Stainless Protection

Salfumant

The product sold in Spain as "Salfumant" is chlorhydric acid (HCl for the chemist) at 24% or 25% concentration.
Should be available anywhere on the world.

<hr width=100% size=1>
 
From your description it appears to me your stainless is suffering from Class II Rouge with a preponderance of the hydroxide rather than pure oxide (Orange colour is a dead give-away).

This class of rouge occurs when chlorides (sea spray) or other halides are present. It is corrosion driven and forms on the surface of the stainless steel at the place where the passive layer is breached. It appears more often on unpassivated and mechanically polished surfaces and may display tubercles (surface inperfections resembling small warts). The stainless steel under these tubercles (if present) will be very shiny and may be pitted. When material from this rouge is analyzed, chlorides or other halides usually are present. The rouge can be easily removed by grinding or polishing, but most often is removed using an acid solution. Citric acid is a good cleaning agent and will repassivate the stainless steel, but if chlorides are present the surface will rouge again.

Class II Rouge forms in a two stage reaction, the first is the dissolution of the chromium oxide passive layer, the second the oxidation of the iron in the substrate:
Cr2O3 + 6Cl- + 6H2O --> 2CrCl3(aq) + 6OH-
2Fe + 4H2O --> 2FeO(OH) + 3H2
This reaction is self-perpetuating by the chloride reacting with the chromium to form hypochlorous acid as a byproduct, and the hypochlorous acid oxidizing the iron and forming more chloride.

Increasing the molybdenum content of the stainless steel increases the resistance to chloride attack. Likewise, replacing the iron in stainless steel with nickel improves the corrosion resistance. This is the progression of alloys with increasing resistance to chloride attack: Type 304L (least), Type 316L, Type 317L, Type 317LM, Alloy 625, Alloys C-276 and C 22 (highest). Whenever a stainless steel system comes in contact with an acid chloride there is a potential for rouging. A pH > 7 solution will have less potential for rouging than pH < 7. Even momentary exposure to an acid chloride solution may set the stage for this type rouging reaction especially if the stainless steel surface is rough.

Mechanically polished surfaces are worse than electropolished surfaces because of the microscopic crevices resulting from smeared metal from the polishing operation. Electropolishing removes these microscopic crevices and produces a passive layer with a higher Cr: Fe ratio. The crevices create concentration cells where the acid chloride solutions may be retained and continue to react, even if the system is given a high pH rinse. Use of a strong surfactant in the rinse will aid in removing the chloride.

So far so good? - now onto the practical cure for your problem....

Pickling and Passivation

Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film. Pickling and passivation are both acid treatments and neither will remove grease or oil. If the fabrication is dirty, it may be neccesary to use a detergent or alkaline clean before pickling or passivation.

Pickling

Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means.
Where the steel has been heated by welding, heat treatments or other means, to the point where a coloured oxide layer can be seen, there is a chromium depleted layer on the surface of the steel underneath the oxide layer. The lower chromium content gives lower corrosion resistance. To restore the best corrosion resistant performance, the damaged metal layer must be removed, exposing a fully alloyed stainless steel surface. Mechanical removal may leave abrasive or other particles embedded (interfering with corrosion performance) or may be impractical, so chemical means are usually employed.
<font color=red>Procedures incorporating pickling solutions of nitric (HNO3) and hydrofluoric (HF) acids remove the scale and the underlying chromium depleted layer and restore the corrosion resistance. Pickling solutions also remove contaminants such as ferrous and ferric oxide particles. </font color=red> Pickling solutions other than mixtures of nitric and hydrofluoric acids exist and can be used for specialised applications.
Pickling pastes, where the solution is mixed with an inert carrier, are commonly used to treat selected areas such as welds.
Pickling involves metal removal and a change or dulling in the visual brightness of the metal.
Electropolishing is a useful alternative to pickling. Metal removal is achieved, but usually results in a bright, smooth and more highly corrosion resistant finish.

Passivation

Passivation is the treatment of the surface of stainless steels, often with acid solutions (or pastes), to remove contaminants and promote the formation of the passive film on a freshly created surface (eg through grinding, machining or mechanical damage).
<font color=red>Common passivation treatments include nitric acid (HNO3) solutions or pastes which will clean the steel surface of free iron contaminants.</font color=red> Care must be taken in selecting and using passivation treatments to ensure the selected treatment will target the contaminant. Passivation will also aid in the rapid development of the passive oxide film on the steel's surface. Passivation does not usually result in a marked change in appearance of the steel surface.
Both pickling and passivation solutions can employ dangerous acids that can damage both the operator and the environment if not handled correctly. Stainless pickling acids are highly corrosive to carbon steel.
It is essential that all acids are thoroughly removed by rinsing the component after completing the process. Residual hydrofluoric acid will initiate pitting corrosion.
It may be advantageous to neutralise the acid with an alkali before the rinsing step.

The corrosion resistance of the stainless steel is also affected by the roughness of the surface after polishing, with a marked decrease of the corrosion resistance as the surface roughness increases above a Ra value of about 0.0005mm. This roughly corresponds to the surface produced by grinding with 360 grit abrasives.
Either passivation or electroplating can be used to improve the corrosion resistance of polished surfaces.

Safety

Pickling and passivation use strong acids, and normal precautions for safety should be followed. Consult Materials Safety Data Sheets and product packaging for detailed advice.

The chemicals needed are readily available from your local welding supply company in the form of ready made pastes.

The above should solve your problem without the use of high priced, diluted products (aimed at the general public) found in local swindlerys and the like.

I use the above pastes for a medium to long term fix but hell, what do I know - I'm only a metallurgist by profession with a boat as a hobby.

<hr width=100% size=1>
hammer.thumb.gif
 
Cliff, this must be the best & most comprehensive answer to the problem I've come across, giving the what/why as well as the solution. Thanks for taking the time to put it down.

Regards,

Richard.

<hr width=100% size=1>
 
Wasn't wasted effort Cliff, I thought it was worthwhile too (the brass one on the other thread as well).

John

<hr width=100% size=1>I am the cat but I am only 6.
 
Another suggestion for polishing pulpits/ pushpits, stantions and railings etc.
Have you tried Metalace from Enginewise? A restoring, polishing and buffing tape ‘system’ for removing light corrosion, salt encrustation, staining and general crud from stainless steel and plated surfaces. Its easy to use, a length of the polishing tape is cut off, wound twice around the thing to be polished and given a good shoe shine action – the result 360 degrees of shiny surface. If you can’t get it from your local chandler, look at their website www.enginewise.co.uk

Regards,

Paul Prince


<hr width=100% size=1>
 
Second that Cliff. I have been in boatbuilding and shipbuilding all my life so I know what to do (cos a metallurgist and welding engineer told me!) but never precisely why until now! All you need to do now is explain crevice corrosion due to oxygen depletion in stainless steels used underwater to me in equally clear terms :-) .....


<hr width=100% size=1>
 
Especially for boatmike - O.K. you asked for it so here goes....

Corrosion Theory
Before we discuss Crevice Corrosion, a basic understanding of Corrosion theory is necessary.
Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases.

Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes, a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.

Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidized, some more easily than others. A tabulation of the relative strength of this tendency is called the galvanic series. Knowledge of a metal's location in the series is an important piece of information to have in making decisions about its potential usefulness for structural and other applications.

The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons that are consumed by a secondary process (cathodic reaction).

These two processes have to balance their charges. The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion.

The electrons produced by the corrosion reaction will need to be consumed by a cathodic reaction in close proximity to the corrosion reaction itself. The electrons and the hydrogen ions react to first form atomic hydrogen, and then molecular hydrogen gas. If the acidity level is high (low pH), this molecular hydrogen will readily become a gas as it is demonstrated by exposing a strip of zinc to a sulfuric acid solution.

As hydrogen forms, it could inhibit further corrosion by forming a very thin gaseous film at the surface of the metal. This "polarizing" film can be effective in reducing water to metal contact and thus in reducing corrosion. Yet it is clear that anything which breaks down this barrier film tends to increase the rate of corrosion. Dissolved oxygen in the water will react with the hydrogen, converting it to water, and destroying the film.

High water velocities tend to sweep the film away, exposing fresh metal to the water. Similarly, solid particles in the water can brush the hydrogen film from the metal. Other corrosion accelerating forces include high concentrations of free hydrogen ions (low pH) which speed the release of the electrons, and high water temperatures, which increase virtually all chemical reaction, rates. Thus a variety of natural and environmental factors can have significant effects on the corrosion rate of metals, even when no other special conditions are involved.

Crevice Corrosion

Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps. Crevice corrosion is initiated by changes in local chemistry within the crevice:
a. Depletion of inhibitor in the crevice
b. Depletion of oxygen in the crevice
c. A shift to acid conditions in the crevice
d. Build-up of aggressive ion species (e.g. chloride) in the crevice

As oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between crevice (microenvironment) and the external surface (bulk environment). The chronology of the aggravating factors leading to a full blown crevice is detailed below. The cathodic oxygen reduction reaction cannot be sustained in the crevice area, giving it an anodic character in the concentration cell. This anodic imbalance can lead to the creation of highly corrosive micro-environmental conditions in the crevice, conducive to further metal dissolution.

This results in the formation of an acidic micro-environment, together with a high chloride ion concentration.

All forms of concentration cell corrosion can be very aggressive, and all result from environmental differences at the surface of a metal. Even the most benign atmospheric environments can become extremely aggressive.

The most common form is oxygen differential cell corrosion. This occurs because moisture has a lower oxygen content when it lies in a crevice than when it lies on a surface. The lower oxygen content in the crevice forms an anode at the metal surface. The metal surface in contact with the portion of the moisture film exposed to air forms a cathode.

The chronology of crevice corrosion is as follows

Autocatalytic process - Stage one
Stage one of a crevice formation
At time zero, the oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere.

Calculation example
Assuming the total surface area is 100 cm2
Assuming the current density is 0.022 mA cm-2 for both the anodic and cathodic reactions.

The total anodic current (corrosion reaction) of 2.2 mA is absolutely equal to the total cathodic current while the corrosion penetration rate is related to 22 mA cm-2 that, using the convertion chart shown below, translates into 0.255 mm y-1 or 10 mpy for a steel specimen.

For all intent and purpose such a corrosion rate is considered benign for most applications.

Autocatalytic process - Stage two

Stage two of a crevice formation
Because of the difficult access caused by the crevice geometry, oxygen consumed by normal uniform corrosion is very soon depleted in the crevice. The corrosion reactions now specialize in the crevice (anodic) and on the open surface (cathodic). The large cathodic surface (Sc) vs. anodic surface (Sa) ratio (Sc/Sa) that forms in these conditions is a definitive aggravating factor of the anodic (corrosion) reaction.

Calculation example

Assuming the total surface area is still 100 cm2 with 99 cm2 occupied by the cathodic area and only 1 cm2 corresponding to the crevice or anodic surface. In reality this imbalance can be larger or smaller depending on local geometry characteristics.

The surface area ratio would then be 99/1 or 99.
Assuming the initial current density of 0.022 mA cm-2 is the same for the cathodic reaction this would mean that the total cathodic current would be equal to 99 cm2 x 0.022 mA cm-2 or 2.18 mA.

This also mean that the corrosion or anodic reaction has to provide the electrons to satisfy the cathodic demand.

The total anodic current would therefore now be 2.18 mA and the anodic current density be 2.18 mA/1cm2 or 2.18 mA cm-2.

Consulting the conversion table we find that this current density now corresponds to a corrosion penetration rate of 25 mm y-1 or 990 mpy for the same steel specimen.

This oversimplified example illustrates one aspect of a crevice situation. The next step in a crevice situation stabilizes it into a permanent problem that will need drastic corrective measures.

Stage three of a crevice formation

In stage three of the crevice development a few more accelerating factors fully develop:
The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products. The pH in a crevice can reach very acidic values, sometimes equivalent to pure acids.

The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. See, for example, how the corrosion of steel is affected as a function of water pH.

The corrosion products seal even further the crevice environment.

The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in the environment, such as chlorides and sulfates, that can be corrosive in their own right.

Corrosion Rate Conversion

The following charts provide a simple way to convert data between the most common corrosion units in usage, i.e. corrosion current (mA cm-2) , mass loss (g m-2 day-1) and penetration rates (mm y-1 or mpy) for all metals or for steel
mA cm-2 mm year-1 mpy g m-2 day-1
mA cm-2 1 3.28 M/nd 129 M/nd 8.95 M/n
mm year-1 0.306 nd/M 1 39.4 2.74 d
mpy 0.00777 nd/M 0.0254 1 0.0694 d
g m-2 day-1 0.112 n/M 0.365 /d 14.4 /d 1

where: mpy = milli-inch per year
n = number of electrons freed by the corrosion reaction
M = atomic mass
d = density

Trust that is clear - remember I'll be asking questions later.

<hr width=100% size=1>
hammer.thumb.gif
 
http://www.corrosion-doctors.org/Localized/Crevice.htm

but here with links, pretty pictures etc

<hr width=100% size=1>Me transmitte sursum, caledoni
 
Wow! Er thanks!
Seriously though... I have printed that off and will keep it to read later, I think I sort of understand but will possibly ask questions later as you are obviously very knowledgable.
As a Mechanical Engineer I started out long ago in the aircraft industry (when we still had one) and converted to the Shipbuilding and Boatbuilding industries in turn.
(Started at the top and worked down you might say!)
It has always been interesting to me that the boatbuilding industry relies very heavily in the principle that "It worked last time so lets do it again" or "Oops! that does not work lets change it and try again" with the customer as the guinea pig.
Lately we have had a move towards "Cheaper is better and sells product" leading to lots of very poor boats that fly in the face of traditional knowledge built down to a price. Having been involved in properly disciplined value analysis I believe a scientific approach to things can result in better and lighter boats without resorting to poor practice and wrong materials if designers listen to experts like yourself. Unfortunately boats are designed at best by a Naval Architect with a general knowledge of things and perhaps (but not often) with the input of a hydrodynamicist and some form of structural designer (in a past life me) who perhaps applies a degree of finite element analysis if you are very lucky. Then they get in the hands of others who build them and all hell breaks loose. Mechanical and electrical systems are minimal considerations and correct choice of materials becomes secondary to cost. Its a sad old world and it's getting worse.
I guess the only way to stop the rot is to educate prospective owners in what to look for. I have always been very wary of any form of stainless steel below the waterline. Some so called "marine grades" are better than others, but personally I would rather see none at all! My own boat (built myself of course) has monel rudder shafts, bronze bearings, bronze skin fittings, and nothing underwater that is not protected by adjacent anodes. Being a catamaran I have the luxury of being able to lift my prop and drive leg out of the water when not being used to propel the boat. Does not stop corrosion of course but reduces electrolytic action. Last winter when out of the water next to a nameless boat from a well known maker, the owner had noticed mine and asked how I avoided corrosion.... When looking at his I noted a stainless steel propshaft (probably 316) with a bronze prop (nothing unusual so far) But the P bracket was welded aluminium with a brass (yes I swear brass) bearing. All bolts were stainless steel (even in the aluminium) He had anodes held on by stainless steel bolts too. All his skin fittings were brass (what variety god knows) with brass gate valves inside. The rudder bearings were nylon (ughhh!) on a 316 (?) shaft that was only tenuously attached to the rudder (approx 5 degrees movement either way) This boat was only 3 years old......
Do you find this frustrating too? Or as I suspect do you work in a more disciplined industry and sail for entertainment keeping your opinions to yourself until asked for as you were to good effect on this forum? (Wish I had developed that knack. Telling it as it is usually gets me into trouble!)
Either way thanks again for the detail. Very interesting and useful....




<hr width=100% size=1>
 
For those interested there are many good sites with info on corrosion and those like Brendan_s linked to are simple for the lay person to understand.

<hr width=100% size=1>
hammer.thumb.gif
 
You must log in or register to reply here.

Share this page

Top