and produces a film that contains a reaction product of the acetylenic alcohol.

Migahed et al. noted that 6-methyl-5-[m-nitrostyryl]-3-mercapto-1,2,4-triazine as corrosion inhibitor reduced anodic dissolution of mild steel in 12% HCl 'remarkably'. The strong adsorption ability of this compound can be attributed to the presence of multiple adsorption centers of nitrogen as well as π-donor moieties.

The best corrosion inhibition efficiency is obtained in the range of about 1.5% by weight of the organic inhibitor, and the same percentage of any synergist.

A promising mixture may be Li et al.'s reported combination of a Mannich base with propargyl alcohol and an "organic synergist" [possibly a PEG/PPG block-copolymer nonyl phenyl ether] Their experimentation indicated that the optimal exposure time is about 14 hours at 90 C, the molar ratio of reaction materials is 1∶2∶4 for amine, aldehyde and ketone respectively, at a concentration of about 1%, and the pH value is kept low at 2 to 3 (corresponding to about 20% HCl).

hullmat991 · **Joined:** Sun Dec 28, 2014 11:12 pm **Posts:** 232

Shame you just can't fill the boiler with dilute HCl and let it sit for a while, and then flush it. That would certainly take care of it but then you might have much more corrosion after the process.

Dave · **Posted:** Tue Oct 11, 2016 11:29 pm

Bob, that's a lot of compressed information that makes my eyes glaze over. Certainly concrete and specific in and of itself, but not comprehensible from the layman's perspective enough to help create a strategy for solving the problem. My first wife could.... let's not go there.

No, I'm not going to dump some amount of acid into the boiler and hope it only makes the scale and not any steel disappear........many people have gotten into some real trouble trying this when they don't have real expertise on hand, and some real experts screw up now and then also.

Overmod · **Joined:** Thu May 24, 2012 1:37 pm **Posts:** 2492

Quote:

Bob, that's a lot of compressed information that makes my eyes glaze over. Certainly concrete and specific in and of itself, but not comprehensible from the layman's perspective enough to help create a strategy for solving the problem."

I confess I didn't post it to solve your problem; I posted it so that others on the forum could discuss what the best inhibitors for a "dilute HCl wash" that does not involve expensive and perhaps wrong-size-fits-all/nostrum boiler potions.

The key issue with doing this is that there will be nooks and crannies in the scale, and in the boiler construction, that preferentially trap acid in places that the subsequent washout and passivation steps might miss. This is a principal reason why inhibited acid, not "HCl/muriatic acid" from the hardware store, is the only kind of acid that is ever used in a riveted boiler. One take-home point outside MEGO level is that, for the right type and 'package' of inhibitors, their concentration may be only at most a couple percent by weight, and they will work wherever the acid might have infiltrated during a given wash procedure, so there will be no little corrosion surprises when you get the tubes out. (I am tempted to ask whether you wouldn't want to get the scale off the tubes as well as the shell if you were planning to safe-end them for economical reuse...)

For Dave's particular concern, dilute (inhibited) acid for a 'running scale reduction' wouldn't be needed, because his timeline to a point where a full-on cleaning is quite short. I'd like to extend the discussion, now that it's been started, to ways of reducing scale with the locomotive not receiving heavy boiler attention quickly. One example would be the degree of 'cleaning' that would not produce major aggregation of loosened hard scale in the water legs, or scale 'pieces' of a size that would cause heat-transfer problems in the legs and have to be broken up with rods for washout...